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An Mw 6.2 earthquake occurred in Suzu, northeastern Noto Peninsula, Japan, on 5 May 2023, followed by many aftershocks. Before this mainshock‐aftershock sequence, an intense earthquake swarm lasted in the vicinity for 2.5 years. Here, we estimated the rupture process of the Mw 6.2 mainshock and relocated >20,000 surrounding small earthquakes. The results show that systematic upward migration occurred via a complex network of faults in the preceding swarm period and that the mainshock rupture was initiated near the shallow end of the swarm earthquakes. The mainshock rupture propagated farther updip, followed by many aftershocks in the shallow extension. Upward fluid movement likely caused systematic upward earthquake migration from a depth of 18–5 km. The present results indicate the importance of monitoring swarm events since large (M > 6) and dangerous earthquakes can occur during such swarms. Plain Language Summary: Growing evidence suggests that fluid movements at depth can cause earthquake swarms in the overriding plates of subduction zones. However, there is less evidence of fluid involvement in the occurrence of large (M > 6) earthquakes, and it has been reported that the generation environments of large earthquakes and swarm earthquakes tend to be different. During intense swarm activity in the northeastern Noto Peninsula, Japan, starting at the end of 2020, an M6.2 earthquake occurred on 5 May 2023, followed by many aftershocks. To clarify the occurrence mechanism of the M6.2 mainshock, we relocated >20,000 small earthquakes and estimated the rupture process. The results showed a systematic upward migration of earthquakes in the preceding swarm period on the deep side of the mainshock fault (depth from 20 to 12 km). The mainshock rupture initiated near the shallow end of the swarm activity and propagated updip (depth from 12 to 9 km). Many aftershocks followed in shallower part of the same fault (depth from 10 to 5 km). Upward fluid migration likely caused the observed systematic upward earthquake migration throughout the sequence. The results indicate that earthquakes as large as M > 6 can be triggered by fluid migration during enhanced seismicity in swarms. Key Points: An Mw 6.2 event initiated near the shallow end of a fault where the preceding earthquake swarm showed systematic upward migrationThe Mw 6.2 rupture propagated farther updip, followed by many shallow aftershocks on the faultUpward fluid migration can trigger earthquakes as large as Mw > 6 [ABSTRACT FROM AUTHOR]

Isolating the source of nontidal oceanographic noise in seafloor pressure data is critical for improving the use of these data for seafloor geodetic applications. Residuals between nearby bottom pressure records have typically been used to remove the nontidal components, as these are largely common‐mode. To evaluate the similarities between pairs of observed bottom pressure records at a range of water depths, we calculate the standard deviations of the time series of residuals between data from all site pairs, installed in the Hikurangi subduction zone offshore New Zealand. We find that the magnitude of the standard deviation depends more on relative water depth than the distance between sites. This confirms the result of previous studies from Cascadia that nontidal components are more similar along isobaths even if the distance between sites is large. Furthermore, in order to reduce noises, the required depth difference between site pairs also varies with site depths. Plain Language Summary: Coherent signals of ocean bottom pressure are observed along common water depths within an ocean bottom pressure array offshore New Zealand. We statistically evaluated the similarity of the seafloor pressure collected in 2014 offshore the North Island's east coast, where the Pacific Plate dives or "subducts" along the Hikurangi subduction zone beneath the North Island. This is important for removal of noise caused by oceanographic processes, which must be done to detect centimeter‐level vertical movement of the seafloor crust during slow slip events using seafloor pressure records. We measured the similarity of pairs of seafloor pressure records at a range of water depths. Similar to previous studies offshore the Cascadia subduction zone, our results confirm that seafloor pressure records from similar depths (but at large horizontal distances from each other) can be used effectively to reduce oceanographic noise in sea floor pressure data to reveal the seafloor crustal deformation. Key Points: The similarity of the nontidal components of bottom pressure varies with relative water depth, rather than distance between sitesBottom pressure data show that reference sites at similar depths will optimize oceanographic noise removal for seafloor geodetic studiesOceanographic models with baroclinicity represent the observed depth dependence of the nontidal variations better than barotropic models [ABSTRACT FROM AUTHOR]

