Centroid moment tensor inversion of shallow very low-frequency earthquakes off the Kii Peninsula, Japan, using a three-dimensional velocity structure model Shunsuke TAKEMURA, Takanori MATSUZAWA, Takeshi KIMURA, Takashi TONEGAWA and Katsuhiko SHIOMI DOI: 10.1029/2018GL078455 Key Points: We conducted CMT inversions of shallow very low-frequency earthquakes using a 3D velocity structure model and onshore seismic network Suitable 3D velocity structure model enable us to obtain accurate CMT solutions of events before oceanfloor observations Our CMT solutions suggest seismic slip around the very shallow part of plate boundary beneath the accretionary prism toe Abstract We conducted centroid moment tensor inversions of shallow very low frequency earthquakes (SVLFEs) that occurred around off the southeast Kii Peninsula, Japan. A set of Green's functions was evaluated using numerical simulations of seismic wave propagation in a three‐dimensional velocity structure model, including the subducting Philippine Sea slab and the accretionary prism. We used observed seismograms for periods of 20–50 s at an onshore broadband network, which enable us to investigate the long‐term activity of SVLFEs. Estimated centroid locations and focal mechanisms of SVLFEs, including SVLFEs that occurred prior to the deployment of seafloor observations, were very similar to those estimated using seafloor records. Low‐angle thrust faulting mechanisms were concentrated around the accretionary prism toe. SVLFE activity in April 2016 was concentrated further west in an area of previous activity. Our results imply seismic slip around the very shallow part of the Philippine Sea plate boundary. Green’s function catalog of Takemura et al. 2018 GRL Green’s functions of each source grid and basis moment tensor were stored in the directories, whose names are related with epicenter location (13X.XE°, 3X.X°N) and basis moment tensor (MX). In each directory, SAC format velocity seismograms of F-net and DONET1 stations are stored. EB, NB and UB of F-net seismograms (Y, X and Z of DONET1 seismograms) are corresponding to ones in East-West, North-South and Up-Down motions, respectively. A low pass filer with frequency less than 0.125 Hz was applied to each trace. In SAC header, station locations, sampling rate and data length are described. Seismic sources of five basis moment tensors except for an isotropic one (M1~M5 of Figure 1 in Kikuchi and Kanamori 1991) were assumed at each source grid, which was basically assumed on the PHS boundary at a horizontal interval of 0.1°. A cosine-type moment rate function with duration of tau s was employed in the stored Green’s functions.

{"references": ["Takemura, S., Matsuzawa, T., Kimura, T., Tonegawa, T., & Shiomi, K. (2018). Centroid moment tensor inversion of shallow very low frequency earthquakes off the Kii Peninsula, Japan, using a three-dimensional velocity structure model. Geophysical Research Letters, 45. https://doi.org/10.1029/2018GL078455"]}