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We investigated the effects of the propagation path and site amplification of shallow tremors along the Nankai Trough. Using far‐field S‐wave propagation from intraslab earthquake data, the amplification factors at the DONET stations were 5–40 times against an inland outcrop rock site. Thick (∼5 km) sedimentary layers with VS of 0.6–2 km/s beneath DONET stations have been confirmed by seismological studies. To investigate the effects of thick sedimentary layers, we synthesized seismograms of shallow tremors and intraslab earthquakes at seafloor stations. The ratios of the maximum amplitudes from the synthetic intraslab seismograms between models with and without thick sedimentary layers were 1–2. This means that thin lower‐velocity (<0.6 km/s) sediments just below the stations primarily control the estimated large amplifications. Conversely, at near‐source (≤20 km) distances, 1‐order amplifications of seismic energies for a shallow tremor source can occur due to thick sedimentary layers. Multiple S‐wave reflections between the seafloor and plate interface are contaminated in tremor envelopes; consequently, seismic energy and duration are overestimated. If a shallow tremor occurs within underthrust sediments, the overestimation becomes stronger because of the invalid rigidity assumptions around the source region. After 1‐order corrections of seismic energies of shallow tremors along the Nankai Trough, the scaled energies of seismic slow earthquakes were 10−10–10−9 irrespective of the region and source depth. Hence, the physical mechanisms governing seismic slow earthquakes can be the same, irrespective of the region and source depth. Plain Language Summary: The deployment of campaigns and permanent ocean bottom seismometers (OBSs) has enabled us to investigate the activity and physical properties of offshore seismic phenomena. Our knowledge of offshore subsurface structures is still limited; consequently, many studies have used conventional analysis methods with the simplest assumptions. Using observed and synthetic seismograms near the Nankai Trough, we found a limitation in the conventional analysis method applied to OBS data. Thick sedimentary layers, which have been confirmed by seismological studies along the Nankai Trough just below the OBSs, cause an approximately 1‐order overestimation of source parameters for seismic phenomena occurring around the shallow plate boundary. This overestimation may have occurred during the seismic energy estimation of shallow slow earthquakes in Hikurangi, Costa Rica, and Mexico. After correcting for the effects of thick sedimentary layers, we found that the scaled energies of seismic slow earthquakes were 10−10–10−9 irrespective of the region and source depth. This suggests that the physical mechanisms governing seismic slow earthquakes can be the same, regardless of region and source depth. Key Points: Effects of path and site on the seismic energy estimation of slow earthquakes at shallow plate boundaries were investigatedThe assumption of far‐field body waves without thick sediments causes an overestimation of seismic energies for shallow tremorsScaled energies of seismic slow earthquakes at both shallow and large depths range from 10−10 to 10−9 [ABSTRACT FROM AUTHOR]

For a more quantitative discussion of slow earthquake activity, we evaluated the detectable limits of very low frequency earthquakes (VLFEs), which are seismic slow earthquakes observed in very low-frequency (< 0.05 Hz) bands in the Nankai subduction zone. We performed numerical simulations using a local 3-D model and used the observed noise level of permanent broad-band seismometers. First, we investigated the effects of the source-time functions on the maximum amplitudes of the VLFE signals at a certain station. The maximum amplitudes of the VLFE signals were controlled by the VLFE moment rate. The detectable limit of VLFEs at each source location can be defined as the lowest moment rate of detectable VLFEs, which radiate signals larger than the noise levels of any component at ≥ 3 stations. For inland seismometers only, the detectable limits of VLFEs at deep (30–40 km) and shallow (≤ 10 km) depths were 1012–1012.3 and 1012.7 N·m s−1, respectively. Due to the geometrical spreading of VLFE signals and large noise levels in horizontal components, offshore seismometers improved the detectability of shallow VLFEs in regions where seismometers were densely deployed. Based on our detectability and published catalogues, shallow slow earthquakes are less active south-southwest off the Kii Peninsula, where geodetic studies expect mechanical coupling. [ABSTRACT FROM AUTHOR]

