Characteristics of relocated hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its aftershocks with a three-dimensional seismic velocity structure

Saeko Kita

doi:10.1186/s40623-019-1100-0

Abstract

I relocated the hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its surrounding area, using a three-dimensional seismic structure, the double-difference relocation method, and the JMA earthquake catalog. After relocation, the focal depth of the mainshock became 35.4 km. As previous studies show, in south-central Hokkaido, the Hidaka collision zone is formed, and anomalous deep and thickened forearc crust material is subducting at depths of less than 70 km. The mainshock and its aftershocks are located at depths of approximately 10 to 40 km within the lower crust of the anomalous deep and thickened curst near the uppermost mantle material intrusions in the northwestern edge of this Hidaka collision zone. Like the two previous large events, the aftershocks of this event incline steeply eastward and appear to be distributed in the deeper extension of the Ishikari-teichi-toen fault zone. The highly inclined fault in the present study is consistent with a fault model by a geodetic analysis with InSAR. The aftershocks at depths of 10 to 20 km are located at the western edge of the high-attenuation (low-Qp) zone. These kinds of relationships between hypocenters and materials are the same as the 1970 and 1982 events in the Hidaka collision zone. The anomalous large focal depths of these large events compared with the average depth limit of inland earthquakes in Japan could be caused by the locally lower temperature in south-central Hokkaido. This event is one of the approximately M7 large inland earthquakes that occurred repeatedly at a recurrence interval of approximately 40 years and is important in the collision process in the Hidaka collision zone.

Highlights

On September 6, 2018, a large earthquake occurred in the central and eastern Iburi regions of Hokkaido, beneath the northeastern (NE) Japan arc, in NE Japan (Fig. 1)
These figures showed that the aftershocks and mainshock are located on the northwestern edge of the thickened crust materials related to the Hidaka collision zone beneath the NE Japan arc
I relocated the hypocenters of microearthquakes, including the aftershocks and mainshock of the 2018 M6.7 inland type event in south-central Hokkaido, using a high-resolution three-dimensional seismic velocity structure

Summary

Introduction

On September 6, 2018, a large earthquake occurred in the central and eastern Iburi regions of Hokkaido, beneath the northeastern (NE) Japan arc, in NE Japan (Fig. 1). The Japan Meteorological Agency (JMA) magnitude of this event is 6.7 and its focal depth is approximately 37 km. The NE Japan arc meets the Kuril arc in the center of Hokkaido Island, which has collided with the Kuril forearc at the Hidaka. In this area, large inland earthquakes with magnitudes of approximately M7 have occurred repeatedly at intervals of approximately 40 years, including the 1931 Mj 6.8, the 1932 Mj 7.0 (Utsu 1999; Ichikawa 1971), and two recent large events—one in 1970 and one in 1982—at depths of 20–30 km for the 1970 Mj 6.7 Hidaka earthquake (Ichikawa 1971; Moriya 1972) and 18–35 km for.

Results

Discussion

Conclusion