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