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Abstract We conducted seismic tomography to estimate the seismic velocity structure and to evaluate the spatiotemporal distribution of interplate earthquakes of the Kii Peninsula, central Honshu, Japan, where the Tonankai and Nankai megathrusts are located. Microearthquakes were quantitatively detected by using the data from a cable-type seafloor seismic observation network, completed in 2015. Our velocity model was consistent with the previous 2-D active-source surveys, which reported the areal extent of key structures: a high-velocity zone beneath Cape Shionomisaki, a subducted seamount off Cape Muroto, and the subducted Paleo-Zenith Ridge. The absence of any other subducted seamount with the same or larger spatial scale, than the identified key structures, was confirmed. Our velocity model also revealed that there was not a simple relationship between areas of large coseismic slip or strong interplate coupling and areas of high velocity in the overriding plate. Relocated hypocenters widely ranged from the upper plate to within the slab, while the most active region was attributed to the oceanic crust in the aftershock region of 2004 off-Kii earthquake. Compared with the results from the land-based observation network, the accuracy of the focal depth estimation was substantially improved. Furthermore, we identified the seismic activity in the vicinity of the plate boundary and determined 14 locations for interplate seismicity areas. They were primarily distributed in the range of seismogenic zone temperature (150–350 °C) along the plate boundary and were located outside of the strong interplate coupling zone. Several active areas of interplate earthquakes exhibited clustered activity during the periods of slow-slip events, observed and accompanied with shallow very-low-frequency earthquakes. Thus, regular interplate microearthquakes became active at the plate boundary in the conjunction with slow slip. In summary, as regular earthquakes provide a more accurate source location than slow earthquakes and can detect events of smaller magnitude, monitoring such interplate earthquakes may reveal spatiotemporal variations in the stick–slip conditions on the plate boundary.

Abstract We estimate the hypocenter locations and the centroid moment tensor solutions of the shallow very-low-frequency earthquake (sVLFE) activity that occurred in the Kumano-nada region of the Nankai Trough megathrust zone in central Japan from December 2020 to January 2021. Using seafloor observation data, we examined the detailed spatio-temporal distribution of the sVLFE activity. During this episode, the activity area was within the vicinity in which the sVLFE activity has been observed in the past and can be divided into two major parts. The sVLFE activity started from the eastern side and remained there for the first 5 days and then migrated to the western side via secondary expansion. The eastern active area is located just below the outer ridge and coincides with the location where the paleo-Zenith Ridge subducted. The western activity area is centered between the outer wedge and the outer ridge with the primary active area being at the outer wedge. Comparing the activity in the eastern and western areas, the eastern side is more active, but the individual moment releases on this side are smaller than those on the western side. This may indicate a difference in the fluid pressure along the plate boundary between the eastern and western areas. After the second expansion of the active area, we observed several migration patterns within the expanded area with a faster velocity than those of the initial and second expansions. The direction of these migrations is opposite to that of the first and second expansions. This indicates that the fluid pressure and/or stress level in the sVLFE generation region changed with time within this episode. Furthermore, many waveforms with sVLFE characteristics were observed at only one or a few observation points near the trough axis in the middle to latter half of January 2021. This indicates the occurrence of small-scale sVLFEs in the vicinity of the trough axis at the end of this sVLFE episode. Graphical Abstract

In our recent study, we detected the pore pressure change due to the slow slip event (SSE) in March 2020 at the two borehole stations (C0002 and C0010), where the other borehole (C0006) close to the Nankai Trough seems not because of instrumental drift for the reference pressure on the seafloor to remove non-crustal deformation such as tidal and oceanic fluctuations. To overcome this problem, we use the seafloor pressure gauges of cabled network Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) stations nearby boreholes instead of the reference by introducing time lag between them. We confirm that the time lag is explained from superposition of theoretical tide modes. By applying this method to the pore pressure during the SSE, we find pore pressure change at C0006 about 0.6 hPa. We also investigate the impact of seafloor pressure due to ocean fluctuation on the basis of ocean modeling, which suggests that the decrease of effective normal stress from the onset to the termination of the SSE is explained by Kuroshio meander and may promote updip slip migration, and that the increase of effective normal stress for the short-term ocean fluctuation may terminate the SSE as observed in the Hikurangi subduction zone.

We have detected an event of pore pressure change (hereafter, we refer it to “pore pressure event”) from borehole stations in real time in March 2020, owing to the network developed by connecting three borehole stations to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) observatories near the Nankai Trough. During the pore pressure event, shallow very low-frequency events (sVLFEs) were also detected from the broadband seismometers of DONET, which suggests that the sVLFE migrated toward updip region along the subduction plate boundary. Since one of the pore pressure sensors have been suffered from unrecognized noise after the replacement of sensors due to the connecting operation, we assume four cases for crustal deformation component of the pore pressure change. Comparing the four possible cases for crustal deformation component of the volumetric strain change at C0010 with the observed sVLFE migration and the characteristic of previous SSEs, we conclude that the pore pressure event can be explained from SSE migration toward the updip region which triggered sVLFE in the passage. This feature is similar to the previous SSE in 2015 and could be distinguished from the unrecognized noise on the basis of t-test. Our new finding is that the SSE in 2020 did not reach very shallow part of the plate interface because the pore pressure changes at a borehole station installed in 2018 close to the trough axis was not significant. In the present study, we estimated the amount, onset and termination time of the pore pressure change for the SSE in 2020 by fitting regression lines for the time history. Since the change amount and duration time were smaller and shorter than the SSE in 2015, respectively, we also conclude that the SSE in 2020 had smaller magnitude that the SSE in 2015. These results would give us a clue to monitor crustal deformation along the Nankai Trough directly from other seafloor observations.