We detected shallow very low frequency earthquakes (VLFEs) off the Cape Muroto and Kii Channel in the Nankai subduction zone and estimated their moment rate functions. Combining the new and previously estimated catalogs, we obtained the comprehensive catalog of shallow VLFE moment rate functions along the Nankai Trough. We defined the shallow VLFE swarms and investigated the scaling relationships of their cumulative moments, activity area, and durations in each region. Detected swarms were considered candidates for shallow slow slip events. A similar scaling relationship was observed between the cumulative moments and activity areas, irrespective of regions. It indicates similar stress drops in each region. However, the relationship between the cumulative moments and durations varied. This difference was explained by the along‐strike variations in the faulting conditions of shallow slow earthquakes, such as material or hydrological properties. Plain Language Summary: Slow earthquakes are characterized as slips much slower than similar‐size regular earthquakes. Although interactions between the shallow slow earthquakes and large tsunamigenic earthquakes have often been discussed, our knowledge of the source characteristics and spatial variations of the shallow slow earthquakes is still limited. In this study, we quantitatively investigated the activity characteristics of shallow, very low frequency earthquakes (VLFEs) along the Nankai Trough. Activity areas and released cumulative moments of shallow VLFE swarms exhibited a similar scaling law irrespective of regions. However, the duration and cumulative moments of the swarms varied in each region. These characteristics can provide key information on the faulting conditions of slow earthquakes in shallow plate boundaries. Key Points: Comprehensive detection and source parameter estimations of shallow very low frequency earthquake swarms along the Nankai TroughThe scaling relationship between the activity areas and cumulative moments of the swarms roughly follows Mo ∝ A3/2Along‐strike variations of the scaling law for swarm durations reflect differences in the faulting conditions of slow earthquakes [ABSTRACT FROM AUTHOR]

The frontal megathrust of the Nankai Trough subduction zone is recognized as a seismogenic fault based on a record of frictional heating, but the underlying micromechanical processes that act on the fault surface are poorly known. Here we present a layer of fault gouge ∼2 mm thick within a core drilled across the megathrust, in which smectite‐rich siltstone has been transformed into a preferentially oriented illite aggregate. The nearly complete breakdown of smectite is consistent with fast frictional heating on this fault; however, the microtextures of the gouge and its surroundings are asserted one produced experimentally by slow slip. We suggest that slow slip with small shear strain has overprinted the textures produced by the previous faster and larger slip. This interpretation based on microtectonic evidence suggests a slow slip around the frontal megathrust took place during slow down, afterslip, or interseismic as observed now going in subduction zone. We suggest that the illite‐dominated gouge is conditionally stable, likely to shift from rapid to slow slip at different times. Plain Language Summary: Recent dense seismic observations have revealed that both fast and slow earthquakes occur in the Nankai Trough megathrust, but the micro‐processes on the fault surface have not yet been clarified. In this study, we performed three‐dimensional structural analysis, electron microscopy observation, and microdomain X‐ray mineral analysis of the Nankai Trough frontal megathrust sample, which are presumed to be the earthquake generating fault. The results show that smectite is lost due to high velocity slip, and a composite planar fabric with relatively slow and small deformation is observed to override the fast deformation. Our observations suggest that the fault may experience both of fast and slow slip at different times. Key Points: Microscopic and mineral analysis were performed on the Nankai Trough frontal megathrust sample collected by drillingThe development of the Y‐P‐R1 fabric was observed, along with traces of fast deformation accompanied by the disappearance of smectiteTwo types of deformations registered in the gouge suggest rapid to slow slip at different times on the same fault [ABSTRACT FROM AUTHOR]

