17:15 〜 18:30
[SCG49-P02] Precise aftershock distribution of the 2019 off-Yamagata earthquake using anchored-buoy type ocean bottom seismometers
キーワード：2019年山形県沖地震、余震観測、係留式海底地震計
A earthquake with a JMA magnitude of 6.7 and a depth of approximately 14 km occurred in the Japan Sea off Sakata, Yamagata on June 18, 2019. A maximum seismic intensity was 6+ and tsunami warning was issued to Yamagata, Niigata and Ishikawa prefectures. The mainshock has a source mechanism of reverse fault type with compression axis of WNW-ESE direction. It is estimated that the earthquake relates to the Niigata-Kobe tectonic zone in the eastern margin of the Japan Sea. Sine the source area of the earthquake is positioned in marine area, seafloor seismic observation is indispensable to obtain precise distribution of aftershocks. Therefore, we decided to make a marine seismic observation in the source region of the earthquake and temporary land seismic observation in the costal area near the source region. Because the source area has water depths of less than 100 meters where fishery activity is high, it is difficult to perform seafloor seismic 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 made the seafloor observation using the developed OBSs.
We adopted the ocean bottom recorder (OBX-750), Geospace Inc, USA for the seafloor observation. OBX-750 has three-component velocity sensitive seismometer with a natural frequency of 15 Hz (GS-ONE OMNI) and a hydrophone. Because the sensors can observe properly in any attitude, the OBX-750 has no leveling system for seismic observation. To monitor the attitude of the package, two orthogonal tiltmeters and an azimuth meter were installed. The equipped hydrophone has a flat response in frequency range of greater than 10 Hz. The signals from the scientific sensors were digitized with a resolution of 24 bits and continuously stored to a memory. The recording duration is typically one month. The OBX-750 has small size (52 x 21 x 11 cm) and light weight (11 kg in air and 4 kg in water). The timing is based on an OVCXO (oven voltage-controlled crystal oscillator). For deployment in shallow water depth, we used anchored-buoy system which has an advantage that the buoy on the surface informs boats of the position of the system. At an end of the rope, a 2-kg weight for stable deployment was attached, and the OBX-750 was connected with a distance of 1.5 meters from the end. Two anchors (8 kg) for originally fix of a small boat were attached with 2 m interval from the recordeer. Anchors to fix a small boat are useful to prevent movement of the system by winds and waves. The rope is 175 m long and a weight (2 kg) was attached to sink the rope. A buoy is connected to the system using rope of 20 m long. We deployed three anchored buoy system in the source region with intervals of 5 km and 8.5 km. Water depths were approximately 80 m. We chose low-gain amplifier due to shallow water depth and a sampling frequency was set to 500 Hz. The systems were deployed on July 5 and we recovered the systems on July 13. Unfortunately, it was confirmed that one buoy could not be seen on the sea surface at the deploying position on July 12. Two anchored-buoy OBS were successfully recovered and we obtained the data. Temporary land seismic stations had three-component seismometer with a natural frequency of 1 Hz and the data including the period of the seafloor observation were retrieved.
Arrival times of P- and S-waves were read from the records of OBSs and land stations, and we located hypocenters using one-dimensional velocity structure which was estimated from results of the seismic surveys. Arrival times picked from the OBS data needed station corrections for precise location. The aftershocks are distributed in depth range from 3 km to 12 km and along a plane dipping to southeast. The plane formed by the aftershocks is consistent with the focal mechanism of the mainshock.
We adopted the ocean bottom recorder (OBX-750), Geospace Inc, USA for the seafloor observation. OBX-750 has three-component velocity sensitive seismometer with a natural frequency of 15 Hz (GS-ONE OMNI) and a hydrophone. Because the sensors can observe properly in any attitude, the OBX-750 has no leveling system for seismic observation. To monitor the attitude of the package, two orthogonal tiltmeters and an azimuth meter were installed. The equipped hydrophone has a flat response in frequency range of greater than 10 Hz. The signals from the scientific sensors were digitized with a resolution of 24 bits and continuously stored to a memory. The recording duration is typically one month. The OBX-750 has small size (52 x 21 x 11 cm) and light weight (11 kg in air and 4 kg in water). The timing is based on an OVCXO (oven voltage-controlled crystal oscillator). For deployment in shallow water depth, we used anchored-buoy system which has an advantage that the buoy on the surface informs boats of the position of the system. At an end of the rope, a 2-kg weight for stable deployment was attached, and the OBX-750 was connected with a distance of 1.5 meters from the end. Two anchors (8 kg) for originally fix of a small boat were attached with 2 m interval from the recordeer. Anchors to fix a small boat are useful to prevent movement of the system by winds and waves. The rope is 175 m long and a weight (2 kg) was attached to sink the rope. A buoy is connected to the system using rope of 20 m long. We deployed three anchored buoy system in the source region with intervals of 5 km and 8.5 km. Water depths were approximately 80 m. We chose low-gain amplifier due to shallow water depth and a sampling frequency was set to 500 Hz. The systems were deployed on July 5 and we recovered the systems on July 13. Unfortunately, it was confirmed that one buoy could not be seen on the sea surface at the deploying position on July 12. Two anchored-buoy OBS were successfully recovered and we obtained the data. Temporary land seismic stations had three-component seismometer with a natural frequency of 1 Hz and the data including the period of the seafloor observation were retrieved.
Arrival times of P- and S-waves were read from the records of OBSs and land stations, and we located hypocenters using one-dimensional velocity structure which was estimated from results of the seismic surveys. Arrival times picked from the OBS data needed station corrections for precise location. The aftershocks are distributed in depth range from 3 km to 12 km and along a plane dipping to southeast. The plane formed by the aftershocks is consistent with the focal mechanism of the mainshock.