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The application of deep‐learning‐based seismic phase pickers has surged in recent years. However, the efficacy of these models when applied to monitoring volcano seismicity has yet to be fully evaluated. Here, we first compile a data set of seismic waveforms from various volcanoes globally. We then show that the performances of two widely used deep‐learning pickers deteriorate systematically as the earthquakes' frequency content decreases. Therefore, the performances are especially poor for long‐period earthquakes often associated with fluid/magma movement. Subsequently, we train new models which perform significantly better, including when tested on two data sets where no training data were used: volcanic earthquakes along the Cascadia subduction zone and tectonic low‐frequency earthquakes along the Nankai Trough. Our model/workflow can be applied to improve monitoring of volcano seismicity globally while our compiled data set can be used to benchmark future methods for characterizing volcano seismicity, especially long‐period earthquakes which are difficult to monitor. Plain Language Summary: Earthquake activity at volcanic regions is often monitored to indicate volcanic activity. Identifying the time when the energy radiated from an earthquake source arrives at a seismometer is essential for locating the earthquake, which can be difficult for volcanic earthquakes because of high noise levels, high event rates, and obscured onsets. Previous studies have demonstrated that deep learning can excel in picking the arrival times of regular earthquakes. However, it is unclear how sensitive these detectors are to earthquakes in volcanic regions. Here, we first compile a data set of earthquakes from various volcanoes globally. We then show that existing deep‐learning‐based detectors can miss a large fraction of these earthquakes, especially those without an abrupt change in signal amplitude. We then provide two new models which can better detect volcanic earthquakes than existing models. Our model/workflow can be applied to improve monitoring of volcanic earthquakes globally. Key Points: We compile a data set of seismic waveforms from various volcanic regions globallyWe show that existing deep‐learning phase pickers' performances deteriorate with decreasing earthquake frequency contentOur retrained models perform better and are more generalizable for monitoring volcano seismicity, especially long‐period earthquakes [ABSTRACT FROM AUTHOR]

Dense and broad‐coverage ocean‐bottom observation networks enable us to obtain near‐fault displacement records associated with an offshore earthquake. However, simple integration of ocean‐bottom strong‐motion acceleration records leads to physically unrealistic displacement records. Here we propose a new method using a Kalman filter to estimate coseismic displacement waveforms using the collocated ocean‐bottom seismometers and pressure gauges. First, we evaluate our method using synthetic records and then apply it to an offshore Mw 6.0 event that generated a small tsunami. In both the synthetic and real cases, our method successfully estimates reasonable displacement waveforms. Additionally, we show that the computed waveforms improve the results of the finite fault modeling process. In other words, the proposed method will be useful for estimating the details of the rupture mechanism of offshore earthquakes as a complement to onshore observations. Plain Language Summary: Ocean‐bottom observations enable us to obtain near‐fault displacement records associated with an offshore earthquake. However, simple integration of ocean‐bottom acceleration records leads to physically unrealistic displacement records. Here we propose a new method to estimate offshore coseismic displacement waveforms. First, we evaluate our method using synthetic records and then apply it to an offshore earthquake that generated a small tsunami. In both cases, our method successfully estimates reasonable displacements. Additionally, we show that the computed waveforms improve the results of the earthquake source modeling process. In other words, the proposed method will be useful for estimating the details of the rupture mechanism of offshore earthquakes as a complement to onshore observations. Key Points: Propose a new method to estimate the near‐fault displacement waveform associated with an offshore earthquakeOur method adopts the Kalman filter approach to combine the collocated strong‐motion seismometer and ocean‐bottom pressure gaugeSince the obtained displacement waveforms have fault rupture information, it can improve the finite fault model [ABSTRACT FROM AUTHOR]

This study probes the lithosphere‐asthenosphere system beneath 155 Ma Pacific seafloor using teleseismic S‐to‐p receiver functions at the Pacific Lithosphere Anisotropy and Thickness Experiment project ocean‐bottom‐seismometers. Within the lithosphere, a significant velocity decrease at 33–50 km depth is observed. This mid‐lithospheric discontinuity is consistent with the velocity contrast between the background mantle and thin, trapped layers of crystallized partial melt, in the form of either dolomite or garnet granulite. These melts possibly originated from deeper asthenospheric melting beneath the flanks of spreading centers, and were transported within the cooling lithosphere. A positive velocity increase of 3%–6% is observed at 130–155 km depth and is consistent with the base of a layer with partial melt in the asthenosphere. A shear velocity decrease associated with the lithosphere‐asthenosphere boundary at 95–115 km depth is permitted by the data, but is not required. Plain Language Summary: Using seismic waves from distant earthquakes recorded by seismometers deployed on the seafloor, we investigated the seismic velocity structure beneath old Pacific seafloor. Within the lithosphere, we see a decrease in velocity at depths of roughly 40 km, which can be modeled as crystallized layers of partial melt that were emplaced in the lithosphere when it was young. The data indicate the presence of a velocity increase at depths of 130–155 km, and are consistent with (but do not require) a velocity decrease at depths of 95–115 km. These boundaries could mark the lower and upper boundaries of a layer in the asthenosphere that contains higher fractions of partially molten rock. Key Points: Lithosphere and asthenosphere beneath 155 Ma western Pacific crust are measured by S‐to‐p receiver functions using the Pacific Lithosphere Anisotropy and Thickness Experiment ocean‐bottom seismometers dataA mid‐lithospheric discontinuity at ∼40 km depth is observed and can be explained by layers of crystallized partial meltA partial melt‐bearing layer from 95 to 155 km depth is consistent with the receiver function observations [ABSTRACT FROM AUTHOR]

Small meteoroids that enter Earth's atmosphere often go unnoticed because their detection and characterization rely on human observations, introducing observational biases in space and time. Acoustic shockwaves from meteoroid ablation convert to infrasound and seismic energy, enabling fireball detection using seismoacoustic methods. We analyzed an unreported fireball in 2022 near the Azores, recorded by 26 seismometers and two infrasound arrays. Through polarization analyses, array methods, and 3‐D ray‐tracing, we determined that the terminal blast occurred at 40 km altitude, ∼60 km NE of São Miguel Island. This location matches an unidentified flash captured by a lightning detector aboard the GOES‐16 satellite. The estimated kinetic energy is ∼10−3 kT TNT equivalent, suggesting a 10−1 m object diameter, thousands of which enter the atmosphere annually. Our results demonstrate how geophysical methods, in tandem with satellite data, can significantly improve the observational completeness of meteoroids, advancing our understanding of their sources and entry processes. Plain Language Summary: Every year, hundreds to thousands of small near‐Earth objects, known as meteoroids, enter Earth's atmosphere. Their hypersonic entry speed and break‐up can generate flashes known as fireballs and associated shockwaves that can reach the ground. However, it is only the largest objects breaking up above populated areas that we typically see or hear, or that are captured by dedicated camera systems. Many of the smaller meteoroids go unnoticed. This observational bias limits our understanding of these objects and how they enter Earth's atmosphere. Here, we report on a fireball that broke up over the Northern Atlantic Ocean in June 2022 and was recorded on a network of seismometers that record sensitive ground motion and infrasound sensors that "hear" low‐frequency sound waves. Our analyses of these data show a small (40 cm diameter) meteoroid exploded at around 40 km altitude and 60 km northeast of São Miguel Island. Crucially, a flash recorded by a lightning mapper aboard a weather satellite provides us with the exact time of the explosion. To the best of our knowledge, this event is one of few documented cases of a fireball detected solely by geophysical means without relying on initial reports from human observers or photographic/video evidence. Key Points: We use seismic and infrasound data to characterize a previously unreported fireball over the North Atlantic Ocean in June 2022The fireball was detected by the Geostationary Lightning Mapper onboard the GOES‐16 satellite, giving a precise constraint on blast timeSeismoacoustic data, in tandem with satellite observations, can help to improve our observational completeness of near‐Earth objects [ABSTRACT FROM AUTHOR]

