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Volcanic eruptions carry essential information on the dynamics of volcanic systems. Studies have documented variable eruption styles and eruptive surface deformation. However, co‐eruptive subsurface structural changes remain poorly understood. Here we characterize the seismic velocity changes from July 2019 to July 2023 at Great Sitkin Volcano in the central Aleutian volcanic arc, using single‐station ambient noise interferometry at five three‐component seismic stations. Coincident with the lava effusion since late July 2021, about two months after the explosive eruption on 26 May 2021, we observe a sustained velocity increase, most prominently to the northwest of the caldera. We attribute this velocity increase to the structural changes with magma extrusion, with the spatial variation controlled by the geometry of the magma system or the property of shallow volcaniclastics. Our findings offer insights into understanding co‐eruptive structural modifications at active volcanoes. Plain Language Summary: Volcanic eruptions provide important insights into how volcanoes work and the potential risks they pose. This study looks at the ongoing eruption at Great Sitkin Volcano in the central Aleutian volcanic arc to better understand the changes happening beneath the surface. We measure the changes in the velocity of seismic waves, with data from five seismometers on the island. We collected data from July 2019 to July 2023, covering different eruptive stages and multiple lava effusion episodes during the eruption that started on 26 May 2021. Coincident with the lava effusion since late July 2021, we notice a significant increase in seismic velocities, most prominently at a station northwest of the volcano's central crater. We conclude that this increase reflects the structural change in the shallow subsurface due to magma extrusion, with deflation of the edifice and closure of cracks. The findings in this study help to improve our understanding of how volcanoes behave during eruptions. Key Points: We observe prominent, spatially variable increases in seismic velocity during lava effusions at Great Sitkin Volcano since late July 2021The sustained co‐eruptive increase in seismic velocity reflects structural changes with crack closures as the consequence of magma extrusionThe distinct spatial variation of the velocity increase may be controlled by reservoir geometry or the strength of shallow volcaniclastics [ABSTRACT FROM AUTHOR]

This study presents a 3D regional modeling of seismic geology and shear wave velocity (Vs) in Metro Vancouver for seismic microzonation and hazard prediction. Leveraging an extensive geodatabase compiled from invasive and non-invasive in situ data, including lithological logs and seismic field data, we delineated four major geological units: Holocene post-glacial and Pleistocene inter/glacial sediments, and Tertiary sedimentary and Pre-Tertiary Coast Mountain plutonic rocks. Seismic geology model integrates the four primary geological formations, leveraging significant impedance-based surfaces derived from meticulously analyzed borehole stratigraphic logs and Vs depth profiles sourced from 2333 georecords, enhancing its depth and accuracy. Through a meticulous comparison with established interpreted geological cross-sections, we have reaffirmed the robustness and reliability of our seismic geology modeling approach. A numerical 3D “geotechnical layer” Vs model with 11 isovelocity surfaces was developed using 688 Vs depth profiles. Comparison with microtremor amplification spectra confirms our 3D models' reliable use in predicting site amplification. We find that the combination of local geology (thicknesses) and Vs information outperforms prediction in fundamental peak frequency compared to using only local geology combined with regional Vs information. Our study contributes to advancing understanding of seismic hazards in Metro Vancouver, highlighting the importance of incorporating localized seismic site conditions for precise regional seismic hazard assessments.

Abstract Volcanic eruptions carry essential information on the dynamics of volcanic systems. Studies have documented variable eruption styles and eruptive surface deformation. However, co‐eruptive subsurface structural changes remain poorly understood. Here we characterize the seismic velocity changes from July 2019 to July 2023 at Great Sitkin Volcano in the central Aleutian volcanic arc, using single‐station ambient noise interferometry at five three‐component seismic stations. Coincident with the lava effusion since late July 2021, about two months after the explosive eruption on 26 May 2021, we observe a sustained velocity increase, most prominently to the northwest of the caldera. We attribute this velocity increase to the structural changes with magma extrusion, with the spatial variation controlled by the geometry of the magma system or the property of shallow volcaniclastics. Our findings offer insights into understanding co‐eruptive structural modifications at active volcanoes.