Recent studies have shown that ocean‐bottom pressure gauges (OBPs) can record seismic waves in addition to tsunamis and seafloor permanent displacements, even if they are installed inside the focal area where the signals are extremely large. We developed a method to extract dynamic ground motion waveforms from near‐field OBP data consisting of a complex mixture of various signals, based on an inversion analysis along with a theory of tsunami generation. We applied this method to the OBP data of the 2011 Tohoku‐Oki earthquake. We successfully extracted the low‐frequency vertical seismograms inside the focal area (f < ∼0.05 Hz), although those of the Mw ∼9 megathrust earthquake had never previously been reported. The seismograms suggested two dominant energy releases around the hypocenter. The seismic wave signals recorded by the near‐field OBP will be important not only to reveal earthquake ruptures and tsunami generation processes but also to conduct real‐time tsunami forecasts. Plain Language Summary: During tsunami generation, different types of waves such as ground motions, ocean acoustic waves, and tsunamis coexist inside the focal area, forming complicated wavefields and pressure changes at the sea bottom. This study developed a method to appropriately decompose the complicated ocean‐bottom pressure gauge (OBP) waveforms into ground motion and tsunami signals. Our method was applied to the near‐field OBP data of the 2011 Tohoku‐Oki earthquake to extract the near‐field seismic motion waveform which had never been reported previously. The waveform suggested a complex earthquake rupture process along the plate boundary, in which the rupture happened twice near the hypocenter. The seismic wave signals recorded by the near‐field OBP will be important not only to reveal the processes of the earthquake rupture and tsunami generation but also to issue tsunami alarms. Key Points: We develop a method to extract low‐frequency ground motion including permanent deformation from ocean‐bottom pressure gauge (OBP) dataWe obtain the seismograms inside the focal area of the 2011 Tohoku‐Oki EQ, which suggest two dominant energy releases around the hypocenterHigh‐frequency near‐field OBP signals should be utilized more widely for geophysical research as well as real‐time tsunami forecasting [ABSTRACT FROM AUTHOR]

Sedimentary basins are known to amplify and increase the duration of ground motions that accompany earthquakes. A similar phenomenon is expected, but not as well documented, in low seismic‐velocity accretionary prisms along subduction margins. In this study, we report anomalously long duration of long‐period ground motions observed in the North Island of New Zealand during seismic wave propagation from the M7.8 Kaikoura earthquake ∼600 km away. Unique waveform data captured by strong‐motion, high‐rate GPS and ocean bottom pressure sensors reveal that long‐period ground motions lasted longer than 450 s in the northeastern North Island. These waveforms indicate one of the longest durations of long‐period (>10 s) ground motions ever recorded at similar epicentral distances for comparable, large earthquakes. To understand the underlying mechanism, we use numerical simulations of seismic wave propagation. We find that a velocity model that includes an accretionary prism, modeled as a large‐scale (∼30,000 km2) wedge characterized by extremely low seismic wave speeds, can explain the observed long durations of long‐period ground motions as the reverberations of seismic waves within the low‐velocity wedge. We show that the long duration of long‐period ground motions leads to prolonged dynamic stressing on the plate interface, likely accentuating the triggering of slow slip that occurred following the Kaikoura earthquake. Accretionary prisms characterized by extremely low seismic velocities may enhance the generation of tsunami earthquakes and dynamic triggering of slow slip events observed in the northern Hikurangi and other subduction margins. Plain Language Summary: In this study, we discovered an unusually long duration (up to 8 minutes) of ground shaking near Gisborne, on the east coast of the North Island of New Zealand during the magnitude 7.8 Kaikoura earthquake in 2016, using data from accelerometer, GPS and ocean bottom pressure sensors. This is arguably the longest duration of long‐period ground shaking ever documented from similar magnitude 7–8 earthquakes worldwide. To resolve why this occurred, we use numerical simulations of seismic wave propagation during the Kaikoura earthquake to show that the unusually long ground shaking near Gisborne was caused by the presence of large‐scale (approximately 150,000 cubic kilometers) body of sediments offshore and beneath the northeastern North Island. This long‐duration shaking was also responsible for triggering a large slow slip event on the Hikurangi subduction zone offshore Gisborne, over 600 km away from the Kaikoura earthquake. Our result has important local and global implications for ground shaking hazard in areas with similar geological characteristics to those in the northeastern North Island, and improves understanding of processes that can trigger slow slip events and earthquakes on subduction zones. Key Points: Anomalously long duration of long‐period ground motions was observed during seismic wave propagation from the 2016 M7.8 Kaikoura earthquakeWaveform modeling indicates the presence of a thick, unusually low seismic‐velocity wedge of material in the northeastern North Island of NZLong‐duration ground motions caused by this wedge would enhance dynamic triggering of slow slip as well as hazard posed by ground shaking [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]

Ocean bottom pressure (OBP) observations are a powerful tool for determining vertical crustal displacements, especially due to earthquakes and slow earthquakes, with centimeter‐level resolution. In these studies, removal of oceanographic noise (tens of centimeters) is required to identify centimeter‐level crustal deformation. We undertake barotropic modeling to remove oceanographic signals from data from an OBP array deployed offshore New Zealand in 2014/2015. We show that removing the nontidal component calculated from a barotropic ocean model reduces the variance in the data by about 66% and provides a feasible means to resolve pressure changes due to crustal deformation during the slow slip events. We also discuss the vertical displacements from slow slip events that occurred in late September to mid‐October 2014, and we outline our procedure for processing OBP data. Plain Language Summary: We developed a new method for determining pressure changes due to slow slip events (SSEs) on offshore subduction plate boundaries by using information from the ocean bottom pressure records and numerical simulation. We use an oceanographic model to correct the seafloor pressure data for oceanographic signals, so that centimeter‐level vertical deformation of the seafloor during the SSEs can be isolated. We show that this method can be used to identify SSEs that occurred off the coast of New Zealand in 2014. Our results indicate that our ocean model can be a useful tool to use ocean bottom absolute pressure gauge data to resolve crustal deformation. Key Points: The oceanographic corrections from our model help to reduce noise in ocean bottom pressure data recorded during slow slip eventWe show a barotropic oceanographic model can be used to reduce the variance in seafloor pressure measurements by about 66%Our oceanographic model is particularly valuable for the shallower‐water sites [ABSTRACT FROM AUTHOR]

