Spatial variations in seismicity characteristics in and around the source region of the 2019 Yamagata-Oki Earthquake, Japan

Taku Ueda, Aitaro Kato, Yosihiko Ogata, & Lina Yamaya

Published August 15, 2020, SCEC Contribution #10650, 2020 SCEC Annual Meeting Poster #081

The 2019 Mj 6.7 Yamagata-Oki Earthquake occurred off the coast of Yamagata Prefecture, Japan, on 18 June 2019. This earthquake occurred adjacent to the northeastern edge of the source region of the 1964 Niigata Earthquake. Few aftershocks occurred in the source region of the Yamagata-Oki earthquake right after the Niigata earthquake, and the recent seismicity rate in this region is extremely low (Kusano and Hamada, 1991).

We investigate spatial variation in characteristics of seismicity and discuss which physical process controls seismicity. We applied the HIerarchical Space-Time Epidemic Type Aftershock Sequence (HIST-ETAS) model (e.g., Ogata, 2004) considering the spatial dependence of each parameter of the Spece Time ETAS model (e.g., Ogata, 1998), to a hypocenter catalog compiled by the Japan Meteorological Agency in and around the Yamagata-Oki earthquake rupture region. We compare spatial variations in background seismicity rate (μ) and aftershock productivity (K), estimated from the HIST-ETAS model, with the geophysical features in the study region.

The μ-value is high along the eastern margin of the Sea of Japan, and these areas correlate well with those possessing a high geodetic E-W strain rate (Meneses-Gutierrez and Sagiya, 2016). The two major earthquakes occurred in and around high μ-value and E-W strain rate, suggesting that the μ-value may be a valuable parameter for evaluating seismic hazard across the Japanese Archipelago. Furthermore, the source region of the Yamagata-Oki earthquake has higher K-value, lower b-value, and lower seismic-wave velocity (Matsubara et al., 2019) than that of the Niigata earthquake. We interpret this low-velocity zone as a well-developed damaged rock that resulted in a reduction in the b-value and an increase in K-value based on laboratory experiments and numerical results (Amitrano, 2003); this damage makes the rock more ductile at the macroscopic scale. The higher ductility in the source region of the Yamagata-Oki earthquake may have acted as a soft barrier against the propagation of dynamic rupture during the Niigata earthquake.

Key Words
The 2019 Yamagata-Oki Earthquake, HIST-ETAS model, Background seismicity rate, Brittle-ductile

Citation
Ueda, T., Kato, A., Ogata, Y., & Yamaya, L. (2020, 08). Spatial variations in seismicity characteristics in and around the source region of the 2019 Yamagata-Oki Earthquake, Japan. Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology