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
Highlights
IntroductionGeodetic observation data show that several zones of strong coupling between the overriding and subducting plates exist in the Nankai forearc (Nishimura et al 2018; Yokota et al 2016)
Introduction The NankaiTrough located in central Japan is a wellknown megathrust zone where M ~ 8 class earthquakes have repeatedly occurred with intervals of 100–200 years, including the 1944 Tonankai earthquake (Mw = 7.9) (e.g., Ando 1975) (Fig. 1a)
We conclude that most of the detected shallow very-low-frequency earthquake (sVLFE) occur along the plate interface, we could not reject the possibility that a part of sVLFE occur along the splay faults
Summary
Geodetic observation data show that several zones of strong coupling between the overriding and subducting plates exist in the Nankai forearc (Nishimura et al 2018; Yokota et al 2016) Adjusted areas of these strong coupling zone are well correlated with the source area of slow earthquakes, whose activities indicate that the intraplate coupling changes with time (Obara and Kato 2016; Behr and Bürgmann 2021). Even though Takemura et al (2019a, b) determined a centroid moment tensor (CMT) for shallow very-low-frequency earthquakes (sVLFEs), they assumed that all sVLFEs occurred along the plate boundary fault. As these events occur outside the network, their estimated locations have large errors in the depth and along-dip directions. It is necessary to investigate in detail the sVLFE locations without assuming a focal depth and to discuss the relationship between their depths and the plate boundary
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