Geyser and volcano monitoring suffer from temporal, geographic, and instrumental biases. We present a recording bias identified through multiyear monitoring of Steamboat Geyser in Yellowstone National Park, USA. Eruptions of Steamboat are the tallest of any geyser in the world and they produce broadband signals at two nearby stations in the Yellowstone National Park Seismograph Network. In winter, we observe lower eruption signal amplitudes at these seismometers. Instead of a source effect, we find that environmental conditions affect the recorded signals. Lower amplitudes for 23–45 Hz frequencies are correlated with greater snow depths at the station 340 m away from Steamboat, and we calculate an energy attenuation coefficient of 0.21 ± 0.01 dB per cm of snow. More long‐term monitoring is needed at geysers to track changes over time and identify recording biases that may be missed during short, sporadic studies. Plain Language Summary: What we learn about geysers and volcanoes depends on when, where, and how we are able to monitor them. Here we present a case study of how seasonal changes affect data recorded on a seismometer, which is an instrument that measures ground motion. The world's tallest geyser, Steamboat Geyser in Yellowstone National Park, has intense eruptions that eject a mixture of water and steam. The eruptions are powerful enough to cause tiny ground motions from sound waves that begin in the air and then transfer into the ground. In the winter, we see smaller ground motions at two nearby seismometers. This might imply that Steamboat's eruptions are weaker in the winter; however, winter in Yellowstone comes with snow, and snow is good at absorbing sound wave energy. We find that smaller ground motions occur when snow depths are greater, and that the strength of ground motions should not be used to directly compare eruption intensity. Few geysers around the world are monitored with scientific equipment for long periods of time. Our result highlights the need for more of this type of monitoring so that we can identify biases that may be missed during shorter investigations. Key Points: Seismic signals from Steamboat Geyser eruptions contain ground‐coupled airwaves at distances up to 2.2 kmSnow dampens airwave arrivals leading to lower apparent signal amplitudes in winterInterpretation of eruption signals at both geysers and volcanoes should take changing environmental factors into account [ABSTRACT FROM AUTHOR]

Off Cape Muroto area, along the Nankai Trough in southwest Japan, is a typical area with adjacent occurrences of slow and megathrust earthquakes. High‐resolution monitoring of slow earthquakes is necessary to understand tectonic conditions. In the off Muroto area, distributed acoustic sensing (DAS) measurement, which provides high‐density strain data, has been conducted using offshore fiber‐optic cable. We observed shallow tremors, a type of slow earthquakes, using DAS measurement for the first time. The characteristics of the tremor signals recorded by DAS were longer durations than those recorded in seismographs and composed of several phases with apparent velocities of several hundreds of m/s to several km/s that are coherent only in tens of meters. By combining DAS and seismograph data, we located these tremors around a subducted seamount peak. Therefore, spatial relationship between slow earthquakes and structural characteristics is suggested. This is a pioneering study of slow earthquake observation using DAS. Plain Language Summary: Distributed acoustic sensing (DAS) measurement is a recent technology that uses a fiber‐optic cable as a strain sensor array. As DAS provides high‐density observation data, it has often been used for the observation of regular earthquakes in recent years. However, few studies have used DAS measurement to observe slow earthquakes, which have longer characteristic durations compared to regular earthquakes. We observed shallow tremors, a type of slow earthquake, using DAS for the first time off Cape Muroto, where slow and huge regular earthquakes occur in the neighboring areas. The characteristics of tremor signals in DAS data are long durations and composition of several phases with variable apparent velocities that are coherent only in tens of meters. We located tremors by picking the arrival of signals by visually checking the waveforms of DAS and an offshore seismograph network and locating them at the point where the difference between synthetic and observed arrival times was the smallest. Most tremors occurred around 134.7°E, 32.8°N, which corresponds to a subducted seamount peak; therefore, a spatial relationship between slow earthquakes and a subducted seamount is suggested in this region. Key Points: We first observed shallow tremors by distributed acoustic sensing (DAS) using offshore fiber‐optic cable in the Nankai TroughTremor signals of DAS are composed of several phases of variable apparent velocities that are coherent only in tens of metersSpatial relationship between shallow tremors and subducted seamount is suggested along the dip direction [ABSTRACT FROM AUTHOR]

Tremors are a type of slow earthquake with long‐duration signals compared to ordinary earthquakes. The long signals have been considered to solely reflect their long source process. However, here, we provide evidence suggesting that the source processes of tremors are not always long. We refer to these observations as short‐duration tremors. They were recorded by ocean‐bottom seismometers placed very close to the source. Although these tremors exhibit a short‐duration signal when recorded near the source, they exhibit a typical long‐duration signal elsewhere. Our numerical simulations demonstrate that the features can be captured by considering a strongly scattering medium around their source. One such structure could be small low‐velocity inclusions distributed around the seismic source. The inclusions may represent the seismic expression of geologically detected aquifers in tremor source regions. Furthermore, this medium could be embedded along the slow earthquake fault zone and play a critical role in their source process. Plain Language Summary: At least two types of earthquakes occur in shallow subduction zones: ordinary earthquakes and tremors. Tremors are known to exhibit long signal duration compared to ordinary earthquakes. To date, tremors' long‐duration signal has been solely interpreted by their source process. Here, we discovered tremors that exhibit short duration signals when recorded close from the source which we referred to as "short‐duration tremors". They suggest that tremors' source process is not always long and structural effects may partially form the typical long‐duration signals. We performed numerical simulations on elastic wave propagation and demonstrated that the observations can be qualitatively reproduced by assuming a strongly scattering material surrounding the seismic source. On the other hand, it has been reported based on ocean bottom drilling project that tremor source region may consist of patchily distributed aquifers. The inclusions in our model may be the seismic expressions of the geologically detected aquifers. Further, such a structure could be embedded along the slow‐earthquake fault zone and play a key role in their source process. Key Points: Short‐duration tremors in ocean‐bottom seismometer records suggest that the source process of tremors is not always longShort‐duration tremors can be interpreted by placing a strongly scattering medium around their sourceSuch a medium embedded along the slow earthquake fault zone could play a key role in the tremor source process [ABSTRACT FROM AUTHOR]

Knowledge of Martian crust and uppermost mantle aid us studying the planet's evolution. NASA's InSight mission provides seismic data being used to reveal the interior structure. Most studies have focused on the crustal structure beneath InSight lander, but the seismic structure of other regions has remained poorly known. We use surface‐wave data to investigate the crustal structure of a large region along the Medusa Fossae Formation and the dichotomy. We adopt the largest‐magnitude marsquake (S1222a) that has been recorded, which provides both Rayleigh‐ and Love‐wave signals. We measure and jointly invert these surface‐wave fundamental‐mode group velocities from ∼15 to 40 s to estimate the average 1D isotropic velocity models. These models includes a high‐velocity layer at ∼7‐km depth, which could be due to a regional basaltic activity or regional stress. Our models also indicate that a common intra‐crustal structure (∼12–40 km depth) may exist in this region along the dichotomy. Plain Language Summary: NASA's Mars exploration mission, InSight, brought a seismometer module that recorded numerous marsquakes. These marsquake recordings reveal the crustal structure beneath the InSight lander. To study the crustal structure of other regions, one can utilize a type of seismic waves, the surface waves. The largest marsquake event observed during InSight provides surface‐wave signals with a high signal‐to‐noise ratio. By analyzing these signals, we investigate the average crustal structure between the epicenter and the InSight landing site in a region near the equator and along the planet's dichotomy. We find a high‐velocity layer at about 7 km depth compared to the layers above and below, which could be due to a regional flood basalt or the regional crustal compressional stress. Our result also exhibits the similar crustal structure (from around 12 to 40 km depth) as the structure beneath the InSight lander, which indicates the possibility of a similar intra‐crustal structure existing along the Martian dichotomy. Key Points: Joint inversion of the S1222a Rayleigh‐ and Love‐wave group velocities provides 1D isotropic velocity models of the regional Martian crustThese models indicate a high‐velocity layer at ∼7 km depth which could be due to the regional basaltic activity or the regional stress fieldExcept for the high‐velocity layer, these models are similar to the crustal structure beneath InSight lander from ∼12 to 40 km depth [ABSTRACT FROM AUTHOR]

The shallow crustal structure of Mars records the evolutionary history of the planet, which is crucial for understanding the early Martian geological environment. Until now, seismic constraints on the Martian crust have come primarily from the receiver functions (RFs). However, analysis of the Mars RFs did not focus on the shallow structure (1–5 km) so far due to the limitation of the signal‐to‐noise ratio at high frequencies for most events. Here, we take advantage of the S1222a and six other marsquakes, which exhibit high signal‐to‐noise ratios, to probe the shallow structure of Mars. We observe a converted S‐wave at approximately 1 s after the direct P‐wave in the high‐frequency P‐wave RFs. This suggests a discontinutity at 2‐km depth between highly fractured and more coherent crustal materials. Plain Language Summary: The Martian shallow crustal structure is essential for understanding the geological evolution of Mars. The InSight lander successfully deployed a seismic station on Mars in late 2018, aiming to investigate the internal structure of Mars. Since most marsquakes detected previously have a low signal‐to‐noise ratio (SNR) at high frequencies, most seismic analyses do not focus on the shallow structure of Mars (1–5 km). However, when the InSight seismometer was near the end of its observational lifetime, a large marsquake occurred on sol 1222 with significant high‐frequency energy, far more than the noise level, allowing us to study the Martian shallow structure. We calculate the high‐frequency P‐wave receiver function (RF) of S1222a and extract a converted S‐wave at approximately 1 s after the direct P‐wave. To confirm the result, we also compute P‐wave RFs for high SNR events that occurred before. We observe this ∼1‐s signal in the high‐frequency P‐wave RFs of two additional large events as well. Combined with the geological analysis adjacent to the InSight lander, we attribute this 1‐s converted S‐wave to a discontinuity at approximately 2 km depth, probably corresponding to the bottom of highly fractured crustal materials beneath the InSight landing site. Key Points: We calculate high‐frequency P‐wave receiver functions (RF) from InSight seismic data of seven marsquakes with high signal‐to‐noise ratiosThe high‐frequency RFs exhibit a converted S‐wave at approximately 1 sThe ∼1‐s converted S‐wave suggests a discontinuity at a depth of approximately 2 km beneath the InSight lander [ABSTRACT FROM AUTHOR]