We present SeisMIC, a fast, versatile, and adaptable open-source software to estimate seismic velocity changes from ambient seismic noise. SeisMIC includes a broad set of tools and functions to facilitate end-to-end processing of ambient noise data, from data retrieval and raw data analysis via spectrogram computation, over waveform coherence analysis, to post-processing of the final velocity change estimates. A particular highlight of the software is its ability to invert velocity change time series onto a spatial grid, making it possible to create maps of velocity changes. To tackle the challenge of processing large continuous datasets, SeisMIC can exploit multithreading at high efficiency with an about five-time improvement in compute time compared to MSNoise, probably the most widespread ambient noise software. In this manuscript, we provide a short tutorial and tips for users on how to employ SeisMIC most effectively. Extensive and up-to-date documentation is available online. Its broad functionality combined with easy adaptability and high efficiency make SeisMIC a well-suited tool for studies across all scales.

Microcracks in geomaterials cause variations in the elastic moduli under applied strain, thereby creating seismic wave velocity variations. These are crucial for understanding the dynamic processes of the crust, such as fault‐zone damage, healing, and volcanic activities. Solid earth tides have been used to detect seismic velocity changes responding to crustal‐scale deformations. However, no prior research has explored the characteristics of the seismic velocity variations caused by large‐scale tidal deformation. To systematically evaluate the tidal response to velocity variations, we developed a new method that utilized the flexibility of a state‐space model. The tidal response was derived from hourly stacked noise autocorrelations using a seismic interferometry method throughout Japan. In particular, large tide‐induced seismic velocity changes were observed in the low S‐wave velocity region of the shallow crust. Overall, the tidal responses of velocity variations can provide new insights into the response mechanisms of the shallow crust to applied strain. Plain Language Summary: Rock deformations can open or close microcracks in rocks along with varying their elastic moduli under an applied strain. The temporal variations in the elastic moduli of rocks alter the seismic wave velocity, which can be monitored to provide information on the strain applied to the crust. This is crucial for understanding the geological processes in fault zones and volcanic regions. To utilize the seismic velocity variations for monitoring how much the Earth's structure deforms, the response of the seismic velocity to the deformations must be assessed. The deformation of the Earth's surface caused by lunar and solar gravity, called solid Earth tides, has been used to study seismic velocity variations in response to crustal deformation. However, only a limited number of regions have been studied for the tidal response of the seismic velocity, and the characteristics of its variations caused by tidal deformation were not yet apparent. This study measured the tidal responses to seismic velocity variations throughout Japan with reliable estimations. Notably, the tide‐induced seismic velocity variations tend to increase in the low S‐wave velocity region. Overall, these results provide new insights into the response mechanisms of the shallow crust to deformations. Key Points: The spatial distribution of seismic velocity changes caused by tides was determined using a dense network of seismic stations in JapanThe tidal response of velocity variations was extracted from ambient noise using an extended Kalman filter with a Maximum Likelihood methodStrain‐velocity sensitivities tend to increase at a low S‐wave velocity in the shallow crust [ABSTRACT FROM AUTHOR]

