Centroid Moment Tensor Inversion of Shallow Very Low Frequency Earthquakes Off the Kii Peninsula, Japan, Using a Three-Dimensional Velocity Structure Model

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 20-50s 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. Plain Language Summary We investigated locations and focal mechanisms of shallow very low frequency earthquakes (SVLFEs) that occurred off the southeast Kii Peninsula, Japan, during September 2004, March 2009, and April 2016. SVLFEs may be linked to the occurrence of large megathrust earthquakes. We only used onshore broadband seismograms, since seafloor observations are still limited and not common in other subduction zones. To analyze SVLFEs precisely, we used synthetic seismograms obtained via numerical simulation with a 3-D model, rather than a conventional 1-D model. Focal mechanisms and locations of SVLFEs were precisely estimated, even for those without seafloor observations. Low-angle thrust faulting solutions were concentrated around the accretionary prism toe. Our results suggest seismic slip around the very shallow part of the Philippine Sea plate boundary.