Barometers floating on high‐altitude balloons in the relatively clement cloud layer on Venus could detect and characterize acoustic waves generated by seismic activity, avoiding the need for high‐temperature electronics required for surface seismology. Garcia et al. (2022, https://doi.org/10.1029/2022GL098844) recently demonstrated the detection of low‐frequency sound (infrasound) caused by earthquakes of magnitudes 7.3 and 7.5 from stratospheric balloons nearly 3,000 km away from the epicenter. They provided a preliminary demonstration of earthquake magnitude and location inversion, and the determination of S‐ and Rayleigh wave velocities using only their acoustic signature. Large earthquakes produce low‐frequency seismic waves that penetrate the interiors of planets; their detection at continental‐scale distances from a high‐vantage point demonstrates the feasibility of balloon‐based investigations of Venus' interior. We contextualize these results within the effort to perform seismology on Venus from balloons, discuss its limitations, and share perspectives on open research questions in this area. Plain Language Summary: Scientists have peered inside Earth, the Moon, and Mars by recording how seismic waves pass through them. Venus' interior is a mystery, though, because the heat and pressure at its surface have thus far prevented direct measurement of seismic activity. Balloons with pressure recorders higher up in the Venusian atmosphere, where it is much cooler, can listen for the sounds the seismic waves make as a proxy for directly measuring ground motion. Before sending the balloons to Venus, it is important to make sure that this idea would actually work. The fact that (Garcia et al., 2022, https://doi.org/10.1029/2022GL098844) were able to record sound from large earthquakes thousands of kilometers away on Earth means that one should be able to do the same thing on Venus as well. While there are still details to work out, listening for quakes on Venus from balloons just got a major boost here on Earth. Key Points: A network of stratospheric balloons captured infrasound from several large earthquakesThis is the first detection of seismic activity from airborne sensors at continental distancesA similar network on Venus could capture and geolocate seismic activity [ABSTRACT FROM AUTHOR]

A meteor event on 11 December 2016 in southern Spain was accompanied by an audible boom and felt vibrations. Acousto‐seismic coupling of the near‐field pressure wave has been recorded by an unusually large number of 44 local seismic stations. Considering the ERA5 atmospheric temperature and wind speed model, we can attribute the seismic detections to a point source at 38 km height, coincident with the most luminous fragmentation event recorded by meteor cameras. We show and analyze waveforms, spectrograms and signal polarization along a dense, temporary broadband transect in the area. Seismic records show important differences between them, like large variability in polarization, amplitudes and frequency content, emphasizing the role of local effects. The duration and complexity of waveforms can be attributed to path and site effects, including multipathing through small‐scale atmospheric heterogeneity and scattering of the incoming acoustic wavefield at local topography. Plain Language Summary: The evening of 11 December 2016, a bright meteor was observed in southern Spain, followed by an audible boom and felt vibrations. The vibrations were recorded at 44 seismometers in a ∼110 × 100 km2 area. In order to locate the signal origin, we consider the atmospheric temperature and wind speed conditions at that time, and find that the wave arrival times are consistent with a source at 38 km height. The source can be associated with the principal explosion event of the meteor recorded by the cameras. A special aspect of this meteor fall is the density of the seismic observations, revealing large variability of the recordings at nearby stations. The observations suggest that the complexity of recorded waveforms is driven by the small‐scale atmospheric structure, local topography and obstacles around the recording stations, as well as the peculiarities of ground coupling of atmospheric acoustic waves. Key Points: A bright meteor event near Granada has been recorded at 44 seismic stations at local distancesSeismic location of the terminal burst using the ERA5 atmospheric model agrees with triangulation from regional meteor camerasWaveforms show large differences between nearby stations, suggesting an important imprint of local coupling and multipathing effects [ABSTRACT FROM AUTHOR]

Seismic instruments such as broadband seismometers and distributed acoustic sensors (DAS) have a demonstrated potential for wide‐scale and continuous in situ monitoring of near‐surface environmental and anthropogenic processes. DAS is attractive for development as a multi‐geophysical observatory due to the prevalence of existing fiber infrastructure in regions with environmental, cultural, or strategic significance. Here we present results from a multi‐seasonal acquisition of DAS data on a seafloor cable in the Beaufort Sea, Alaska. During a November 2021 data collect we captured the rapid transition of ambient noise characteristics from an "ice‐free" state to an "ice‐bound" state. A sea ice formation front was plainly visible on the DAS record and was observed to propagate 20 km seaward over a period of 8 hr. Satellite‐based instrumentation were unable to record this event due to cloud cover, low light conditions, and orbital frequency. Plain Language Summary: Distributed acoustic sensing (DAS) is an emerging technology that uses fiber optic cable to mimic a high resolution array of one dimensional seismometers. Distributed acoustic sensors is compatible with any single‐mode fiber optic cable, including seafloor telecommunications cables. Previous recordings of DAS data on seafloor cables have demonstrated that this technology is able to detect and track individual ocean waves, surface vessels, and whales. In this first‐ever acquisition of DAS data from a seafloor beneath a frozen ocean, we show that DAS is also capable of mapping sea ice coverage with unprecedented spatial resolution and can track sea ice formation and breakup up in near real time. Key Points: Seafloor distributed acoustic sensing can distinguish between sea ice and open water on the basis of endemic wave modesThis technology is capable of high resolution, near real time imaging of sea ice dynamics, such as rapid freezing of marginal iceThis can complement large scale, low resolution satellite‐based imaging in regions of high scientific or operational importance [ABSTRACT FROM AUTHOR]

The mild degassing and effusion that characterizes open‐vent volcanoes can be interrupted by episodes of sustained explosive activity known as paroxysms. Here, we present observations from a seismic array deployment during the 2021 eruption of Mt. Etna, Italy. During the observation period, degassing dominated surface activity at the central and northeast summit craters; lava flows, Strombolian explosions, and fire fountaining, accompanied by ash plumes, characterized eruption in the southeast sector of Mt. Etna. Seismic array locations showed changes associated with shifts in the style and location of activity across multiple craters at Mt. Etna. We observed changes in array locations between the north‐northeast and southeast directions that consistently anticipated the onset of paroxysmal activity in the southeast sector. Our results demonstrate the potential of seismic arrays to resolve vent‐specific activity and shed light on precursory patterns leading up to paroxysmal activity. Plain Language Summary: In this study we present analyses of seismic data collected at Mt. Etna, Italy, during a period of paroxysmal eruptive activity in the summer of 2021. Throughout the period under investigation, activity at the volcano was characterized by shifts in both style and location across the multiple summit craters. Analyses of seismic data recorded by a cluster of tightly spaced seismometers, or seismic array, allowed to track the location of seismic tremor throughout the study period. We show, with unprecedented resolution, that migration of the tremor source systematically anticipated the onset of the most intense phases of eruption. Our results are consistent with previously proposed models of a branched conduit system within the shallow plumbing at Mt. Etna; different branches of the conduit system feed activity from different vents within the multiple summit craters, each with a distinctive style of eruption. The results of our study demonstrate that seismic arrays can shed light on precursory patterns leading up to paroxysmal activity at Mt. Etna, and other similar volcanoes worldwide. We surmise that seismic arrays are a powerful monitoring tool with clear potential to improve volcano early warning practices and protocols. Key Points: Seismic arrays track degassing and explosive activity at multi‐vent volcanoesSystematic shifts in seismic tremor locations anticipate the onset of paroxysmal activity at Mt. EtnaShifts in tremor locations and style of surface activity at Mt. Etna are consistent with previous models of a branched conduit system [ABSTRACT FROM AUTHOR]