A slow slip event (SSE) and tectonic tremors occurred approximately 1.5 month prior to the occurrence of the 2011 Tohoku‐Oki earthquake near the Japan Trench. Although we cannot completely rule out other sources of noise near the observation site, slow earthquakes would demonstrate the nucleation of a fast slip event from prior slow earthquake activity. Here we report on tectonic tremor activity detected from short‐period ocean‐bottom seismometer (OBS) data with a modified frequency scanning method that allows robust tectonic tremor detection by incorporating information from multiple frequency passbands in order to suppress false detections. We have identified at least five tectonic tremor sequences on the OBS records from the end of January 2011 to just before the occurrence of the Tohoku‐Oki earthquake in March 2011. To reveal the spatial distributions of tectonic tremors before the Tohoku‐Oki earthquake, we then calculated tremor energy, with corrected site amplification, from the identified tectonic tremor time window at each OBS that detected tremors. Possible sources of tremors were distributed in trenchward side of the SSE fault prior to the main shock. These tremors could be modulated by SSE to take into account consistent timing between tremor and SSE term, and consistency of spatial locations between estimated tremor sources and SSE faults. This spatiotemporal pattern in tectonic tremor activity demonstrates a significant separation of occurrence sources between tectonic tremors and regular earthquakes. Key Points: A modified frequency scanning method allowed investigation of tectonic tremors occurring before the largest Tohoku‐Oki foreshockThis method revealed details of tremor activity in the rupture area of Tohoku‐Oki earthquake, especially close to the Japan TrenchTemporal and spatial changes in tectonic tremor activity reflected slow slip event expansion processes that occur before large earthquakes [ABSTRACT FROM AUTHOR]

The Te Araroa earthquake occurred on 2 September 2016 (local time) offshore of the northeastern coast of New Zealand's North Island (Mw 7.1). When this event occurred, ocean bottom pressure gauges (OBPs), installed ~170 km south of the source area, clearly recorded direct tsunami from the source to OBPs (~ −1.5 cm), and tsunami from coastal reflections (~2 cm). We estimate the centroid location that best reproduces the OBP waveforms. When using the direct wave alone, the centroid location is poorly constrained, with a horizontal uncertainty of ~100 km. By combining both direct and reflected tsunami waveforms, we obtain a centroid location near the Global Centroid Moment Tensor centroid (~80 km northeast from the coast) with smaller uncertainty (~40 km). We also estimate the earthquake source dimension (length and width) and found that the models using coastal reflections require a source dimension larger than ~30 km long. Based on the slip distribution obtained by the finite fault inversion, we obtain an energy‐based stress drop ΔσE of 1.0 MPa, consistent with typical earthquake stress drop values. This study shows that the information added by coastal reflected tsunami provides much tighter constraints on the centroid location, source dimension, and stress drop of offshore earthquakes, which is difficult to obtain from the onshore seismic data alone. Future studies should utilize the information provided by coastal reflected waves to improve earthquake source modeling using ocean bottom pressure data. Key Points: Tsunami from coastal reflected waves associated with the 2016 Te Araroa EQ are clearly observed by offshore ocean bottom pressure gaugesWe greatly reduce uncertainties in the centroid location and fault dimensions of the Te Araroa EQ using tsunami reflected from the coastLater tsunami arrivals should be used more widely to extract earthquake source information in the future [ABSTRACT FROM AUTHOR]

We examined the dynamic pressure change at the seafloor to estimate the centroid moment tensor solutions of the largest and second largest foreshocks ( Mw 7.2 and 6.5) of the 2011 Tohoku-Oki earthquake. Combination of onshore broadband seismograms and high-frequency (~20-200 s) seafloor pressure records provided the resolution of the horizontal locations of the centroids, consistent with the results of tsunami inversion using the long-period (≳10 min) seafloor pressure records although the depth was not constrained well, whereas the source locations were poorly constrained by the onshore seismic data alone. Also, the waveforms synthesized from the estimated CMT solution demonstrated the validity of the theoretical relationship between pressure change and vertical acceleration at the seafloor. The results of this study suggest that offshore pressure records can be utilized as offshore seismograms, which would be greatly useful for revealing the source process of offshore earthquakes. [ABSTRACT FROM AUTHOR]

The article presents the observations of the 2011 Tohoku-Oki Earthquake at 19 ocean bottom seismometers (OBS) sites located off Miyagi region. The researchers retrieved 17 OBS from March to November 2011 in emergency research cruises. The retrieved OBSs have recorded the seismic wave data, which were used for relocation of the hypocenters of the foreshock, mainshock, and aftershocks.