5:15 PM - 6:45 PM
[U15-P18] Seismic activity off Joetsu, Niigata Prefecture, Japan, from Dec. 2008 to Oct. 2009 using long-term ocean bottom seismometers (OBSs)
Keywords:2024 Noto Peninsula Earthquake, Joetsu-oki, Seismic activity, Long-term Ocean Bottom Seismometer, Mechanism
The Noto Peninsula earthquake (Mj=7.6) occurred on January 1, 2024 in a strain concentration zone along the eastern margin of the Japan Sea. This earthquake is considered to have occurred within a series of active earthquake swarms that have been occurring since around December 2020. This earthquake occurred in the coastal area of the Noto Peninsula, and its aftershocks extended into the Joetsu-oki region, which is considered to be an area where it is difficult to accurately determine regular seismic activity prior to the main shock due to the lack of a regular seismic network on the seafloor.
The Earthquake Research Institute of the University of Tokyo carried out aftershock observations using OBSs and active-source seismic survey around the source region of the 2007 Chuetsu-oki earthquake (Shinohara et al., 2008; Nakahigashi et al., 2011). As a result, it was clarified that many aftershocks occurred in the upper crust after the 2007 Chuetsu-oki earthquake. However, since the strain concentration zone extends into the oceanward, we deployed a Long-term Ocean Bottom Seismometer (LT-OBSs) in the Joetsu-oki region from December 2008 to October 2009 for the purpose of accurately understanding regular seismic activity.
In this study, P- and S-wave readings were taken manually based on the JMA centralized seismic source list during the observation period. We also determined the epicenters of 1,230 earthquakes with three or more P-wave readings and one or more S-wave reading, because we found that many microearthquakes occurred that were not captured by the land-based observation network. The hypoMH (Hirata and Matsu'ura, 1987) was used for source determination. The 1D structural model used for the source determination is based on the results of the integrated sea-land crustal structure survey conducted in 2007 (Nakahigashi et al., 2011), which traversed the observation area. The source determination took into account delay of arrival time due to sedimentary layers at each observatory. Of these, 1101 earthquakes were accurately source-determined with horizontal and depth errors within 1 km and 2 km, respectively. Based on the initial epicenters obtained here, we obtained the three-dimensional velocity structure in the sea area containing the strain concentration zone. The double-difference tomography method by Zhang and Thurber (2003) was used to estimate the seismic velocity structure.
The analysis revealed the following: 1) Seismic activity appears to be active around the 2007 Chuetsu-oki earthquake, but also in the area northwest of the 2007 Chuetsu-oki earthquake, earthquakes appear to be relatively active at depths of 8 to 13 km. 2) Seismic activity also appears to be relatively active in the area to the northwest at depths of 8-13 km, which is considered to correspond to the upper crust. 2) Further to the northwest, seismic activity in the Toyama Trough is low and almost no earthquakes have occurred. On the other hand, there is an area of seismic activity at depths of 15-18 km further to the northwest, which appears to correspond to the northeastern edge of the Noto Peninsula earthquake aftershock zone.
We will continue to estimate the mechanism solution of regular earthquakes that occur in the Joetsu-oki region and discuss it with seismic activity and crustal structure.
The Earthquake Research Institute of the University of Tokyo carried out aftershock observations using OBSs and active-source seismic survey around the source region of the 2007 Chuetsu-oki earthquake (Shinohara et al., 2008; Nakahigashi et al., 2011). As a result, it was clarified that many aftershocks occurred in the upper crust after the 2007 Chuetsu-oki earthquake. However, since the strain concentration zone extends into the oceanward, we deployed a Long-term Ocean Bottom Seismometer (LT-OBSs) in the Joetsu-oki region from December 2008 to October 2009 for the purpose of accurately understanding regular seismic activity.
In this study, P- and S-wave readings were taken manually based on the JMA centralized seismic source list during the observation period. We also determined the epicenters of 1,230 earthquakes with three or more P-wave readings and one or more S-wave reading, because we found that many microearthquakes occurred that were not captured by the land-based observation network. The hypoMH (Hirata and Matsu'ura, 1987) was used for source determination. The 1D structural model used for the source determination is based on the results of the integrated sea-land crustal structure survey conducted in 2007 (Nakahigashi et al., 2011), which traversed the observation area. The source determination took into account delay of arrival time due to sedimentary layers at each observatory. Of these, 1101 earthquakes were accurately source-determined with horizontal and depth errors within 1 km and 2 km, respectively. Based on the initial epicenters obtained here, we obtained the three-dimensional velocity structure in the sea area containing the strain concentration zone. The double-difference tomography method by Zhang and Thurber (2003) was used to estimate the seismic velocity structure.
The analysis revealed the following: 1) Seismic activity appears to be active around the 2007 Chuetsu-oki earthquake, but also in the area northwest of the 2007 Chuetsu-oki earthquake, earthquakes appear to be relatively active at depths of 8 to 13 km. 2) Seismic activity also appears to be relatively active in the area to the northwest at depths of 8-13 km, which is considered to correspond to the upper crust. 2) Further to the northwest, seismic activity in the Toyama Trough is low and almost no earthquakes have occurred. On the other hand, there is an area of seismic activity at depths of 15-18 km further to the northwest, which appears to correspond to the northeastern edge of the Noto Peninsula earthquake aftershock zone.
We will continue to estimate the mechanism solution of regular earthquakes that occur in the Joetsu-oki region and discuss it with seismic activity and crustal structure.