About 10% of Europe's surface area is prone to rapid flooding of rivers confined in valleys. The devastating potential of such floods is exacerbated by the deficits of existing gauging networks, including low station densities and recording frequencies, and lack of information beyond stage height. Here, we use seismic data of the July 2021 Ahrtal flood, Germany, to extract information to complement sparse hydrometric data, and to reconstruct the rapid evolution of this fatal event. We show that a seismic station can deliver essential flood metrics such as magnitude, propagation velocity and debris transport rate. These seismic products provide high resolution insight to the non‐linear flood behavior. We argue that an approach combining distributed low‐cost seismometers with existing seismic stations, can provide important real time data on future catastrophic floods and associated hazards in upland catchments, offering precious response time also in currently ungauged landscapes. Plain Language Summary: Rapidly evolving floods are a major hazard for 10% of European landscapes. They are hard to adequately detect and describe by the classic gauge station scheme, but seismic sensors provide a valuable alternative to this difficulty. A seismometer can sense a flood like the devastating one that hit the Ahr valley, Germany, in July 2021 up to 1.5 km away. The seismic footprint of the flood allows to provide information on flood magnitude, velocity and trajectory at sub‐minute resolution and at near real time. We show how this new approach can be utilized for future flood protection. Key Points: Seismic sensing of valley confined floods improves classic detection approachesNear‐real time information on flood magnitude, trajectory, and velocityGate keeper seismometer networks can improve flood risk management in Europe [ABSTRACT FROM AUTHOR]

In April 2022, a seismic swarm near Mt. Edgecumbe in southeast Alaska suggested renewed activity at this transform fault volcano, which was last active ≈800 years ago. Previously, thin rhyolitic tephras were deposited 5 and 4 ka. Satellite radar data from 2014 to 2022 resolves line‐of‐sight rapid inflation up to 7.1 cm/yr beginning in August 2018. Bayesian modeling suggests a transcrustal system of a deflating (−0.528 km3) dipping sill at 20 km depth recharging a magma chamber at 10 km (0.222 km3). A near‐vertical conduit could capture the volume difference without noticeable surface deformation. Reanalyzed seismicity, recorded 25 km away, shows increases since July 2019. Magma ascent through ductile material and brittle strain release in a stressed overburden could explain the time delay. Cloud‐native open data and workflows enabled discovery and analysis of this signal within days after going unnoticed for >3 years. Plain Language Summary: In April 2022, a cluster of earthquakes, detected near Mt. Edgecumbe in Southeast Alaska, suggested magmatic activity. As the volcano is near the major Queen‐Charlotte Fairweather fault that separates the Pacific and North American plates, similar earthquakes were previously assumed to be tectonic. Since magmatic activity at volcanoes is often accompanied by ground deformation, we analyzed available satellite radar data going back to fall of 2014. This reveals ongoing crustal motion toward the satellite of up to 7.1 cm/yr starting in August 2018. Modeling of the deformation suggests that magma sourced from gently dipping tabular body at 20 km depth is pooling at about 10 km depth. Since the nearest seismograph is 25 km away, we record only large seismicity, which increases in July 2019, but we perhaps missed some lower energy seismicity due to this distance. We believe that we are observing magma rising through malleable crust into an existing magmatic system and that the observed earthquakes are created as the overlying rock adjusts to the increased magmatic pressure. The observed activity is rare, especially in similar tectonic settings, and presents an opportunity to better understand the reactivation of dormant volcanoes. Key Points: InSAR reveals up to 7.1 cm/yr of constant line‐of‐sight inflation since August 2018 at Mt. Edgecumbe with microseismicity since July 2019Bayesian modeling suggests a deflating sill at 20 km and an inflating magma chamber at 10 km depth; a narrow conduit likely connects themWe expect continued deformation with future rate changes due to supply or visco‐elastic dynamics as observed at similar systems [ABSTRACT FROM AUTHOR]

Small‐scale convection beneath the oceanic plates has been invoked to explain off‐axis nonplume volcanism, departure from simple seafloor depth‐age relationships, and intraplate gravity lineations. We deployed 30 broadband ocean bottom seismometer stations on ∼40 Ma Pacific seafloor in a region notable for gravity anomalies, measured by satellite altimetry, elongated parallel to plate motion. P‐wave teleseismic tomography reveals alternating upper mantle velocity anomalies on the order of ±2%, aligned with the gravity lineations. These features, which correspond to ∼300°–500°K lateral temperature contrast, and possible hydrous or carbonatitic partial melt, are—surprisingly—strongest between 150 and 260 km depth, indicating rapid vertical motions through a low‐viscosity asthenospheric channel. Coherence and admittance analysis of gravity and topography using new multibeam bathymetry soundings substantiates the presence of mantle density variations, and forward modeling predicts gravity anomalies that qualitatively match observed lineations. This study provides observational support for small‐scale convective rolls beneath the oceanic plates. Plain Language Summary: Covered by kilometers of water and therefore hard to access, Earth's oceanic tectonic plates have several features we cannot explain. Among these are linear undulations ("rolls") in the strength of gravitational acceleration at the sea surface. Using data from a rare underwater seismic experiment, we have produced 3‐D maps of seismic properties of the Earth's sub‐surface in a location of clear gravity rolls. We find linear blobs of fast and slow material in the mantle beneath the oceanic plate, parallel to the gravity features. These represent cold sinking and warmer rising material, revealing a highly dynamic convective system underneath the plate, which has long been theorized but not previously directly observed at this scale. Key Points: We use ocean bottom seismometer data to image elongated upper mantle P‐wave velocity anomalies, of order ±2%, striking parallel to free air gravity lineationsThese features extend >200 km deep, requiring thermal anomalies and small‐fraction partial melts related to volatiles in the asthenosphereAnomalies are inferred to arise from small‐scale convective cells beneath the plate with a planform parallel to absolute plate motion [ABSTRACT FROM AUTHOR]

We conduct a high‐resolution teleseismic receiver function investigation of the subducting plate interface within the Alaskan forearc beneath Kodiak Island using data collected as part of the Alaska Amphibious Community Seismic Experiment in 2019. The Kodiak node array consisted of 398 nodal geophones deployed at ∼200–300 m spacing on northeastern Kodiak Island within the southern asperity of the 1964 Mw9.2 Great Alaska earthquake. Receiver function images at frequencies of 1.2 and 2.4 Hz show a coherent, slightly dipping velocity increase at ∼30–40 km depth consistent with the expected slab Moho. In contrast to studies within the northern asperity of the 1964 rupture, we find no evidence for a prominent low‐velocity layer above the slab Moho thick enough to be resolved by upgoing P‐to‐S conversions. These results support evidence from seismicity and geodetic strain suggesting that the 1964 rupture connected northern (Kenai) and southern (Kodiak) asperities with different plate interface properties. Plain Language Summary: We use 398 portable seismometers that were deployed as part of the Alaska Amphibious Community Seismic Experiment to image the boundary between the subducting Pacific plate and the base of the North American plate. The seismometers, spaced ∼200–300 m apart, were stationed on Kodiak Island in 2019 within the southern rupture area of the 1964 Mw9.2 Great Alaska earthquake. We analyze conversions from compressional to shear waves from distant earthquakes to understand the conditions of the plate interface. Our results show a dipping velocity increase at ∼30–40 km depth at the expected location of the Pacific slab crust‐mantle boundary. In contrast to prior results from the northern 1964 rupture zone, we do not find a low‐velocity layer on the subducting plate. Our results indicate that the 1964 rupture connected segments of the Alaskan subduction zone with different plate interface properties. Key Points: We present receiver function imaging from a dense three‐component nodal array deployment on Kodiak Island above the subducting Pacific PlateA clear slab Moho conversion is found but, in contrast to the Kenai Peninsula, there is no coherent low‐velocity layer atop the slabThe 1964 Great Alaska Earthquake ruptured across structural segments with different plate interface properties [ABSTRACT FROM AUTHOR]

A significant amount of stress in the Cascadia subduction zone (CSZ) is released by different forms of slow earthquakes including tremors, low‐frequency earthquakes, and very low‐frequency earthquakes (VLFEs)—all occurring onshore at the deeper part of the transition zone. Their activity offshore, however, remains elusive. We discover widespread occurrence of discrete VLFEs offshore Cascadia using ocean‐bottom seismometers. Barring occasional regular fast earthquakes, VLFEs are the only seismic stress indicators offshore. Using centroid moment tensor inversion and matched filtering, we detect sequences of 12 distinct families of VLFEs. The VLFEs in northern CSZ have focal mechanisms consistent with the area's subduction zone deformation. Interestingly, the VLFEs in the southern part of CSZ show strike‐slip faulting, attributed to offshore faults, sediment consolidation, subduction bending, and transpressional regimes created by complex plate tectonics. It challenges a canonical view of the seismogenic zone in Cascadia characterized by a frictionally homogeneous fault segment producing only regular fast earthquakes. Plain Language Summary: Very low‐frequency earthquakes (VLFEs) are a form of slow earthquakes which release significant amount of stress along the fault. Cascadia subduction zone shows little ordinary microseismic activity offshore and near the trench. We discover widespread and frequent occurrence of VLFEs offshore Cascadia. We observe 12 distinct VLFEs repeating over a time span of 6 months in 2014 along offshore northern and central Cascadia. The VLFE activity found in this study indicates that slow earthquakes occur in an area known for producing large megathrust earthquakes, potentially altering the stress condition, and influencing generation of damaging earthquakes. The offshore VLFEs are likely caused by the complex plate tectonics processes in this area suggesting a more frictionally heterogenous subduction fault than previously thought. Key Points: Widespread very low‐frequency earthquakes (VLFEs) offshore the Cascadia subduction zoneVLFEs serve as a crucial stress indicator in this area where seismicity is sparseVLFE distribution suggests that the seismogenic zone in Cascadia may host slow earthquakes [ABSTRACT FROM AUTHOR]

