Abstract
An earthquake with a magnitude of 6.7 occurred in the Japan Sea off Yamagata on June 18, 2019. The mainshock had a source mechanism of reverse-fault type with a compression axis of WNW–ESE direction. Since the source area is positioned in a marine area, seafloor seismic observation is indispensable for obtaining the precise distribution of the aftershocks. The source area has a water depth of less than 100 m, and fishing activity is high. It is difficult to perform aftershock observation using ordinary free-fall pop-up type ocean bottom seismometers (OBSs). We developed a simple anchored-buoy type OBS for shallow water depths and performed the seafloor observation using this. The seafloor seismic unit had three-component seismometers and a hydrophone. Two orthogonal tiltmeters and an azimuth meter monitored the attitude of the package. For seismic observation at shallow water depth, we concluded that an anchored-buoy system would have the advantage of avoiding accidents. Our anchored-buoy OBS was based on a system used in fisheries. We deployed three anchored-buoy OBSs in the source region where the water depth was approximately 80 m on July 5, 2019, and two of the OBSs were recovered on July 13, 2019. Temporary land seismic stations with a three-component seismometer were also installed. The arrival times of P- and S-waves were read from the records of the OBSs and land stations, and we located hypocenters with correction for travel time. A preliminary location was performed using absolute travel time and final hypocenters were obtained using the double-difference method. The aftershocks were distributed at a depth range of 2.5 km to 10 km and along a plane dipping to the southeast. The plane formed by the aftershocks is consistent with the focal mechanism of the mainshock. The activity region of the aftershocks was positioned in the upper part of the upper crust. Focal mechanisms were estimated using the polarity of the first arrivals. Although many aftershocks had a reverse-fault focal mechanism similar to the focal solution of the mainshock, normal-fault type and strike–slip fault type focal mechanisms were also estimated.Graphical
Highlights
Introduction The2019 Yamagata-oki earthquake, determined by the Japan Meteorological Agency (JMA) with a magnitude (MJMA) of 6.7 and a depth of approximately 14 km occurred in the Japan Sea off Sakata, Yamagata prefecture on June 18
Since the source regions of the large earthquakes occurring in the Japan Sea are positioned below marine areas, seafloor seismic observations were carried out to determine the distribution of their aftershocks (Urabe et al 1985; Aoyagi et al 1998; Shinohara et al 2008; Yamada et al 2008)
A tsunami with a height of 11 cm was observed at Tsuruoka, Yamagata prefecture
Summary
Introduction The2019 Yamagata-oki earthquake, determined by the Japan Meteorological Agency (JMA) with a magnitude (MJMA) of 6.7 and a depth of approximately 14 km occurred in the Japan Sea off Sakata, Yamagata prefecture on June 18. Since the source regions of the large earthquakes occurring in the Japan Sea are positioned below marine areas, seafloor seismic observations were carried out to determine the distribution of their aftershocks (Urabe et al 1985; Aoyagi et al 1998; Shinohara et al 2008; Yamada et al 2008). The Niigata earthquake that occurred in the adjacent region (Kusano and Hamada 1991) prior to the 2019 Yamagata-oki earthquake and is believed to had have a similar focal mechanism of reverse-fault type with a steep westward-dipping plane and a gradual eastward-dipping plane (Hirasawa 1965; Abe 1975) From these reasons, we decided to conduct a marine seismic observation in the source region of the 2019 Yamagata-oki earthquake in order to obtain exact lateral and depth distribution of the aftershocks of the mainshock with land seismic observations in the coastal area close to the source region. This paper describes the development of the anchored-buoy type OBS, observations using the developed OBSs for shallow water, and the precise aftershock distribution, with particular focus on the depths of the aftershocks using marine seismic stations just above the aftershock region
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