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

Taku Ueda,Lina Yamaya,Yosihiko Ogata,Aitaro Kato

doi:10.1186/s40623-020-01325-9

Abstract

The 2019 {M}_{j} 6.7 Yamagata-Oki earthquake occurred adjacent to the northeastern edge of the source region of the 1964 {M}_{j} 7.5 Niigata earthquake, offshore of Yamagata Prefecture, Japan. Few aftershocks occurred in the source region of the Yamagata-Oki earthquake immediately following the Niigata earthquake, and the recent seismicity rate in this region is low compared with the source region of the Niigata earthquake. This spatial variation in seismicity may allow us to elucidate plausible physical processes that shape the spatiotemporal evolution of these shallow-crustal environments. Here, we investigate the spatial variations in seismicity characteristics by applying the HIerarchical Space–Time Epidemic Type Aftershock Sequence (HIST-ETAS) model to an earthquake catalog compiled by the Japan Meteorological Agency for events in and around the Yamagata-Oki earthquake rupture region. We compare spatial variations in the background seismicity rate and aftershock productivity estimated from the HIST-ETAS model with the geophysical features in the study region. The background seismicity rate is high along the eastern margin of the Sea of Japan and correlates well with a previously identified zone that possesses a high geodetic E–W strain rate. The two major earthquakes occurred in and around a high E–W strain rate zone, suggesting that the background seismicity rate may serve as a key parameter for evaluating seismic hazard across the Japanese Archipelago. Furthermore, the source region of the Yamagata-Oki earthquake has a higher aftershock productivity and lower seismic-wave velocity than that of the Niigata earthquake. We interpret this low-velocity zone to be a well-developed damaged rock that resulted in an increase in aftershock productivity based on previous laboratory experiments and numerical results; 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 worked as a barrier against the propagation of dynamic rupture that occurred during the Niigata earthquake.

Highlights

The 2019 Mj (Japan Meteorological Agency (JMA) magnitude) 6.7 Yamagata-Oki earthquake occurred off the coast of Yamagata Prefecture, Japan, at ~ 14 km depth, on 18 June 2019 (Fig. 1)
These previous studies imply that investigations of the spatial variations in given seismicity parameters can allow us to infer the physical processes that shape the spatiotemporal evolution of these shallow-crustal environments
We estimated the seismicity parameter uncertainties at each location to evaluate the significance of the relative differences in each parameter between the source regions of the Yamagata-Oki and Niigata earthquakes

Summary

Introduction

The 2019 Mj (Japan Meteorological Agency (JMA) magnitude) 6.7 Yamagata-Oki earthquake occurred off the coast of Yamagata Prefecture, Japan, at ~ 14 km depth, on 18 June 2019 (Fig. 1). Zeng et al (2018) demonstrated that spatial variations in the background seismicity rate in California correlate with the maximum shear strain rate, and Ogata (2004) suggested that aftershock productivity K (one of the parameters that define the modified Omori law) is high on the boundary of the source region hosting M 7-class earthquakes along the offshore Tohoku region. These previous studies imply that investigations of the spatial variations in given seismicity parameters (i.e., background seismicity rate, aftershock productivity, and aftershock decay rate) can allow us to infer the physical processes that shape the spatiotemporal evolution of these shallow-crustal environments. We discuss spatial variations in the seismicity parameters, with a focus on the active tectonic zone and a relative abundance of ductility inferred from rock deformation mechanics

Methods

Results

Discussion

Conclusion