Single‐frequency microseisms (SFMs) have been revealed to be only generated in shallow water for decades, while some recent studies reported them in a deep ocean. Using the continuous waveform data recorded by ocean bottom seismometers, we investigate the deep ocean SFMs in the northern Philippine Sea. Based on the spectrum analysis, we find that the SFMs can be detected in deep ocean and the detection is time variable. To determine the source locations of SFMs, we perform polarization analysis and calculate the cross‐correlation coefficients between SFMs on vertical components and ocean wave energy spectra considering the attenuation of SFMs. Both the polarization and correlation results show that the sources nearby stations dominate the observed Rayleigh‐wave SFMs though distant sources also contribute. Our investigation suggests that the SFMs can be generated in a deep ocean likely by infragravity ocean waves interacting with seafloor topography, which is strengthened by strong ocean storms. Plain Language Summary: Microseisms are seismic noise caused by ocean waves impacting on the seafloor and recorded by seismic stations. Those of microseisms have similar periods of 10–20 s as ocean waves are called single‐frequency microseisms (SFMs), which are generally considered to be excited only in shallow water. Here we show that SFMs can be detected in a deep ocean from time to time especially when strong ocean storms impact. Combining the source azimuthal distributions of SFMs as well as the correlations between SFMs and ocean wave energy, we demonstrate that the derived dominating sources of abyssal SFMs are local, mostly nearby stations. The generating mechanism of the local SFMs is likely that long‐period ocean waves strengthened by strong ocean storms impact with the rugged seafloor topography. Key Points: The signals of single‐frequency microseisms can be detected in deep ocean greater than 2,000 m with temporal featuresThe Rayleigh‐wave single‐frequency microseisms (SFMs) observed are revealed to be mainly excited by the localized sourcesInfragravity ocean waves strengthened by storms and seafloor topography play important roles in the generation of deep ocean SFMs [ABSTRACT FROM AUTHOR]

Closely spaced, multi‐strand ridge transform faults (RTFs) accommodate relative motions along fast spreading mid‐ocean ridges. However, the relations between RTFs and plate spreading dynamics are poorly understood. The Quebrada system is one of the most unique RTF systems at the East Pacific Rise, consisting of four transform faults connected by three short intra‐transform spreading centers (ITSCs). We use seven‐months of ocean bottom seismograph data to study the Quebrada system, and find abundant earthquakes unevenly distributed among three active faults. We identify two deep, diffuse seismicity clouds at the inside corners of the ITSC‐transform fault intersections, and one seismically active fracture zone. The observations suggest a complex regional plate‐motion pattern, including possible heterogeneous rotations within the Quebrada system. Evolution of multi‐strand RTFs may have resulted from a strong three‐dimensional local thermal and fluid effects, while the RTFs may have also regulated regional tectonics, forming an intricate feedback system. Plain Language Summary: Mid‐ocean ridge transform faults (RTFs) are plate boundaries that offset adjacent mid‐ocean ridges. At fast spreading mid‐ocean ridges, such as the East Pacific Rise (EPR), closely spaced, multi‐strand RTFs are often connected by two or more short intra‐transform spreading centers (ITSCs). However, physical processes accommodating plate spreading along such multi‐strand RTF systems and the inter‐relations between the fault system and the tectonic dynamics are not well understood. Quebrada is one of such multi‐strand RTFs at the EPR. We utilize seven‐month seismic data from ocean bottom seismographs of a 2008 experiment to investigate the seismotectonics of the region. We find intriguing, abundant seismicity on one of the fracture zones, contradicting the traditional view that fracture zones are seismically quiescent. Further, we identify two diffuse seismicity clouds penetrating the uppermost mantle at the inside corners of the ITSC‐transform fault intersections, implying complex interactions among ITSCs, transform faults, and their surrounding structure. From these observations, we infer that there are rotational motions within the Quebrada fault system, which have caused slip along the fracture zone and facilitated fluid circulations to produce deep, diffuse seismicity. We speculate that there is a complex feedback system between the multi‐strand RTFs and local three‐dimensional tectonic processes. Key Points: Four segments of Quebrada transform system show significantly different seismic intensitiesOne fracture zone segment appears to be seismogenic indicating local plate rotationDeep clouds of seismicity extend into the uppermost mantle at spreading center‐ transform fault intersections [ABSTRACT FROM AUTHOR]

The aftershock distribution of the 2014 Mw 8.1 Iquique earthquake offshore northern Chile, identified from a long‐term deployment of ocean bottom seismometers installed eight months after the mainshock, in conjunction with seismic reflection imaging, provides insights into the processes regulating the updip limit of coseismic rupture propagation. Aftershocks updip of the mainshock hypocenter frequently occur in the upper plate and are associated with normal faults identified from seismic reflection data. We propose that aftershock seismicity near the plate boundary documents subduction erosion that removes mass from the base of the wedge and results in normal faulting in the upper plate. The combination of very little or no sediment accretion and subduction erosion over millions of years has resulted in a very weak and aseismic frontal wedge. Our observations thus link the shallow subduction zone seismicity to subduction erosion processes that control the evolution of the overriding plate. Plain Language Summary: To better understand the controls on shallow seismicity and subduction erosion following large subduction earthquakes, we use marine recordings of the Mw 8.1 2014 Iquique earthquake aftershocks and long‐offset multi‐channel seismic data. By comparing the aftershock locations and seismic imaging, we observe that most aftershocks occurred in the upper continental plate and abruptly stopped in the frontal forearc. The amplitude characteristics of upper‐crust reflections indicate a fractured and fluid‐filled outer forearc, which is associated with the absence of aftershocks. Large‐scale faulting, as evidenced by disrupted reflections in the seismic image, can be correlated to upper plate seismicity. We propose that the aftershocks updip of the main earthquake area reflect active subduction erosion processes. Key Points: We investigate structure and seismicity at the updip end of the 2014 Iquique earthquake rupture using amphibious seismic dataSeismicity updip of the 2014 Iquique earthquake occurs over a broad range likely interpreted to be related to the basal erosion processesCoseismic stress changes and aftershocks activate extensional faulting of the upper plate and subduction erosion [ABSTRACT FROM AUTHOR]

The seismometer deployed by the InSight lander measured the seismic velocity of the Martian crust. We use a rock physics model to interpret those velocities and constrain hydrogeological properties. The seismic velocity of the upper ∼10 km is too low to be ice‐saturated. Hence there is no cryosphere that confines deeper aquifers and possibly no aquifers locally. An increase in seismic velocity at depths of ∼10 km could be explained by a few volume percent of mineral cement (1%–5%) in pore space and may document the past depth of aquifers. Plain Language Summary: Large amounts of water may be stored in the Martian crust and episodically released to flood the surface. Where this water exists, and even why, is uncertain. The seismometer on the InSight lander measured the velocity of seismic waves in the Martian crust. The presence of ice and water affects seismic velocity. We argue that the measurements preclude a layer of ice‐filled crust that confines liquid water in an aquifer. Key Points: We interpret the seismic wave velocity of the Martian crust measured by InSightWe quantity the effects of ice and water on seismic velocity using a rock physics modelMeasurements preclude a layer of ice‐filled crust that confines liquid water in an aquifer [ABSTRACT FROM AUTHOR]

