5:15 PM - 7:15 PM
[SSS12-P10] Focal mechanisms at the source area of the 2024 Noto Peninsula earthquake estimated from dense seafloor seismic observation data
The source area of the 2024 Noto peninsula earthquake extends about 150 km from the west to off northeast of the Noto peninsula. Since the M7.6 earthquake on Jan. 1, 2024, seismicity remains active throughout the source area. JAMSTEC and universities in Japan have conducted a seafloor seismic observation for one year. From June to Sep. 2024, 80 ocean bottom seismographs (OBSs) were deployed at the northeastern part of the source area. The 40 OBSs among them were deployed at 2km interval along a seismic survey line (No et al. 2025, JpGU). This study examined focal mechanisms using the dense seismic observation data from June to Sep. 2024.
This study first relocated the hypocenters using manually inspected arrival times of P and S-waves. Using the relocated hypocenters, we estimated the focal mechanisms from the P-wave polarities. Uncertainties of nodal planes tended to be small because of the dense OBS data. Earthquakes near the survey lines (M>2.3) showed various focal mechanisms including reverse, normal and strike-slip. Some reverse fault events showed complex polarity distributions that imply contributions of the non-double-couple (DC) component. We also observed the case that most of the polarity data along the survey lines were located near the nodal planes. This contradicted with large wave amplitudes of P-wave, and implied the contribution of non-DC component.
The study on focal mechanisms from Jan. to Feb. 2024 (Shinohara et al. under review) has already pointed out that the non-DC component may have affected focal mechanism estimation. Akuhara et al. (2025, JpGU) quantitively examined the non-DC component by analyzing P-wave amplitudes. Our polarity data also implied the contribution of the non-DC component for many events. We may need to conduct wave amplitude analyses to examine details of focal mechanisms and the contribution of non-DC component.
This study first relocated the hypocenters using manually inspected arrival times of P and S-waves. Using the relocated hypocenters, we estimated the focal mechanisms from the P-wave polarities. Uncertainties of nodal planes tended to be small because of the dense OBS data. Earthquakes near the survey lines (M>2.3) showed various focal mechanisms including reverse, normal and strike-slip. Some reverse fault events showed complex polarity distributions that imply contributions of the non-double-couple (DC) component. We also observed the case that most of the polarity data along the survey lines were located near the nodal planes. This contradicted with large wave amplitudes of P-wave, and implied the contribution of non-DC component.
The study on focal mechanisms from Jan. to Feb. 2024 (Shinohara et al. under review) has already pointed out that the non-DC component may have affected focal mechanism estimation. Akuhara et al. (2025, JpGU) quantitively examined the non-DC component by analyzing P-wave amplitudes. Our polarity data also implied the contribution of the non-DC component for many events. We may need to conduct wave amplitude analyses to examine details of focal mechanisms and the contribution of non-DC component.