Fluid migration in subduction zones is a key controlling factor of slow and megathrust earthquakes at plate boundaries. During the migration, seismic velocity and heterogeneous structures in its pathways may be temporarily perturbed, preferably triggering slow earthquakes. Here, we show that transient changes of seismic heterogeneity occurred 0–9 months before shallow slow earthquakes in the Nankai subduction zone, Japan, using very long‐term (6–10 years) records of ambient seafloor noise. The heterogeneity changes preceding to shallow slow earthquakes were observed near the margin of the source region, while concurrent changes primarily occurred in the source region. We propose that the heterogeneity changes are attributed to dynamic fluid migration, and the difference in timings reflects the pore pressure level in the corresponding source region. When fluids are supplied to a source region under relatively low pressure, fluids are leaked out from its downdip or updip side, and slow earthquakes occur not immediately but with a time delay of at most 9 months. In the high pore pressure case, slow earthquakes occur immediately with fluid migration from the source region. This study suggests that the heterogeneous seismic structure is possibly changed by fluid migration before slow earthquakes in the Nankai subduction zone. Plain Language Summary: While the linkage between fluid and slow earthquakes occurring at the plate boundary of subduction zones has been revealed, such a relationship for very shallow parts close to trench axes remains elusive. In this study, we investigated the temporal variations of seismic velocity and heterogeneous structure beneath the seafloor in the Nankai subduction zone, Japan, using the seismographs observed by a permanent cabled network starting in 2010. The intensities of seismic heterogeneous structures change 0–9 months prior to the shallow slow earthquakes, implying that the heterogeneous structure is altered by fluid migration that affects the slow earthquake generation. In the case of the fluid migration preceding to slow earthquakes, fluids are supplied to the source region with relatively low pore pressure levels, and the pore pressure level there increases until the timing of the slow earthquake generation. In contrast, the case of concurrent fluid migration corresponds to the fluids supplied to the source region with a relatively high pore pressure level, and slow earthquakes occur at the immediate timing of the fluid supply. These results suggest that monitoring heterogeneous seismic structures potentially contributes to further understanding of the spatio‐temporal relationship between fluid migration and shallow slow earthquakes. Key Points: Transient changes of seismic heterogeneities occurred 0–9 months before slow earthquakes in the shallow Nankai subduction zoneWe consider that the heterogeneity changes are caused by fluid migrationThe difference in timings between fluid migration and slow earthquakes reflects the pore pressure levels at the source regions [ABSTRACT FROM AUTHOR]

Slow earthquakes follow a power‐law size distribution with an exponential taper for the largest events. We investigated changes in the size distribution of shallow tectonic tremor events during two prolonged tremor episodes (>1 month) along the Nankai trough and found that the slope of the size distributions increased while the cut‐off magnitudes decreased late during each episode, as tremor activity waned. Interpreting these changes with the two‐dimensional probabilistic cell automaton model of slow earthquakes, we found that a decrease in event ignition probability or an increase in energy dissipation during slip can qualitatively explain the observed changes. These changes imply that a decrease in accumulated stress or pore‐fluid pressure on the fault interface occurred during each tremor episode. Because the tremor source migrates during an episode, the changes in the size distribution parameters can be attributed to spatial variations or temporal changes in the source characteristics. Plain Language Summary: The size distribution of slow earthquakes mostly follows a power law like that of ordinary earthquakes, in which the logarithm of event numbers is negatively proportional to the logarithm of event sizes, with an exponential taper for events larger than a cut‐off magnitude. We investigated the changes in this size distribution during two prolonged episodes of shallow tectonic tremor that occurred on the plate interface along the Nankai trough, southwestern Japan. We found that the ratio of smaller events increased and the cut‐off magnitude decreased as tremor activity decreased late in each episode. We interpreted this observation by using a model of slow earthquakes that divides a source fault into small cells and updates slip on each cell probabilistically. The model can explain the changes in the tremor size distribution by a decrease in the probability of event ignition or an increase in the energy dissipation during fault slip. This result implies that the accumulated stress or the pore‐fluid pressure on the source fault decreased when the tremor was less active. Because the tremor source migrates during the course of each episode, these changes indicate that the source characteristics of tremor vary at different times or locations. Key Points: The slope and cut‐off magnitude of event size distribution changed during episodes of shallow tectonic tremor along the Nankai troughThe 2D probabilistic cell automaton slow earthquake model can explain these changes by changes in event ignition probability or energy dissipation during slipObserved changes imply release of accumulated stress or decrease of pore fluid pressure on the fault interface during each tremor episode [ABSTRACT FROM AUTHOR]