Detailed crustal and uppermost mantle structure is imaged for the first time utilizing ∼4 years of broadband seismic data newly collected in the Timor‐Leste and Nusa Tenggara Timor region of Indonesia. We apply three techniques, ambient noise tomography, teleseismic P wave receiver function, and coda autocorrelation, to resolve a 3D Vs model and Moho structure. Our tomographic images show low‐velocity anomalies (<30 km) beneath Timor related to underthrusted Gondwana sequence from the Australian plate, which are vertically offset by the high‐velocity backstop of the Banda forearc terrane. The structure progressively changes along strike, reflecting different collisional stages developed as a result of the oblique convergence. At greater depth, we detect seismically fast lithospheric mantle (>30 km) and the arc‐ward dipping Moho beneath Timor, both interpreted to be from the Australian plate. Our findings provide direct structural evidence of the Australian continental margin at lithospheric depths beneath the Banda Arc collisional zone. Plain Language Summary: The accretion of partial, or entire, volcanic arcs to a continental margin as they collide with a continent is one of the major processes in which new material is added to the crust over geological time. It also plays an important role in recycling crustal material back into the mantle via possible continental deep subduction. The Banda Arc–Australian continent collision currently taking place in SE Asia is one of the most classic examples of such process, yet its detailed deep structure that is critical to our understanding on how it evolves is still unclear due to the previous lack of dense data coverage. Resulting from a new seismic experiment consisting of 30 seismometers across the collisional zone and utilizing three complementing imaging techniques, we detect direct structural evidence of the underthrusted Australian passive margin at lithospheric depths with different components (continental crust, lithospheric mantle, and their boundary) beneath the Timor island. Key Points: Australian continental margin is detected beneath Timor by passive seismic imaging with new broadband data along the Banda ArcAcross arc structural variations in the crust reveal the collision of the Australian continent with the Banda forearc terraneThe progressive structural changes along strike reflect different collisional stages as a result of oblique plate convergence [ABSTRACT FROM AUTHOR]

The crust near an extinct mid‐ocean ridge provides unique constraints on how its accretion and deformation responded to the cessation of spreading. Here we present crustal thickness and Vp/Vs measurements beneath 11 Ocean Bottom Seismograph sites that cross the South China Sea's extinct spreading axis. We find that the oceanic crust, which generally had only slight thickness changes once spreading started, abruptly thins at sites close to the extinct ridge axis. Abnormally high Vp/Vs ratios are obtained at several sites south of the ridge, indicating the presence of serpentine. These observations imply that, in its final stage, spreading changed to an ultraslow accretion style. As the axial crust thinned, normal faults and/or detachment faults began to form. Water could penetrate more deeply through these faults, and large fault slip could raise ultramafic peridotites to near or at the seafloor, creating favorable conditions for their enhanced serpentinization. Plain Language Summary: Mid‐ocean ridges can both form and die. As a ridge dies, what happens to melt generation beneath the ridge axis? Clues exist in the oceanic crust near an extinct ridge that was created during the ridge's final stages of seafloor spreading. We explore a well‐known extinct spreading center within the South China Sea. Using ocean bottom seismometers and the receiver function technique, we determine the thickness of the magmatic crust, and a parameter (Vp/Vs) that reflects the structure of the underlying crust. We find that, approaching the extinct spreading axis, the crust, typically uniform in thickness, thins significantly. This indicates that the melt supply decreases relative to spreading rate during the final stages of spreading. The Vp/Vs ratio at three sites nearest to the extinct spreading axis is abnormally high. Our inference is that the spreading rate and magma production has decreased to the point where large crustal faults can develop at several locations along the ridge. These faults provide pathways for seawater to reach and react with the uppermost mantle, forming minerals with higher Vp/Vs ratios which are then uplifted to the seafloor as the faults grow. Key Points: Ocean Bottom Seismograph receiver functions constrain crustal structure near the extinct ridge of the South China SeaThinned crust indicates reduced spreading rate and magma supply during the final stage of spreadingAnomalously high Vp/Vs ratios are likely due to serpentine along faults formed in response to slowing spreading [ABSTRACT FROM AUTHOR]

Since landing on Mars, the NASA InSight lander has witnessed eight Phobos and one Deimos transits. All transits could be observed by a drop in the solar array current and the surface temperature, but more surprisingly, for several ones, a clear signature was recorded with the seismic sensors and the magnetometer. We present a preliminary interpretation of the seismometer data as temperature‐induced local deformation of the ground, supported by terrestrial analog experiments and finite‐element modeling. The magnetic signature is most likely induced by changing currents from the solar arrays. While the observations are not fully understood yet, the recording of transit‐related phenomena with high sampling rate will allow more precise measurements of the transit times, thus providing additional constraints for the orbital parameters of Phobos. The response of the seismometer can potentially also be used to constrain the thermoelastic properties of the shallow regolith at the landing site. Plain Language Summary: The geophysical lander, InSight, has been operating on the surface of Mars since November 2018. Since then, the Martian moons Phobos and Deimos have been partially blocking the Sun, as seen from the InSight landing site, multiple times. Multiple InSight instruments have been measuring the effect of those transits; this surprisingly includes the seismometer and the magnetometer. We conclude that temperature‐induced deformation and tilt are responsible for the seismic measurements. The change observed in the magnetometer measurements are most likely the result of a drop in the solar array currents. We do not observe atmospheric modulations with InSight's weather station during the transit. These observations help constrain orbital parameters of the Martian moons, and the seismometer signal might allow investigating thermoelastic properties of the shallow Martian material. Key Points: Multiple geophysical InSight instruments observe unexpected signals during Phobos transitsLocal ground deformation due to surface temperature drops explain the tilt signals seen by the seismometerThe drop in the solar array currents results in a change of the magnetic field [ABSTRACT FROM AUTHOR]

The Sanriku ocean‐bottom seismometer system uses an optical fiber cable to guarantee real‐time observations at the seafloor. A dark fiber connected to a Distributed Acoustic Sensing (DAS) interrogator converted the cable in an array of 19,000 seismic sensors. We use these measurements to constrain the velocity structure under a section of the cable. Our analysis relies on 24 hr of ambient seismic field recordings. We obtain a high‐resolution 2‐D shear‐wave velocity profile by inverting multimode dispersion curves extracted from frequency‐wave number analysis. We also produce a reflection image from autocorrelations of ambient seismic field, highlighting strong impedance contrasts at the interface between the sedimentary layers and the basement. In addition, earthquake wavefield analysis and modeling help to further constrain the sediment properties under the cable. Our results show for the first time that ocean‐bottom DAS can produce detailed images of the subsurface, opening new opportunities for cost‐effective ocean‐bottom imaging in the future. Plain Language Summary: Distributed Acoustic Sensing (DAS) is a relatively new measurement method that has the potential to convert existing fiber optic communication infrastructure into arrays of thousands of seismic sensors. In this research, we connected a DAS to a cable that was originally installed at the bottom of the ocean to sustain a seismic and tsunami observatory in the Sanriku Region. We show that this new type of measurement can provide reliable information to image and explore the shallow subsurface under this fiber cable. This is the first time such analysis is performed in an oceanic environment, and our methods could be readily exportable to other fiber‐optic cables that are the backbones of our modern telecommunication. Key Points: Ocean‐bottom Distributed Acoustic Sensing is used to image shallow VS structureRayleigh wave phase velocity dispersion curves are extracted from frequency‐wave number analysisReflection image is obtained from autocorrelations of ambient seismic field [ABSTRACT FROM AUTHOR]

Variations in stress drops of earthquakes associated with the April and May 2015 eruption of Axial Seamount, on the Juan de Fuca Ridge, suggest a reduction in crustal strength as a result of the eruption. Seismicity during the inflation and deflation periods was well recorded by ocean bottom seismometers located within and along the caldera. We use these nearby recordings and an empirical Green's function spectral ratio method to obtain corner frequencies for stress drops of earthquakes on caldera ring faults. We find stress drops from 0.6 to 43 MPa for 423 ring fault earthquakes (1.6 ≤ MW ≤ 3.6) and an average stress drop two times higher during the inflation period (6.4 MPa) prior to the eruption, than during the subsequent deflation (3.2 MPa). Stress drops also correlate with spatially varying shear wave speed, possibly reflecting a region of pervasive cracking in the northern caldera. Plain Language Summary: How do earthquakes reflect changes in crustal strength at a volcano? Seismometers on the seafloor at Axial Seamount (an underwater volcano off the coast of the U.S. Pacific Northwest) recorded thousands of earthquakes associated with Axial's 2015 eruption. We investigate changes in earthquake behavior that reveal a reduction in the strength of volcanic ring faults as a result of this eruption. We find that earthquakes release more stress when the volcano inflates before the eruption, than as the volcano deflates during and after the eruption. This suggests that increased crustal damage resulting from the eruption limits stress accumulation. Key Points: Ocean bottom seismic data are used to obtain stress drops of 423 ring fault earthquakes associated with the eruption of a submarine volcanoHigher stress drops during inflation and lower stress drops during deflation suggest decreased fault strength as a result of the eruptionStress drop spatial heterogeneity correlates with changes in shear wave speed, suggesting increased damage in Axial's northern caldera [ABSTRACT FROM AUTHOR]

