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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]

Cross‐correlation analysis was applied to long‐term onshore broadband records from April 2004 to March 2021 to detect and relocate shallow very low frequency earthquakes (VLFEs) southeast off the Kii Peninsula, along the Nankai Trough, Japan. We then determined the moment rate functions of detected shallow VLFEs using the Monte Carlo‐based simulated annealing method. According to this new comprehensive catalog, shallow VLFEs are widespread beneath the accretionary prism toe, but shallow VLFEs with large cumulative moments are localized around the western edge of the paleo‐Zenisu Ridge, which is subducted beneath southeast off the Kii Peninsula. Our results from the long‐term shallow VLFE catalog are well consistent with previous studies in this region, suggesting that heterogeneous structures and stress conditions due to the subducted paleo‐Zenisu Ridge promote the occurrence of shallow slow earthquakes. The relocated shallow VLFE epicenters illustrated three major episodes characterized by a similar activity area and five minor episodes characterized by different areas. The three major episodes exhibited slow frontal migration with different initiation locations, directions, and speeds, as well as several rapid reverse migrations. Episodes of minor activity were distributed in different locations within part of the area of major activity. Different patterns of shallow VLFE migration could reflect temporal changes in the pore‐fluid distribution or stress conditions of the plate boundary. Plain Language Summary: Our knowledge of the physical characteristics of shallow slow earthquakes, which may provide frictional properties on plate boundaries and preparation processes of large interplate earthquakes, is still limited. Thus, we comprehensively analyzed shallow very low frequency earthquakes (VLFEs) southeast off the Kii Peninsula, in the Nankai subduction zone, where large tsunamigenic earthquakes have repeatedly occurred with intervals of approximately 100–200 yr. We found that shallow VLFEs occurred around the western edge of the subducted oceanic ridge. We considered that this ridge causes complex stress and subsurface structures around the plate boundary and consequently promotes slow earthquake activity. Key Points: We estimated the locations and moment rate functions of shallow very low frequency earthquakes (VLFEs) that occurred near the Nankai TroughLarge cumulative moments of shallow VLFEs were concentrated in the region with the subducted oceanic ridgeThe migration of shallow VLFEs exhibits various patterns in terms of initiation, migration direction, and area [ABSTRACT FROM AUTHOR]

The spatiotemporal characteristics of shallow slow earthquake activity are linked to the tectonic environments of shallow plate boundaries. In this work, the spatiotemporal variations of shallow very low frequency earthquake (SVLFE) activity along the Nankai Trough were investigated using a cross‐correlation analysis. The SVLFEs migrated or spread eastward along the strike direction of the trench during large SVLFE episodes. Migrations and clusters of SVLFEs suggest the occurrence of shallow slow slip events. The observed lateral variations in SVLFE activity patterns reflect the heterogeneous distributions of effective strengths at the shallow plate boundary along the Nankai Trough. Migrations and clusters of SVLFEs tended to be concentrated in the regions surrounding the stress accumulation peaks on the Philippine Sea Plate boundary. The stress accumulated in the transitional regions between high‐strength and low‐strength zones can be released by shallow slow earthquakes. Plain Language Summary: Shallow slow earthquakes with rupture speeds between those of regular earthquakes and stable sliding often occur in the shallower extents of expected megathrust rupture zones. Such shallow slow earthquakes are useful for monitoring the slip behavior of shallow plate boundaries. The spatiotemporal variations of shallow slow earthquakes are related to tectonic characteristics including stress accumulation, friction properties, and the presence of pore fluids. To investigate the spatiotemporal variations of shallow slow earthquakes occurring along the Nankai Trough, the activities of shallow very low frequency earthquakes (SVLFEs) were examined using template matching and cross‐correlation relocation analysis. It was found that activity patterns of SVLFEs differed in each analyzed segment due to the different structural characteristics of the shallow Philippine Sea Plate boundary. A detailed spatial distribution revealed that SVLFEs tended to cluster or migrate in the regions surrounding the boundary stress accumulation peaks on the Philippine Sea Plate boundary. The obtained results suggest that shallow slow earthquakes occur and migrate to release accumulated stress in transitional regions between high‐strength (strongly locked) and low‐strength (stable sliding) zones on the shallow plate boundary. Key Points: Matched filter technique revealed details of activity patterns of shallow very low frequency earthquakes (SVLFEs) along the Nankai TroughThe characteristics of SVLFE clusters and migrations reflect frictional properties and possible slow slip events on the plate boundarySVLFEs tended to cluster or migrate in the regions surrounding the shear stress accumulation peaks on the Philippine Sea Plate boundary [ABSTRACT FROM AUTHOR]

The spatial distributions of shallow slow earthquakes are related to stress accumulation and structural characteristics of the shallow plate boundary, which are important for understanding megathrust earthquakes. To investigate spatiotemporal variation of shallow very low frequency earthquake (SVLFE) activity along the Nankai Trough, we conducted centroid moment tensor inversion method incorporating effects of offshore heterogeneous structures. By applying centroid moment tensor method into long‐term onshore seismograms, we obtained spatiotemporal variation of SVLFE activity occurred from June 2003 to May 2018. We find that SVLFE activities are related to the spatial variations of the slip‐deficit rate and pore fluid around the Philippine Sea plate boundary. SVLFEs are effectively activated by mechanical weakening due to rich pore fluid in areas surrounding strongly locked zones. Our results imply that spatial variations of long‐term SVLFE activity provide information on tectonic environments, which could constrain the rupture behavior of shallow plate boundaries during future megathrust earthquakes. Plain Language Summary: The spatial distributions of shallow slow earthquakes are related to stress accumulation and structural characteristics of the shallow plate boundary, which are important for understanding megathrust earthquakes. Despite this importance, conventional methods using onshore seismograms and one‐dimensional Earth models cannot well constrain the location and magnitude of shallow slow earthquakes due to the effects of three‐dimensional (3‐D) offshore subsurface structures. Moreover, offshore observations near hypocenters are still limited. By applying a method incorporating the effects of 3‐D subsurface structures into long‐term onshore seismograms, we show the spatial variations of well‐constrained shallow very low frequency earthquakes (SVLFEs) along the Nankai Trough, including events that occurred prior to offshore observations. We find that SVLFE activities are related to the spatial variations of the slip‐deficit rate and pore fluid around the Philippine Sea plate boundary. SVLFEs are effectively activated by mechanical weakening due to rich pore fluid in areas surrounding strongly locked zones. Our results imply that spatial variations of long‐term SVLFE activity provide information on tectonic environments, which could constrain the rupture behavior of shallow plate boundaries during future megathrust earthquakes. Key Points: We conducted CMT inversions of shallow very low frequency earthquakes (SVLFEs) occurred from June 2003 to May 2018 along the Nankai TroughSpatiotemporal variation of SVLFE activity are related to slip‐deficit rate and pore fluid along the Philippine Sea plate boundarySVLFEs are effectively activated by mechanical weakening due to rich pore fluid in areas surrounding strongly locked zones [ABSTRACT FROM AUTHOR]