Volcanic inflation and deflation often precede eruptions and can lead to seismic velocity changes (dv/v $dv/v$) in the subsurface. Recently, interferometry on the coda of ambient noise‐cross‐correlation functions yielded encouraging results in detecting these changes at active volcanoes. Here, we analyze seismic data recorded at the Klyuchevskoy Volcanic Group in Kamchatka, Russia, between summer of 2015 and summer of 2016 to study signals related to volcanic activity. However, ubiquitous volcanic tremors introduce distortions in the noise wavefield that cause artifacts in the dv/v $dv/v$ estimates masking the impact of physical mechanisms. To avoid such instabilities, we propose a new technique called time‐segmented passive image interferometry. In this technique, we employ a hierarchical clustering algorithm to find periods in which the wavefield can be considered stationary. For these periods, we perform separate noise interferometry studies. To further increase the temporal resolution of our results, we use an AI‐driven approach to find stations with similar dv/v $dv/v$ responses and apply a spatial stack. The impacts of snow load and precipitation dominate the resulting dv/v $dv/v$ time series, as we demonstrate with the help of a simple model. In February 2016, we observe an abrupt velocity drop due to the M7.2 Zhupanov earthquake. Shortly after, we register a gradual velocity increase of about 0.3% at Bezymianny Volcano coinciding with surface deformation observed using remote sensing techniques. We suggest that the inflation of a shallow reservoir related to the beginning of Bezymianny's 2016/2017 eruptive cycle could have caused this local velocity increase and a decorrelation of the correlation function coda. Plain Language Summary: Before eruptions, volcanoes inflate due to the rising magma from below. Previous studies have found that these deformations can lead to small changes in the properties of the surrounding rock. We use passive image interferometry, a method that relies on the omnipresent background vibration of the Earth—mostly induced by the oceans, to measure these changes at the Klyuchevskoy Volcanic Group in Kamchatka, Russia. However, in Kamchatka, this background noise is masked and distorted by small earthquakes and tremors originating from the volcanoes themselves. We combine machine learning techniques with established monitoring methods to find times when these tremors remain similar. Afterward, we use data from these time periods in the conventional way to observe changes in the soil and the rock. Our results show that rain‐ and snowfall and the thickness of the snow cover exert the strongest influence on the properties of the rocks. Additionally, we found that a large magnitude 7.2 earthquake, which struck Kamchatka during our study, caused a slight weakening of the rocks due to microstructural damage. We register changes shortly before an eruption and suggest a connection to the beginning of an eruptive cycle in 2016. Key Points: Fluctuating noise conditions lead to distortions in noise interferometry studies, which we avoid with the help of machine learningThe seismic velocity on Kamchatka is affected by numerous mechanisms, amongst them environmental, tectonic, and volcanic eventsWe observe a velocity increase at Bezymianny during February 2016 and link it to the beginning of the eruptive cycle [ABSTRACT FROM AUTHOR]

Abstract Earth’s crust responds to perturbations from various environmental factors. To evaluate this response, seismic velocity changes offer an indirect diagnostic, especially where velocity can be monitored on an ongoing basis from ambient seismic noise. Investigating the connection between the seismic velocity changes and external perturbations could be useful for characterizing dynamic activities in the crust. The seismic velocity is known to be sensitive to variations in meteorological signals such as temperature, snow, and precipitation as well as changes in sea level. Among these perturbations, the impact of variations in sea level on velocity changes inferred from seismic interferometry of ambient noise is not well known. This study investigates the influence of the ocean in a 3-year record of ambient noise seismic velocity monitoring in the Chugoku and Shikoku regions of southwest Japan. First, we applied a bandpass filter to determine the optimal period band for discriminating among different influences on seismic velocity. Then, we applied a regression analysis between the proximity of seismic station pairs to the coast and the ocean influence, as indicated by the correlation of sea level to seismic velocity changes between pairs of stations. Our study suggests that for periods between 0.0036 to 0.0155 cycle/day (64–274 days), the ocean’s influence on seismic velocity decreases with increasing distance of station pairs from the coast. The increasing sea level deforms the ocean floor, affecting the stress in the adjacent coast. The stress change induced by the ocean loading may extend at least dozens of kilometers from the coast. The correlation between sea level and inland seismic velocity changes is negative or positive. Although it is difficult to clearly interpret the correlation based on a simple model, they could depend on the in situ local stress, orientation of dominant crack, and hydraulic conductivity. Our study shows that seismic monitoring may be useful for evaluating the perturbation in the crust associated with an external load. Graphical abstract

Abstract Microcracks in geomaterials cause variations in the elastic moduli under applied strain, thereby creating seismic wave velocity variations. These are crucial for understanding the dynamic processes of the crust, such as fault‐zone damage, healing, and volcanic activities. Solid earth tides have been used to detect seismic velocity changes responding to crustal‐scale deformations. However, no prior research has explored the characteristics of the seismic velocity variations caused by large‐scale tidal deformation. To systematically evaluate the tidal response to velocity variations, we developed a new method that utilized the flexibility of a state‐space model. The tidal response was derived from hourly stacked noise autocorrelations using a seismic interferometry method throughout Japan. In particular, large tide‐induced seismic velocity changes were observed in the low S‐wave velocity region of the shallow crust. Overall, the tidal responses of velocity variations can provide new insights into the response mechanisms of the shallow crust to applied strain.