The eastern Nankai Trough is a unique site where many shallow, very low frequency earthquakes (sVLFEs) are recorded by nearby broadband ocean bottom seismometers. Here, we estimated the locations and seismic moment tensors (MTs) of sVLFEs based on the low‐frequency (<0.07 Hz) components of the records. Although some sVLFEs exhibited long‐duration signals (>100 s), indicating a degree of source complexity, the MT inversions were limited to events with impulsive and short durations (20–30 s). Nevertheless, cluster analysis confirmed that the MTs of impulsive sVLFEs reasonably represented the overall deformation for an event group including many longer‐duration events. The distribution of MTs indicates that deformations associated with sVLFEs are influenced by a subducted oceanic ridge in this region, which produces an along‐the‐trough variation in a strain accumulation/release pattern and probably controls the spatial patterns of tsunami generation. Plain Language Summary: Locations and focal mechanisms of shallow, very low frequency earthquakes in the Nankai Trough were estimated using near‐field and broadband seismic records. The obtained distribution shows that it is influenced by a subducted oceanic ridge in this region. The seismically active area, or the area where strain accumulation/relase take place, appears to be pressed landward by the subducted oceanic ridge. Thus, tsunamigenic slip may occur closer to the landin the region, compared to regions without the subducted ridge. Key Points: Distribution of shallow very low frequency earthquakes in the Nankai Trough was estimated using near‐field broadband seismic recordsThe distribution infers that stress field and strain accumulation/release pattern in this region is influenced by a subducted oceanic ridgeThe subducted ridge presses seismically active area landward; thus, tsunamigenic slip may occur close to the land compared to other regions [ABSTRACT FROM AUTHOR]

The characteristics of shallow low‐frequency earthquakes (LFEs) are related to stress changes on the shallow plate boundary, which are important for understanding the megathrust earthquake cycle. The 5 September 2004 off the Kii Peninsula earthquakes (MJMA = 7.1, 7.4) occurred near the Nankai trough subduction zone, off southwest Japan. Ocean bottom seismometer observations from 22 September to 30 November 2004 detected many shallow LFEs among the ordinary aftershocks. During the observation period, the frequency of shallow LFEs steadily decayed with the exception of episodic activities, which were very sensitive to stress changes caused by tides and the 23 October 2004 Niigata earthquake (MJMA = 6.8). We also confirmed correlations between shallow LFEs and shallow very low frequency earthquakes, which suggest that these slow events represent the same slip phenomenon. These findings will contribute to clarifying the impact of LFEs on megathrust earthquakes. Key Points: We detected shallow low‐frequency earthquakes (LFEs) after the 2004 off Kii Peninsula earthquakes by using ocean bottom seismometersSome shallow LFEs are associated with very low frequency earthquakes, which suggests both slow earthquakes represent the same phenomenonEpisodic shallow LFEs are stochastic yet sensitive to stress changes caused by both the 2004 Niigata earthquake and tidal effects [ABSTRACT FROM AUTHOR]

Gravity waves impacting ice shelves illicit a suite of responses that can affect ice shelf integrity. Broadband seismometers deployed on the Ross Ice Shelf, complemented by a near‐icefront seafloor hydrophone, establish the association of strong icequake activity with ocean gravity wave amplitudes (AG) below 0.04 Hz. The Ross Ice Shelf‐front seismic vertical displacement amplitudes (ASV) are well correlated with AG, allowing estimating the frequency‐dependent transfer function from gravity wave amplitude to icefront vertical displacement amplitude (TGSV(f)). TGSV(f) is 0.6–0.7 at 0.001–0.01 Hz but decreases rapidly at higher frequencies. Seismicity of strong icequakes exhibits spatial and seasonal associations with different gravity wave frequency bands, with the strongest icequakes observed at the icefront primarily during the austral summer when sea ice is minimal and swell impacts are strongest. Key Points: Rift and icefront icequakes were identifiedIcefront icequakes are most frequent during the austral summer when swell forcing is greatest in the absence of sea iceAn empirical ocean‐to‐ice shelf displacement transfer function is obtained [ABSTRACT FROM AUTHOR]

Understanding deformation in ice shelves is necessary to evaluate the response of ice shelves to thinning. We study microseismicity associated with ice shelf deformation using nine broadband seismographs deployed near a rift on the Ross Ice Shelf. From December 2014 to November 2016, we detect 5,948 icequakes generated by rift deformation. Locations were determined for 2,515 events using a least squares grid‐search and double‐difference algorithms. Ocean swell, infragravity waves, and a significant tsunami arrival do not affect seismicity. Instead, seismicity correlates with tidal phase on diurnal time scales and inversely correlates with air temperature on multiday and seasonal time scales. Spatial variability in tidal elevation tilts the ice shelf, and seismicity is concentrated while the shelf slopes downward toward the ice front. During especially cold periods, thermal stress and embrittlement enhance fracture along the rift. We propose that thermal stress and tidally driven gravitational stress produce rift seismicity with peak activity in the winter. Plain Language Summary: In Antarctica, large bodies of floating ice called ice shelves help prevent ice on land from sliding into the ocean. To predict how Antarctica might respond to climate change, we need to understand how ice shelves interact with the environment, including the atmosphere and the ocean. The largest ice shelf, the Ross Ice Shelf, is over 500,000 km2 in area, making it the largest body of floating ice in the world. In this study, we deployed nine seismographs, the same instruments used to study earthquakes, to monitor vibrations and cracking within the Ross Ice Shelf over a 2‐year period. During that time, the instruments detected nearly 6,000 fracture events along a 120‐km‐long crack in the ice shelf. We compared the timing of the cracking to air temperature data, ocean wave activity, and tides to see whether these factors influenced the crack's behavior. We found that fracture occurs most frequently just after high tide during winter and when the air is very cold. We also found that fracture at the rift is not triggered by ocean waves. This work demonstrates that Antarctic ice shelves are very sensitive to the environment and highlights the need to continue studying them. Key Points: 2,515 icequakes associated with rift deformation on the Ross Ice Shelf are located using a double‐difference location algorithmIcequake timing correlates with tidal phase diurnally and inversely correlates with air temperature on multiday and seasonal time scalesOcean swell, infragravity waves, and a significant tsunami arrival are not correlated with increased rift activity [ABSTRACT FROM AUTHOR]

Characterizing shallow interplate slip is essential for modeling the potential generation of megathrust earthquakes and tsunamis. Large postseismic crustal deformation caused by the 2011 Tohoku‐Oki earthquake suggests the occurrence of shallow afterslip, but it is unclear what kind of slip is dominant at the shallow plate interface. Here we report episodic tremor activity south of the primary rupture area based on ocean bottom seismometer observations. Five tremor episodes with recurrence intervals of ~60 days show migration behavior and shear mechanisms consistent with plate subduction, indicating the occurrences of the episodic slow slip events in the vicinity of the afterslip area. The association of slow slip events and afterslip sheds new light on the frictional nature and stress state of shallow megathrusts and may help predict the possible rupture extent of future earthquakes. Plain Language Summary: Understanding the slip characteristics in the shallow part of subduction zone is essential to evaluate the potential risk of megathrust earthquakes and tsunamis. It remains unclear, however, what type of slip occurs at the shallow plate interface due to the difficulty in observing offshore seismic activity. In this paper, we report newly discovered tremor and slow slip activity in the afterslip region where coseimic rupture did not happen during the 2011 Tohoku‐Oki earthquake, based on a new campaign ocean bottom seismometer observation. We propose that the tremor and slow slip region illuminate the boundary between fast coseismic slip and stable aseismic slip that could limit the rupture extent of future megathrust earthquakes. The observed tremor and slow slip activities also suggest that the interplate stress state has been recovered to the level of that before the Tohoku earthquake. Key Points: We discover tectonic tremor activity in the Japan trench subduction zone based on a new campaign ocean bottom seismometer observationThe characteristics of tremor strongly suggest slow slip events in the vicinity of the afterslip area of the 2011 Tohoku earthquakeTremor and slow slip region may delineate the boundary between seismic and aseismic region along the plate interface [ABSTRACT FROM AUTHOR]

Distributed acoustic sensing (DAS) is a recently developed technique that has demonstrated its utility in the oil and gas industry. Here we demonstrate the potential of DAS in teleseismic studies using the Goldstone OpticaL Fiber Seismic experiment in Goldstone, California. By analyzing teleseismic waveforms from the 10 January 2018 M7.5 Honduras earthquake recorded on ~5,000 DAS channels and the nearby broadband station GSC, we first compute receiver functions for DAS channels using the vertical‐component GSC velocity as an approximation for the incident source wavelet. The Moho P‐to‐s conversions are clearly visible on DAS receiver functions. We then derive meter‐scale arrival time measurements along the entire 20‐km‐long array. We are also able to measure path‐averaged Rayleigh wave group velocity and local Rayleigh wave phase velocity. The latter, however, has large uncertainties. Our study suggests that DAS will likely play an important role in many fields of passive seismology in the near future. Plain Language Summary: Distributed acoustic sensing (DAS) is a newly developed technique that transforms telecommunication fiber optic cables into linear arrays of ground‐motion sensors. It has received much attention in the oil and gas industry recently. In this study, we conduct the Goldstone OpticaL Fiber Seismic experiment in Goldstone, California, and explore the potential of DAS in passive seismology using distant earthquakes. We first show that seismic data from the 10 January 2018 M7.5 Honduras earthquake recorded by DAS are of high fidelity by comparing them with that on a nearby broadband seismometer. We then demonstrate the utility of DAS in passive earthquake seismology, including receiver function analysis, densely distributed travel time measurements, and Rayleigh wave group/phase velocity estimation. These applications can potentially be used to provide high‐resolution structural images of the crust and mantle. Thus, DAS will likely play an important role in many fields of passive seismology in the near future. Key Points: We demonstrate the potential of DAS in teleseismic studies using the GOLFS experiment in Goldstone, CaliforniaDAS recordings can be used for travel time measurement, receiver function, and surface wave analysesDAS will likely play an important role in many fields of passive seismology in the near future [ABSTRACT FROM AUTHOR]

