Spatial Variations in Seismicity Characteristics in and Around the Source Region of the 2019 Yamagata-Oki Earthquake, Japan

Abstract

The 2019 {\text{M}}_{\text{j}} 6.7 Yamagata-Oki Earthquake occurred adjacent to the northeastern edge of the source region of the 1964 {\text{M}}_{\text{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 extremely low compared with that of the surrounding region. 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 shear strain rate. The two major earthquakes occurred in and around an active shear 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, lower b-value, 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 both a reduction in the b-value and 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 soft barrier against the propagation of dynamic rupture that occurred during the Niigata earthquake.

Seasonal Modulation of Crustal Seismicity in Northeastern Japan Driven by Snow Load

“…Aftershocks should be removed via a declustering technique prior to investigating the seasonal modulation of seismicity. Here we apply the Hierarchical Space-Time Epidemic-Type Aftershock Sequence (HIST-ETAS) model (Ogata, 2004;Ogata et al, 2003;Ueda et al, 2021) to the earthquake catalog and the probability that each event is a background event is used as the declustered catalog (Ueda & Kato, 2023;Zhuang et al, 2002). A comparison of the cumulative numbers of total and background earthquakes (Figure 5c) shows that the aftershocks are successfully removed from the original earthquake catalog by the HIST-ETAS model.…”

Análisis y validación experimental de licuefacción de suelos mediante estudio granulométrico

“…Entre los eventos documentados, donde se ha evidenciado la ocurrencia de este fenómeno, se pueden mencionar el terremoto de la ciudad de Niigata (Japón) en 1964, donde un sismo de magnitud 7.6 indujo la licuefacción de suelos en gran parte de la ciudad, causando daños estructurales y volcamientos de edificios, con numerosas pérdidas humanas[4]. El del río Atrato en la región del Urabá Antioqueño (Colombia) en 1992, en el que un sismo de magnitud 6.6 ocasionó la licuefacción de suelos en las orillas del río, provocando grandes deslizamientos y afectaciones civiles en la zona[5].…”