4:30 PM - 4:45 PM
[SSS02-11] Moment tensor inversion using dense short-period OBS array in the off Ibaraki region
Ocean bottom seismometer networks have been recently developed along subduction zones. It is important for understanding stress regime to determine moment tensor mechanisms of small earthquakes occurring along subduction zones. Since small earthquakes generate less low-frequency components, incorporation of high-frequency data observed in the vicinity of the earthquakes at offshore stations is necessary. Although high-frequency data analyses require high-resolution 3-D seismic structure, Yamaya et al. (in prep) resolved fine-scale sedimentary structure of the off Ibaraki region above the acoustic basement with resolutions of ~0.1– 1.0 km vertically and ~10 km horizontally. In this study, we conducted waveform inversion to solve for the moment tensors using this seismic structure.
A dense array of short-period ocean bottom seismometers (OBSs) was deployed in the off Ibaraki region from October 2010 (11 OBSs started from February 2010) to October 2011 (Nakatani et al., 2015). The array consists of 32 OBSs deployed with station intervals of about 6 km. During the observation period, on March 11, the 2011 off the Pacific coast of Tohoku earthquake occurred, and a large number of aftershocks occurred in our study region including the largest M7.6 aftershock. We selected earthquakes (M>3) from the JMA catalog that occurred just beneath and around the array. We conducted waveform inversion using P-waves to determine the moment tensors (Sipkin, 1982). We used 3 types of data filtered at frequency ranges of 0.4–1.0 Hz, 0.4–1.5 Hz, and 0.6–1.5 Hz. We determined the centroid and origin times of the earthquakes by grid search. We used a 3-D seismic structure model constructed by combining detailed shallow structure above the lower crust (Yamaya et al., in prep) and Japan Integrated Velocity Structure Model (Koketsu et al., 2012) for deeper structure. For calculating Green’s function at each grid points, we used seismic waveform propagation code based on the finite difference method (OpenSWPC; Maeda et al., 2017).
We applied the method to ~100 earthquakes to obtain their moment tensor and depth solutions. The solutions are mostly consistent irrespective of the filtered frequency ranges. The earthquakes in the north of the OBS network show mechanisms consistent with the top of the subducting Pacific Plate. In contrast, those locating in the southern region are slightly different from those in the north, which may due to the subducting Philippine Sea Plate.
A dense array of short-period ocean bottom seismometers (OBSs) was deployed in the off Ibaraki region from October 2010 (11 OBSs started from February 2010) to October 2011 (Nakatani et al., 2015). The array consists of 32 OBSs deployed with station intervals of about 6 km. During the observation period, on March 11, the 2011 off the Pacific coast of Tohoku earthquake occurred, and a large number of aftershocks occurred in our study region including the largest M7.6 aftershock. We selected earthquakes (M>3) from the JMA catalog that occurred just beneath and around the array. We conducted waveform inversion using P-waves to determine the moment tensors (Sipkin, 1982). We used 3 types of data filtered at frequency ranges of 0.4–1.0 Hz, 0.4–1.5 Hz, and 0.6–1.5 Hz. We determined the centroid and origin times of the earthquakes by grid search. We used a 3-D seismic structure model constructed by combining detailed shallow structure above the lower crust (Yamaya et al., in prep) and Japan Integrated Velocity Structure Model (Koketsu et al., 2012) for deeper structure. For calculating Green’s function at each grid points, we used seismic waveform propagation code based on the finite difference method (OpenSWPC; Maeda et al., 2017).
We applied the method to ~100 earthquakes to obtain their moment tensor and depth solutions. The solutions are mostly consistent irrespective of the filtered frequency ranges. The earthquakes in the north of the OBS network show mechanisms consistent with the top of the subducting Pacific Plate. In contrast, those locating in the southern region are slightly different from those in the north, which may due to the subducting Philippine Sea Plate.