Ambient noise interferometry is becoming increasingly popular for studying seismic velocity changes. Such changes contain information on the structural and mechanical properties of Earth systems. Application to monitoring, however, is complicated by the large number of processes capable of inducing crustal velocity changes. We demonstrate this at White Island volcano over a 10‐year period containing multiple well‐documented eruptions. Using individual seismic stations, we detect velocity perturbations that we ascribe to volcanic activity, large earthquakes, and seasonality. Distant seismic stations capture widespread nonvolcanic changes that are also present at the volcano. Comparison between velocity changes recorded by distant and local stations then allows us to distinguish volcanic phenomena from seasonality. Through this, we resolve distinct features in ambient noise‐derived velocity changes that relate to volcanic unrest and a phreatic eruption, illustrating the strength of the approach. Plain Language Summary: Detecting small changes in physical properties at volcanoes is important toward effectively forecasting volcanic eruptions. One approach is to monitor the speed of seismic waves at the Earth's surface, which are sensitive to changes in the rock they pass through. At volcanoes, such changes may manifest through subsurface pressure increases preceding eruptions. Correctly interpreting these changes, however, is complicated. Many other, nonvolcanic, processes are also capable of producing similar wave speed perturbations. At White Island volcano, New Zealand, we detect changes associated with volcanic activity, large earthquakes, and weather‐related processes using seismometers located on the volcano. We demonstrate that volcanic and nonvolcanic features can be distinguished by comparing changes recorded at the volcano with those recorded at large distances, where volcanic changes are not expected to be observed. Through this, we resolve distinct differences that can be related to volcanic activity, illustrating the strength of this approach. Key Points: Individual three‐component seismic stations are well suited to monitoring at locations with sparse seismic coverageVolcanic, coseismic, and seasonally induced velocity changes highlight the complexity when interpreting ambient noise resultsDistant seismic stations provide a useful comparison to distinguish between volcanic and nonvolcanic processes [ABSTRACT FROM AUTHOR]

Scientifically valuable information can be learned by listening to the tiny vibrations emanating from a glacier with seismometers. However, this approach has never been employed to better understand glaciers protected from heat by a debris mantle, despite being common in the Himalayas, one of the most glacierized regions in the world. Here we installed a seismic network at a series of challenging high‐altitude sites on a glacier in Nepal. Our results show that the diurnal air temperature modulates the glacial seismic noise. The exposed surface of the glacier experiences thermal contraction when the glacier cools, whereas the areas that are insulated with thick debris do not suffer such thermal stress. Thus, the unprotected ice surface bursts with seismicity every night due to cracking, which gradually damages and weathers the ice. This is the first time such processes have been observed at relatively warm temperatures and outside of the polar regions. Plain Language Summary: It has been realized that much scientifically valuable information can be learned by listening to the tiny vibrations emanating from a glacier with sensitive sensors. However, due to their remoteness and the difficulties in accessing glacial environments, this approach has rarely been employed to better understand these important systems. For example, debris‐covered glaciers, which are protected from heat by a debris mantle, remain to be studied despite being common in the Himalayas, one of the most glacierized regions in the world. Here we installed a seismic network at a series of challenging high‐altitude sites on a glacier in Nepal. Our results show that the diurnal air temperature modulates the glacial seismic activity. A debris mantle dampens the diurnal amplitude of temperature and thus protects the ice from cyclic mechanical damage, whereas debris‐free (exposed) ice experiences intensive near‐surface fracturing early in the morning. This implies that the unprotected ice surface bursts with seismicity every night due to cracking, which gradually damages and weathers the ice. This is the first time such processes have been observed outside of the polar regions. These findings are in agreement with the personal experiences of climbers who felt and heard loud cracks on high‐altitude glaciers at night. Key Points: We present the first passive seismic experiment at a debris‐covered glacierDiurnal air temperature fluctuations influence the broadband seismic noiseDebris cover protects the glacier ice from thermal stress‐induced fracture [ABSTRACT FROM AUTHOR]

Ambient noise interferometry has become an increasingly popular tool for monitoring active volcanoes. We apply this method to investigate seven past eruptive periods at Veniaminof volcano, Alaska. Two of the largest eruptions studied show seismic velocity changes associated with preeruptive , coeruptive, and posteruptive volcanic processes. We develop and implement new analysis techniques to determine how seismic velocity changes at Veniaminof are distributed with depth. Spatiotemporal examination of these seismic velocity changes reveals evidence for the distribution of magma storage and the timescale at which magmatic fluids intrude into and reside within these storage regions in the months preceding eruption. We conduct the depth analysis using data recorded on a single seismometer. The same analysis could be applied to any volcano monitored by at least one seismometer in order to detect magmatic activity indicative of impending eruption, specifically the intrusion and migration of magmatic fluids into the volcanic system. Plain Language Summary: Veniaminof is one of the most active volcanoes in Alaska. Eruptions at the volcano occur frequently and can begin with little to no warning. In this study, we explore new and emerging seismic methods for identifying magmatic activity indicative of eruption. First, we apply the technique of ambient noise interferometry to determine subtle changes in seismic velocity within Veniaminof's subsurface. Such changes in seismic velocity indicate the movement of magmatic fluids into the volcanic system as early as weeks to months prior to the start of the two largest eruptions examined. In the second part of our study, we develop new techniques to determine changes in seismic velocity as a function of both depth and time. This analysis provides evidence for the timescales and depths at which magmatic fluids migrate through and are stored within the crust preceding eruption at Veniaminof. In order to carry out this spatiotemporal analysis, we utilize seismic data recorded on a single seismometer. Thus, the same analysis could be applied at any volcano monitored by even a single seismometer in an effort to identify the intrusion, migration, and storage of magmatic fluids in the time leading to eruption. Key Points: Ambient noise interferometry is a useful method for identifying magmatic activity associated with eruptionSeismic data recorded on a single seismic station can be used to determine changes in seismic velocity as a function time and depthSpatiotemporal changes in seismic velocity yield estimates of the timescale and depths at which magmatic fluids migrate through the crust [ABSTRACT FROM AUTHOR]

Continuous seismic observations across the Ross Ice Shelf reveal ubiquitous ambient resonances at frequencies >5 Hz. These firn‐trapped surface wave signals arise through wind and snow bedform interactions coupled with very low velocity structures. Progressive and long‐term spectral changes are associated with surface snow redistribution by wind and with a January 2016 regional melt event. Modeling demonstrates high spectral sensitivity to near‐surface (top several meters) elastic parameters. We propose that spectral peak changes arise from surface snow redistribution in wind events and to velocity drops reflecting snow lattice weakening near 0°C for the melt event. Percolation‐related refrozen layers and layer thinning may also contribute to long‐term spectral changes after the melt event. Single‐station observations are inverted for elastic structure for multiple stations across the ice shelf. High‐frequency ambient noise seismology presents opportunities for continuous assessment of near‐surface ice shelf or other firn environments. Plain Language Summary: Ice shelves are the floating buttresses of large glaciers that extend over the oceans and play a key role in restraining inland glaciers as they flow to the sea. Deploying sensitive seismographs across Earth's largest ice shelf (the Ross Ice Shelf) for 2 years, we discovered that the shelf nearly continuously sings at frequencies of five or more cycles per second, excited by local and regional winds blowing across its snow dune‐like topography. We find that the frequencies and other features of this singing change, both as storms alter the snow dunes and during a (January 2016) warming event that resulted in melting in the ice shelf's near surface. These observations demonstrate that seismological monitoring can be used to continually monitor the near‐surface conditions of an ice shelf and other icy bodies to depths of several meters. Key Points: High‐frequency (>5 Hz), narrow‐band signals observed on an ice shelf are sensitive to changes in the near‐surface firn layerSpectral peak frequency changes coincide with melt/freeze events on the ice shelf as well as with storm‐driven redistribution snowMelt events have a unique spectral signature and can be modeled in terms of the penetration depth to which these thermal anomalies diffuse [ABSTRACT FROM AUTHOR]