Structural Characteristics of the Nankai Trough Shallow Plate Boundary Inferred From Shallow Very Low Frequency Earthquakes

Abstract

The spatial distributions of shallow slow earthquakes are related to stress accumulation and structural characteristics of the shallow plate boundary, which are important for understanding megathrust earthquakes. To investigate spatiotemporal variation of shallow very low frequency earthquake (SVLFE) activity along the Nankai Trough, we conducted centroid moment tensor inversion method incorporating effects of offshore heterogeneous structures. By applying centroid moment tensor method into long‐term onshore seismograms, we obtained spatiotemporal variation of SVLFE activity occurred from June 2003 to May 2018. We find that SVLFE activities are related to the spatial variations of the slip‐deficit rate and pore fluid around the Philippine Sea plate boundary. SVLFEs are effectively activated by mechanical weakening due to rich pore fluid in areas surrounding strongly locked zones. Our results imply that spatial variations of long‐term SVLFE activity provide information on tectonic environments, which could constrain the rupture behavior of shallow plate boundaries during future megathrust earthquakes.

Characteristics of Shallow Low‐Frequency Earthquakes off the Kii Peninsula, Japan, in 2004 Revealed by Ocean Bottom Seismometers

“…Energy rates were relatively small around station A near the trench axis and relatively high at stations B, E, and F about 20 km landward from the trough axis. This can also be confirmed from the VLFE catalog of Takemura et al (2019). Hypocentral depths were poorly constrained, as shown in Figure S4; however, 1,610 of the 2,315 hypocenters (70%) had calculated depths of 0 km, and 252 more (11%) had 5 km depths.…”

“…This can also be confirmed from the VLFE catalog of Takemura et al (2019). VLFEs are known to occur in connection with slow deformation on the plate boundary or in the accretionary prism (Ito & Obara, 2006;Obara & Ito, 2005;Sugioka et al, 2012;Takemura et al, 2019). The locations of these paired events are in general agreement.…”

“…As shown in Figure 1a, VLFEs observed by the inland F-net stations were associated with LFEs observed by the OBS network. The moment rate in this study was roughly estimated by dividing the seismic moment by the duration from Takemura et al (2019). Figure 3a is a plot of VLFEs in that catalog and LFEs from this study with origin times within 30 s of the VLFEs.…”

“…The VLFE catalog was downloaded from Takemura et al (2019). The VLFE catalog was downloaded from Takemura et al (2019).…”

Migrations and Clusters of Shallow Very Low Frequency Earthquakes in the Regions Surrounding Shear Stress Accumulation Peaks Along the Nankai Trough

“…The remaining CMT solutions with high (>65%) VRs were very similar to those estimated by ocean‐bottom seismometers (Takemura, Matsuzawa, et al, ). Based on the method of Takemura, Matsuzawa, et al (), we synthesized 150‐s velocity seismograms for F‐net stations using 29 well‐constrained CMT solutions (blue focal spheres in Figure a and in Supporting Information in Data Set S1). We used these synthetic seismograms as template waveforms because of the absence of incoherent noise signals.…”