Slow earthquake size distributions have been found to follow either a power law or an exponential law, depending on the data set. We investigated the size distribution of shallow tectonic tremor events along the Nankai trough, southwestern Japan, observed in 2009, 2015, 2016, and 2018 and estimated the seismic energy radiated during the events as a measure of event size. The obtained size distributions mostly followed a power law with a b value of about 1, but an exponential law also fits well the data of the largest events; this result indicates that the size distribution follows the Gutenberg‐Richter law with an exponential taper. If only the largest events were observed, an exponential size distribution would be obtained. This exponential taper could be attributed to a source fault limitation. Because the maximum event size differed among activity periods, the source fault size varied among them, although the source areas largely overlap. Plain Language Summary: The size distribution of ordinary earthquakes mostly follows a power law, in which number of earthquakes reduces to 1/10 when the earthquake magnitude increases in 1 (the Gutenberg‐Richter law), while that of volcanic tremor follows an exponential law, in which event number exponentially decreases as the event size increases. On the other hand, the size distribution of tectonic tremor has not been clarified yet. We investigated the size distribution of shallow tectonic tremor events along the Nankai trough, southwestern Japan. The obtained size distributions mostly followed a power law with a b value of about 1, similar to ordinary earthquakes, but an exponential law also fits well the data of the largest events. This result indicates that the size distribution follows the Gutenberg‐Richter law with an exponential taper. The tapered power law distribution of tremor events implies that the source fault of small events, which follow a power law size distribution, is self‐similar, whereas that of larger events, which follow an exponential law distribution, is limited by the size of the entire fault. Key Points: We investigated the size distribution of shallow tremor events observed along the Nankai troughThe observed size distribution is well explained by a power law with b value about 1 with an exponential taper for the largest eventsThis result implies that tectonic tremor follows a tapered Gutenberg‐Richter distribution [ABSTRACT FROM AUTHOR]

Deep and shallow slow earthquakes occur at the edge of the seismogenic zone in the Nankai subduction zone. Deep slow earthquakes occur under high‐temperature and high‐pressure conditions, whereas shallow slow earthquakes occur at lower temperatures and pressures. Although these two types of slow earthquake show qualitatively similar behaviors, such as slow deformation and depleted high‐frequency seismic signals, their similarities have not yet been evaluated in quantitative ways. In this work, we analyze shallow tectonic tremors accompanied by very low frequency earthquakes off Kii Peninsula, Japan. We estimate the seismic energy of the shallow tectonic tremors by correcting for the site amplifications of ocean bottom seismic stations and seismic attenuation in the shallow accretionary prism. The estimated seismic energy rates of these tectonic tremors are compared with seismic moment rates of accompanying very low frequency earthquakes, revealing that the scaled energy of shallow slow earthquakes is roughly constant, ~10−9 to 10−8, which is 0 to 1 order of magnitude larger than the typical scaled energy of deep slow earthquakes. Key Points: Seismic energy of shallow tectonic tremor is quantified correcting for site amplification and seismic attenuationSeismic energy rates and seismic moment rates of shallow tremors show along‐dip variations, which are similar to deep tremorsThe scaled energy of shallow slow earthquake is 0 to 1 order of magnitude larger than deep slow earthquake [ABSTRACT FROM AUTHOR]

