Slow Slip Events in the Afterslip Area of the 1996 Hyuganada Earthquakes, Southwest Japan, Based on GNSS Data

Abstract:

The Philippine Sea plate is subducting beneath the Amurian plate along the Nankai trough in southwestern Japan. In this area, the Bungo Channel and Hyuganada, it is known that afterslip and slow slip events (SSEs) occur on the plate interface. SSEs repeatedly occur in the Bungo Channel (e.g., Hirose et al., 1999) and afterslip followed two M 6.7 earthquakes in 1996 in the Hyuganada region (e.g., Yagi et al., 2001). In the afterslip area, the occurrence of repeating SSEs has been reported (Yarai and Ozawa, 2013; Ozawa et al. 2017; Takagi et al., 2019), but the timing and detailed spatial relationship between the source areas of the SSEs after the afterslip and the afterslip area are not well known. In this study, we estimate aseismic slip from the Bungo Channel to Hyuganada and discuss about spatiotemporal evolutions of the afterslip and the SSEs in Hyuganada.

We use daily coordinates of GEONET GNSS stations operated by Geospatial Information Authority of Japan (GSI) (F3 solutions: Nakagawa et al., 2009). Coordinate time-series data at 47 stations from Sep. 1996 to Dec. 2001 are analyzed. We assume a geometry of the plate interface on the subducting Philippine Sea plate (Shiomi et al., 2008; Nakanishi et al., 2018) and apply a time-dependent inversion method (Hirose et al., 2014) to the GNSS data to estimate a spatiotemporal slip evolution.

4 events are detected; (1) afterslip of the 1996 Hyuganada earthquakes from Oct. 1996 to Jul. 1997; (2) the 1997 Bungo SSE from Dec. 1996 to Feb. 1998; (3) an SSE in the afterslip area from Sep. 1998 to Apr. 1999; (4) an SSE similar to (3) from Mar. 2000 to Jan. 2001. The slip distributions and magnitudes of (1)-(4) roughly correspond to previous studies. However, in this study, the slip areas of (3) and (4) are almost identical, are about 3/4 as large as that of (1), and are included in the slip area of (1). In addition, (3) begins about 13 months after the end of the afterslip.

Repeated occurrences of the SSEs in the afterslip area and the 13 month-interval after the end of the afterslip provide useful constraints on the frictional property in the area.

Acknowledgments: The GNSS data were provided by GSI. This work was supported by JSPS KAKENHI Grant Number JP16H06474.