Campi Flegrei is a volcanic field located west of Naples (Italy) in a densely populated area. Since 2005, its ground has been rising steadily due to the accumulation of fluids at shallow depths. The inflation of volcanic edifices is a possible precursor of an impending eruption. The uplift is accompanied by increasing seismic activity. This raises concerns about the possibility that the volcano may be on the verge of an eruption. To track the fluid movement, it is possible to monitor subtle changes of velocities of seismic waves by exploring ambient seismic noise. By examining different frequency bands, we can observe velocity changes at different depths. We interpret these changes as a monitoring of depth-dependent deformation in addition to the standard monitoring of surface deformation. We observe a velocity decrease in the long-term trend, presumably due to the extension of the hydrothermal system at shallow depths. To explain the long-term changes, we model a spherical pressure source to simulate volumetric strain changes induced by recent fluid activity. The model explains both, surface and subsurface deformation which leads to the opening of microcracks and pores, resulting in the observed velocity decrease. The short-term velocity changes are mainly driven by temperature or groundwater level changes. Once velocity changes are corrected for seasonal effects, remaining short term velocity changes can be associated with volcanic activity and earthquake swarms. • Monitoring of seismic velocity variations using passive image interferometry. • Long-term velocity change due to volumetric changes of microcracks and pores. • Short-term velocity change due to seasonal temperature and groundwater fluctuations. • Removing seasonal effects reveals short-term velocity changes from volcanic activity. [ABSTRACT FROM AUTHOR]

• Hourly velocity change in the xishan village landslide measured from ambient noise. • Decoupled the velocity anomalies related to different environmental factors. • Revealed the impact of landslide activities on rainfall infiltration. Numerous potential landslides with slow sliding speeds pose a continuous threat to human safety. The movements of these landslides continuously modify hydrogeological conditions and lead to cracks expansion, making it easier for subsequent rainfall to infiltrate into the landslide interior. Monitoring the tiny variations of structures and pore-water pressure in the interior of landslides can help understanding this coupling mechanism and accessing landslide damage risk. Here, we utilized seismic ambient noise interferometry to assess hourly velocity changes that enable us to quantify the response of landslides to external factors at different depths, including earthquakes, rainfall, and mechanical effects from sliding. In a case of Xishan Village Landslide, Sichuan Province, China, we observed an outstanding velocity drop coinciding with the onset of rainfall and bursts in tremor activity in a period in which the landslide experienced vertical displacement. Our observations highlight the new sight to investigate how the mechanical process of landslide movement modifies hydrogeological conditions and promotes rainfall infiltration, as well as assesses the subsequent recovery process. [ABSTRACT FROM AUTHOR]

Earth's crust responds to perturbations from various environmental factors. To evaluate this response, seismic velocity changes offer an indirect diagnostic, especially where velocity can be monitored on an ongoing basis from ambient seismic noise. Investigating the connection between the seismic velocity changes and external perturbations could be useful for characterizing dynamic activities in the crust. The seismic velocity is known to be sensitive to variations in meteorological signals such as temperature, snow, and precipitation as well as changes in sea level. Among these perturbations, the impact of variations in sea level on velocity changes inferred from seismic interferometry of ambient noise is not well known. This study investigates the influence of the ocean in a 3-year record of ambient noise seismic velocity monitoring in the Chugoku and Shikoku regions of southwest Japan. First, we applied a bandpass filter to determine the optimal period band for discriminating among different influences on seismic velocity. Then, we applied a regression analysis between the proximity of seismic station pairs to the coast and the ocean influence, as indicated by the correlation of sea level to seismic velocity changes between pairs of stations. Our study suggests that for periods between 0.0036 to 0.0155 cycle/day (64–274 days), the ocean's influence on seismic velocity decreases with increasing distance of station pairs from the coast. The increasing sea level deforms the ocean floor, affecting the stress in the adjacent coast. The stress change induced by the ocean loading may extend at least dozens of kilometers from the coast. The correlation between sea level and inland seismic velocity changes is negative or positive. Although it is difficult to clearly interpret the correlation based on a simple model, they could depend on the in situ local stress, orientation of dominant crack, and hydraulic conductivity. Our study shows that seismic monitoring may be useful for evaluating the perturbation in the crust associated with an external load. [ABSTRACT FROM AUTHOR]