The first International Ocean Discovery Program (IODP) Core-Log-Seismic Integration at Sea (CLSI@Sea) workshop, held in January–February 2018, brought together an international, multidisciplinary team of 14 early-career scientists and a group of scientific mentors specialized in subduction zone processes at the Nankai Trough, one of the Earth's most active plate-subduction zones located off the southwestern coast of Japan. The goal of the workshop was to leverage existing core, log, and seismic data previously acquired during the IODP's Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), to address the role of the deformation front of the Nankai accretionary prism in tsunamigenic earthquakes and slow slip in the shallow portion of the subduction interface. The CLSI@Sea workshop was organized onboard the D/V Chikyu concurrently with IODP Expedition 380, allowing workshop participants to interact with expedition scientists installing a long-term borehole monitoring system (LTBMS) at a site where the workshop's research was focused. Sedimentary cores from across the deformation front were brought onboard Chikyu , where they were made available for new description, sampling, and analysis. Logging data, drilling parameters, and seismic data were also available for investigation by workshop participants, who were granted access to Chikyu laboratory facilities and software to perform analyses at sea. Multi-thematic presentations facilitated knowledge transfer between the participants across field areas, and highlighted the value of multi-disciplinary collaboration that integrates processes across different spatiotemporal scales. The workshop resulted in the synthesis of existing geophysical, geologic, and geochemical data spanning IODP Sites C0006, C0007, C0011 and C0012 in the NanTroSEIZE area, the identification of key outstanding research questions in the field of shallow subduction zone seismogenesis, and fostered collaborative and individual research plans integrating new data analysis techniques and multidisciplinary approaches. [ABSTRACT FROM AUTHOR]

Abstract: The mainshock‐aftershock sequence is one of the fundamental characteristics of seismogenesis, yet the physical mechanism of aftershock generation remains poorly understood. The simple explanation that aftershocks are caused by the mainshock's redistribution of strain energy is not always applicable, especially for aftershocks within the mainshock slip area, where strain energy is released. Here we show that the genesis of aftershocks can be modeled using a frictionally heterogeneous fault system. We conducted quasi‐dynamic numerical simulations of fault rupture cycles on a finite fault governed by a rate‐ and state‐dependent friction law. Aftershocks are observed around and within the mainshock rupture area when the frictional heterogeneity varies significantly along the fault. On the other hand, aftershocks are not produced when along‐fault variations in the frictional heterogeneity are small, which mimics the observed lack of aftershocks for repeating earthquakes. [ABSTRACT FROM AUTHOR]

Tectonic tremor provides a new source of observations that can be used to constrain the seismic attenuation parameter for ground-motion prediction and hazard mapping. Traditionally, recorded earthquakes of magnitude ~3-8 are used to develop ground-motion prediction equations; however, typical earthquake records may be sparse in areas of high hazard. In this study, we constrain the distance decay of seismic waves using measurements of the amplitude decay of tectonic tremor, which is plentiful in some regions. Tectonic tremor occurs in the frequency band of interest for ground-motion prediction (i.e., ~2-8 Hz) and is located on the subducting plate interface, at the lower boundary of where future large earthquakes are expected. We empirically fit the distance decay of peak ground velocity from tremor to determine the attenuation parameter in four subduction zones: Nankai, Japan; Cascadia, United States-Canada; Jalisco, Mexico; and southern Chile. With the large amount of data available from tremor, we show that in the upper plate, the lower crust is less attenuating than the upper cmst. We apply the same analysis to intraslab events in Nankai and show the possibility that waves traveling from deeper intraslab events experience more attenuation than those from the shallower tremor due to ray paths that pass through the subducting and highly attenuating oceanic cmst. This suggests that high pore-fluid pressure is present in the tremor source region. These differences imply that the attenuation parameter determined from intraslab earthquakes may underestimate ground motion for future large earthquakes on the plate interface. [ABSTRACT FROM AUTHOR]