Configuration and structure of the Philippine Sea Plate off Boso, Japan: constraints on the shallow subduction kinematics, seismicity, and slow slip events

Abstract

We applied tomographic inversion and receiver function analysis to seismic data from ocean-bottom seismometers and land-based stations to understand the structure and its relationship with slow slip events off Boso, Japan. First, we delineated the upper boundary of the Philippine Sea Plate based on both the velocity structure and the locations of the low-angle thrust-faulting earthquakes. The upper boundary of the Philippine Sea Plate is distorted upward by a few kilometers between 140.5 and 141.0°E. We also determined the eastern edge of the Philippine Sea Plate based on the delineated upper boundary and the results of the receiver function analysis. The eastern edge has a northwest-southeast trend between the triple junction and 141.6°E, which changes to a north-south trend north of 34.7°N. The change in the subduction direction at 1-3 Ma might have resulted in the inflection of the eastern edge of the subducted Philippine Sea Plate. Second, we compared the subduction zone structure and hypocenter locations and the area of the Boso slow slip events. Most of the low-angle thrust-faulting earthquakes identified in this study occurred outside the areas of recurrent Boso slow slip events, which indicates that the slow slip area and regular lowangle thrust earthquakes are spatially separated in the offshore area. In addition, the slow slip areas are located only at the contact zone between the crustal parts of the North American Plate and the subducting Philippine Sea Plate. The localization of the slow slip events in the crust-crust contact zone off Boso is examined for the first time in this study. Finally, we detected a relatively low-velocity region in the mantle of the Philippine Sea Plate. The low-velocity mantle can be interpreted as serpentinized peridotite, which is also found in the Philippine Sea Plate prior to subduction. The serpentinized peridotite zone remains after the subduction of the Philippine Sea Plate and is likely distributed over a wide area along the subducted slab.

Long-period ground motion simulation using centroid moment tensor inversion solutions based on the regional three-dimensional model in the Kanto region, Japan

“…In the Kanto region, because the predominant period of long-period ground motion is approximately 6 s, a wellconstrained sedimentary model is required to evaluate long-period ground motion for observed and anticipated large earthquakes. Recently, other regional/local velocity structure models of the sedimentary basin and subducting oceanic plate have been released (e.g., Hirose et al 2008;Takemura et al 2015; Headquarters for Earthquake Research Promotion 2017; Ito et al 2019). The reproducibility of long-period ground motion in the Kanto Basin can be improved through the utilization of other published or adjusted models based on ground motion Fig.…”

Receiver Function Imaging of the Amphibious NE Japan Subduction Zone—Effects of Low‐Velocity Sediment Layer

“…The RF method is a widely used tool to map seismic discontinuities (i.e., reflectivity) such as interfaces of the hydrated oceanic crust (e.g., Bostock et al., 2002). Recently, the RF method is applied to amphibious datasets in SW and central Japan (Akuhara & Mochizuki, 2015; Ito et al., 2019) and the western part of the US (Audet & Schaeffer, 2018; Janiszewski & Abers, 2015; Reeves et al., 2015). In NE Japan, however, RF imagings have been done using only land station data (Kawakatsu & Watada, 2007; Tonegawa et al., 2005).…”

Long-period ground motion simulation using centroid moment tensor inversion solutions based on the regional three-dimensional model in the Kanto region, Japan

“…In the Kanto region because the predominant period of long-period ground motion is approximately 6 s, a well-constrained sedimentary model is required to evaluate long-period ground motion for observed and anticipated large earthquakes. Recently, other regional/local velocity structure models of the sedimentary basin and subducting oceanic plate have been released (e.g., Hirose et al 2008;Takemura et al 2015; Headquarters for Earthquake Research Promotion 2017; Ito et al 2019). The reproducibility of long-period ground motion in the Kanto Basin can potentially be improved through the utilization of other published or adjusted models based on ground motion simulations; this should be the primary focus of future research.…”