Abstract We analyze temporal variations in the travel times of both P and S waves (Vp and Vs) for 14 months at Toyohashi (central Japan) with a continuously operating vibration source that could efficiently produce both P and S waves. Seismic waves produced by the source, which is called the accurately controlled routinely operated signal system (ACROSS), are recorded by three nearby seismic stations, and the travel time variation at each station is estimated using the transfer function calculated from the recorded data. Long-term variations in Vp and Vs are observed and can be interpreted by the change in fluid saturation and crack density of the subsurface rocks. The variation in fluid saturation and crack density are consistent with that in the groundwater level, which is measured at the station nearest to the ACROSS. Short-term responses to rainfalls are observed at the station nearest to the ACROSS system; the interpretation of the changes in crack density and saturation is inconsistent with the ground water observation, partly owing to the initial response to rainfall. This can be interpreted as an air–water mixture within pores or cracks on a fine scale.

The 2018 Kīlauea caldera collapse produced extraordinary sequences of seismicity and deformation, with 62 episodic collapse events which significantly altered the landscape of the summit region. Despite decades of focused scientific studies at Kīlauea, detailed information about the internal structure of the volcano is limited. Recently developed techniques in seismic interferometry can be used to monitor the internal structure of an active volcano more directly by detecting subtle spatiotemporal changes in seismic wave velocity, but their utility relies on accurate interpretations of the underlying phenomena causing those velocity changes. Here, we retrospectively apply repeating‐earthquake‐based seismic interferometry to the 2018 Kīlauea eruption sequence. We find that seismic velocities changed over two distinct time scales: a sudden increase followed by a slower decrease in velocity in the hours following each collapse event, and a gradual, long‐term decrease in velocity over several weeks that ceased approximately 1 month prior to the end of the eruption. Modeling suggests that short‐term changes can be explained by magma reservoir pressurization which specifically closed vertical ring fractures. Long‐term changes are related to subsidence of the caldera and likely include the influence of inelastic strain from the formation of new fractures. These observations provide new insights into the evolution of Kīlauea during its progressive collapse and will inform future interpretations for near‐real‐time monitoring at hazardous volcanoes around the world using similar techniques, especially where a dominant fracture orientation is present. Plain Language Summary: In 2018, the caldera of Kīlauea volcano, Hawaiʻi, collapsed as magma from the summit reservoir system drained to feed the eruption in the lower East Rift Zone. Changes in the area around the magma reservoir beneath Halemaʻumaʻu crater affected the speed of seismic wave propagation, which we measured using naturally occurring earthquakes. Changes in the seismic velocity were caused by the opening and closing of small cracks as the pressure in the reservoir changed and by the creation of a new fault. Our work shows that the orientation of cracks may be an important consideration when interpreting the relation between velocity change and deformation. Key Points: We used repeating earthquakes to derive relative seismic velocity changes with time during the last 2 months of caldera collapseSeismic velocity changes are correlated with deformation (strain) occurring over two distinct time scales with different causesFracture orientation may control which component of strain seismic velocity is sensitive to and important to consider for interpretation [ABSTRACT FROM AUTHOR]

