CMT inversion for small-to-moderate earthquakes applying to dense short-period OBS array at off Ibaraki region

Abstract

We conducted centroid moment tensor (CMT) inversion for small-to-moderate aftershocks in the off Ibaraki region of the 2011 off the Pacific coast of Tohoku earthquake. In this study, we used high-frequency (0.4–1.0 Hz) seismograms from a dense array of short-period ocean bottom seismometers (OBSs) and a reliable three-dimensional (3-D) seismic velocity model. Higher-frequency analysis and dense OBS arrays offer CMT solutions with high spatial resolutions. Since our OBS array observed aftershocks occurring immediately following the 2011 off the Pacific coast of Tohoku earthquake, we determined 536 CMT solutions for small-to-moderate aftershocks in the off Ibaraki region (JMA-scale magnitudes of 2.5–4.0). According to our CMT solutions, characteristics of the aftershock activities in the off Ibaraki region are classified into five groups: (1) thrust earthquakes, which are considered as interplate earthquakes, separated by the large slip area of the 2011 Ibaraki-oki earthquake (the largest [[EQUATION]] 7.6 aftershock of the 2011 off the Pacific coast of Tohoku earthquake) and the tectonic tremors; (2) intraslab strike-slip earthquakes located at the north of the fault area of the 2011 Ibaraki-oki earthquake; (3) intraslab normal-fault earthquakes, which suggest a tensional stress field within the subducting Pacific Plate due to the plate bending by the overriding Philippine Sea Plate; (4) various mechanisms above a subducting seamount, which suggest the 3-D complex fracturs; and (5) normal-fault earthquakes shallower than the interplate earthquakes, which were possibly caused by the heterogeneity of the subducting seamount.

High-frequency S and S-coda waves at ocean-bottom seismometers

“…The effects of heterogeneous structures in offshore regions, such as seawater and oceanic sediments, on low-frequency (< 0.5 Hz) surface waves have been well documented (e.g., Shapiro et al 1998;Noguchi et al 2016;Volk et al 2017; Kaneko et al 2019); however, their effects on high-frequency (> 1 Hz) S and S-coda waves have not been studied. Highfrequency seismic waves enable the investigation of not only the characteristics of shallow and small seismic events (e.g., Yabe et al 2019Yabe et al , 2021Yamaya et al 2022) and shallower high-resolution structural heterogeneities (e.g., Kamei et al 2012; Davy et al 2021), but also seawater-temperature monitoring based on teleseismic T waves (e.g., Wu et al 2020). Figure 1b and 1c show examples of high-frequency seismic waves observed at the DONET and Full Range Seismograph Network of Japan (F-net) stations during an earthquake that occurred at 08:00 on November 30, 2011 (Japan Standard Time; JST) near the Nankai Trough (Fig.…”