Abstract A sequence of eruptions in the lower East Rift Zone (LERZ) of the Kilauea, Hawaii volcano commenced in early May 2018 and caused serious damage and residential evacuation. The post-eruption drainage and collapses of the summit lava lake and caldera suggest a well-connected magmatic plumbing system along the rift zone. How and when the pre-eruptive magmatic processes in such connecting system occurred are, however, still unclear. For that, we apply ambient noise interferometry with seismic data from January 2017 to June 2018 from 12 broadband seismometers to investigate spatiotemporal seismic velocity changes (dv/v) of the upper crust in the Kilauea area. The dv/v variations in three frequency bands (0.3–0.6 Hz, 0.6–0.9 Hz, and 0.9–2.0 Hz) show distinct responses to strong earthquake ground shaking and deep magmatic intrusion processes. Earthquake-induced dv/v drops mainly in the higher two frequency bands imply shallow mechanical changes within the uppermost 1 km of the crust. In contrast, the magma-related dv/v changes can be characterized into three periods of activity: from the December 2017 to March 15, 2018, a dv/v excursion only seen in the lowest frequency band indicates the magmatic intrusion processes taking place at the depth range of 1–4 km, consistent with the proposed depth of the magma reservoir-dike system in the rift zone. The spatial dv/v distribution suggests that the magma may intrude to the deeper summit magma reservoir and in the upper East Rift Zone (UERZ) at this time. From March 15 to April 17 in 2018, the summit inflation recorded by tiltmeters causes the dv/v increases in the higher two frequency bands. After April 17 to the eruption, the accumulating damage of the edifice together with the stronger intrusion activity in the UERZ result in dv/v decreases in all three frequency bands around the summit and UERZ areas. Our observations highlight that the ambient noise interferometry analysis provides an opportunity to image and understand the pre-eruptive processes of reservoir-dike magma system and could be a useful supplement to current volcanic monitoring systems.

Thawing of mountain permafrost in response to rising temperatures degrades the stability of rock walls and thereby affects infrastructure integrity in Alpine terrain. In this study, we use 15 yr of passive seismic data from a single station deployed near a known permafrost body on Mt. Zugspitze (Germany), to monitor freeze‐thaw processes. The recordings reveal a persistent cultural seismic noise source, which we utilize to compute single‐station cross‐correlations and extract relative seismic velocity changes. We find that parts of the cross‐correlations show seasonal velocity variations (≈3% peak‐to‐peak amplitude) and a long‐term velocity decrease (≈0.1%/yr). Comparison with meteorological data and a previous electrical resistivity tomography study suggests that these velocity changes are caused by freeze‐thaw cycles and by permafrost degradation, respectively. The results demonstrate the potential of passive seismology for permafrost monitoring and suggest that denser instrumentation will provide detailed spatio‐temporal insights on permafrost dynamics in future studies. Plain Language Summary: Climate change causes permafrost (year‐round frozen rock) warming and thawing, which destabilizes rock slopes and thus constitutes a hazard potential. However, unlike glacier retreat, permafrost thawing cannot be directly observed from the surface and requires special imaging techniques for monitoring. Here, we use seismic waves generated by cable cars and other man‐made infrastructure to probe permafrost on Mt. Zugspitze (Germany) and track temporal changes over the past 15 yr. Results from a single seismic station show that the seismic wave propagation velocity in the rock is subject to seasonal variations (difference between late winter and late summer of up to 3%) and a long‐term decrease of roughly 0.1%/yr. As the seismic velocity is generally higher in frozen rock compared to unfrozen rock, the seasonal changes can be well explained by seasonal thaw and refreeze, and the long‐term changes by ongoing permafrost thawing. Because passive seismology is labor and cost effective compared to common techniques requiring active signal excitation, seismology constitutes a promising new approach for continuous long‐term permafrost monitoring. Key Points: We use a single seismic station deployed near a permafrost body on Mt. Zugspitze (Germany) to monitor freeze‐thaw processes over 15 yrCross‐correlations between the sensor components reveal seasonal velocity change cycles and a long‐term velocity decreaseThe changes are due to seasonal freeze‐thaw cycles and permafrost degradation, suggesting seismology as an effective permafrost monitoring tool [ABSTRACT FROM AUTHOR]

We analyze temporal variations in the travel times of both P and S waves (Vp and Vs) for 14 months at Toyohashi (central Japan) with a continuously operating vibration source that could efficiently produce both P and S waves. Seismic waves produced by the source, which is called the accurately controlled routinely operated signal system (ACROSS), are recorded by three nearby seismic stations, and the travel time variation at each station is estimated using the transfer function calculated from the recorded data. Long-term variations in Vp and Vs are observed and can be interpreted by the change in fluid saturation and crack density of the subsurface rocks. The variation in fluid saturation and crack density are consistent with that in the groundwater level, which is measured at the station nearest to the ACROSS. Short-term responses to rainfalls are observed at the station nearest to the ACROSS system; the interpretation of the changes in crack density and saturation is inconsistent with the ground water observation, partly owing to the initial response to rainfall. This can be interpreted as an air–water mixture within pores or cracks on a fine scale. [ABSTRACT FROM AUTHOR]

We use cross correlation of the ambient seismic field to estimate seasonal variations of seismic velocity in the Mississippi Embayment and to determine the underlying physical mechanisms. Our main observation is that the δt/t variations correlate primarily with the water table fluctuation, with the largest positive value from May to July and the largest negative value in September/October relative to the annual mean. The δv/v residuals after water level fluctuation correction correlate with air pressure in the short term and follow the trend of temperature in the long term. The δv/v residuals after water level fluctuation and air pressure correction correlate inversely with the wind speed. The correlation coefficients between water table fluctuation and δt/t are independent of the interstation distance and frequency, but high coefficients are observed more often between 0.3 and 1 Hz than between 1 and 2 Hz because high‐frequency coherent signals attenuate faster than low‐frequency ones. The δt/t variations lag behind the water table fluctuation by about 20 days, which suggests that the velocity changes can be attributed to the pore pressure diffusion effect. The seasonal variations of δt/t are azimuthally independent, and a large increase of noise amplitude only introduces a small increase to the δt/t variation. At close distances, the maximum δt/t holds a wide range of values, which is likely related to local structure. At larger distances, velocity variations sample a larger region so that it stabilizes to a more uniform value. We find that the observed changes in wave speed can be explained by a poroelastic model. Key Points: We observe the minimum wave speed from May to July due to the increased pore pressure from the water table fluctuation and the maximum wave speed in September/OctoberThe δt/t correlates primarily with the water table fluctuationA poroelastic model can explain the velocity variations in the crust [ABSTRACT FROM AUTHOR]

A sequence of eruptions in the lower East Rift Zone (LERZ) of the Kilauea, Hawaii volcano commenced in early May 2018 and caused serious damage and residential evacuation. The post-eruption drainage and collapses of the summit lava lake and caldera suggest a well-connected magmatic plumbing system along the rift zone. How and when the pre-eruptive magmatic processes in such connecting system occurred are, however, still unclear. For that, we apply ambient noise interferometry with seismic data from January 2017 to June 2018 from 12 broadband seismometers to investigate spatiotemporal seismic velocity changes (dv/v) of the upper crust in the Kilauea area. The dv/v variations in three frequency bands (0.3–0.6 Hz, 0.6–0.9 Hz, and 0.9–2.0 Hz) show distinct responses to strong earthquake ground shaking and deep magmatic intrusion processes. Earthquake-induced dv/v drops mainly in the higher two frequency bands imply shallow mechanical changes within the uppermost 1 km of the crust. In contrast, the magma-related dv/v changes can be characterized into three periods of activity: from the December 2017 to March 15, 2018, a dv/v excursion only seen in the lowest frequency band indicates the magmatic intrusion processes taking place at the depth range of 1–4 km, consistent with the proposed depth of the magma reservoir-dike system in the rift zone. The spatial dv/v distribution suggests that the magma may intrude to the deeper summit magma reservoir and in the upper East Rift Zone (UERZ) at this time. From March 15 to April 17 in 2018, the summit inflation recorded by tiltmeters causes the dv/v increases in the higher two frequency bands. After April 17 to the eruption, the accumulating damage of the edifice together with the stronger intrusion activity in the UERZ result in dv/v decreases in all three frequency bands around the summit and UERZ areas. Our observations highlight that the ambient noise interferometry analysis provides an opportunity to image and understand the pre-eruptive processes of reservoir-dike magma system and could be a useful supplement to current volcanic monitoring systems. [ABSTRACT FROM AUTHOR]

Broadband seismic networks are becoming more intensive, generating a large amount of data in the long-term collection process. When processing the data, the researchers rely almost on instrument response files to understand the information related to the instrument. Aiming at the process of instrument response recording and instrument response correction, we identify several sources of the instrument response phase error, including pole-zero change, the causality difference in instrument correction method, and the problem of filter coefficient recording. The data time offset range from the instrument response phase error is calculated from one sample point to several seconds using the ambient noise data recorded by multiple seismic stations. With different data delays, the time offset of the noise correlation function is estimated to be 74% to 99% of the data delay time. In addition, the influence of instrument response phase error on the measurement of seismic velocity change is analyzed by using ambient noise data with pole-zero change, and the results show that the abnormal wave velocity with exceeding the standard value is exactly in the time period of the instrument response error, which indicates that the instrument response error affects the study of seismology. [ABSTRACT FROM AUTHOR]

A moderate Mw 5.0 earthquake occurred near Mogul, Nevada (just west of Reno, Nevada), on 26 April 2008. This mainshock was surrounded by notable foreshock and aftershock sequences. Due to the long foreshock sequence, the area was well instrumented at the time of the mainshock. We investigated the foreshock and aftershock sequences for evidence of velocity changes in the structure immediate to the hypocenter and above it in the area of observed strong ground motion. Using autocorrelations of the time periods with nearly continuous foreshocks and aftershocks, we detected a nearly 1% negative change in velocity on recordings of station MOGL which was approximately over the hypocenter of the mainshock. We also observed from these recordings a shift in the spectral peaks to lower frequency following the mainshock, again indicative of a velocity decrease. Due to the different spatial sampling of the two methods, the effects could not be attributed to the same subsurface volume. However, both results are strong evidence for coseismic velocity changes accompanying an earthquake which is very much smaller than those for which previous velocity changes have been reported. We hypothesize that these changes can be observed for even smaller earthquakes, given a fortuitous placement of observing stations. [ABSTRACT FROM AUTHOR]

Coda-wave interferometry is a technique to detect small seismic velocity changes using phase changes in similar waveforms from repeating natural or artificial sources. Seismic interferometry is another technique for detecting seismic velocity changes from cross-correlation functions of ambient seismic noise. We simultaneously use these two techniques to clarify seismic velocity changes at Sakurajima volcano, one of the most active volcanoes in Japan, examining the two methods. We apply coda-wave interferometry to the records of repeated active seismic experiments conducted once a year from 2011 to 2014, and seismic interferometry to the ambient seismic noise data. We directly compare seismic velocity changes from these two techniques. In coda-wave interferometry analyses, we detect significant seismic velocity increases between 2011 and 2013, and seismic velocity decreases between 2013 and 2014 at the northern and eastern flanks of the volcano. The absolute values are at a maximum 0.47 ± 0.06% for 2-4 Hz, 0.24 ± 0.03% for 4-8 Hz, and 0.15 ± 0.03% for 8-16 Hz, respectively. In seismic interferometry analyses, vertical-vertical cross-correlations in 1-2, 2-4, and 4-8 Hz bands indicate seismic velocity increases and decreases during 3 years of 2012-2014 with the maximum amplitudes of velocity change of ±0.3% for 1-2 Hz, ±0.4% for 2-4 Hz, and ±0.2% for 4-8 Hz, respectively. Relative velocity changes indicate the almost annual change. These periodical changes are well matched with volcano deformation detected by GNSS receivers deployed around the volcano. We compare the results from coda-wave interferometry with those from seismic interferometry on the shot days and find that most of them are consistent. This study illustrates that the combined use of coda-wave interferometry and seismic interferometry is useful to obtain accurate and continuous measurements of seismic velocity changes. [ABSTRACT FROM AUTHOR]