5:15 PM - 6:45 PM
[SSS08-P15] Seismic activity in the transition zone between the ordinary earthquake and shallow slow earthquake in Hyuga-nada, southwest Japan
Keywords:Hyuga-nada, ocean bottom seismometer, seismicity
Hyuga-nada is a region where the Philippine Sea Plate subducts beneath the Eurasian Plate, and both ordinary earthquakes and slow earthquakes occur there. Geodetic and seismic observations in the past decade have shown that long-term SSEs occur in the down-dip side of the seismogenic zone (15-30 km depth) where interplate earthquakes occur. Shallow tectonic tremors and shallow VLFEs occur in the up-dip side. The earthquake catalog was provided by the Japan Meteorological Agency (JMA), but for the earthquakes in Hyuga-nada, the accuracy of hypocenter determination is low because the analysis is based on data from land-based seismic stations. In order to determine where each of these phenomena occur, seismic surveys were conducted between 2021-2022, above the transition zone between the normal and shallow seismic zones. We utilized this observational data, performed hypocenter relocation and first-motion focal mechanism analysis, and compared with previous research.
In this study, we used the data obtained by 10 ocean bottom seismometers between March 27, 2021 and January 7, 2022. First, we manually picked P and S phase onsets, the polarity of first motion of the P-wave, and the maximum amplitude of the vertical component for 83 events listed in the JMA catalog. Second, we picked the P to S converted phase onsets and calculated initial station corrections to correct the large travel time delay due to soft and thick sediment at the seafloor. The calculation of hypocenter location was performed by hypoMH (Hirata and Matsu'ura, 1987). We set the Vp/Vs ratio as 1.9 at all depths, which was obtained by making Wadati diagrams. A 1-D velocity model was constructed based on the 2-D P-wave velocity structure, which was obtained by a seismic survey (Arai et al., 2023). To refine the hypocenter determination, the station correction terms were iteratively modified until travel time residuals changed sufficiently small, and 81 events were finally obtained.
The RMS values of the travel time residuals for the P and S phases decreased from 0.311 s to 0.089 s and from 1.059 s to 0.342 s, respectively. Most of the earthquakes have small location errors of 0.5 km in the horizontal direction and 0.4 km in the depth direction, which shows accurate hypocenter distribution compared to the JMA catalog. Hypocenter locations of this study moved horizontally about 10 km to the northwest and became shallower than those of the JMA catalog. Fifteen reliable focal mechanisms obtained using HASH (Hardebeck and Shearer, 2002, 2003) showed high-angle normal faulting.
Comparing analysis results with the P-wave velocity structure of Arai et al. (2023) suggests that most of the earthquakes occurred within the Philippine Sea Plate. The spatio-temporal distribution indicates that the hypocenters migrated from about 2 km below the plate boundary to near the plate boundary. It is possible that intraplate earthquakes occur on the down-dip side of slow earthquakes in Hyuga-nada by the same mechanism as in the Hikurangi subduction zone, where many intraplate earthquakes occur (Warren-Smith et al., 2019; Nishikawa et al., 2021). In the future, we will conduct relative hypocenter determination and verify the results of the analysis including migration, as well as analyze shallow tremors and verify the mechanism of ordinary and slow earthquakes in Hyuga-nada.
Acknowledgements
This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its “The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research)” and “Research Project for Disaster Prevention on the great Earthquakes along the Nankai trough”. We thank the crews of the T/S Nagasakimaru (Faculty of Fisheries, Nagasaki University) and the R/V Kaiyomaru No. 3 (Kaiyo Engineering Co.) for their skillful work and these voyage participants from Kyoto University, Kyushu University, Kagoshima University, Tokyo University of Marine Science and Technology, and Tokyo University. Dr. R. Arai of the Japan Agency for Marine-Earth Science and Technology kindly provided the seismic survey line data of Hyuga-nada in 2020.
In this study, we used the data obtained by 10 ocean bottom seismometers between March 27, 2021 and January 7, 2022. First, we manually picked P and S phase onsets, the polarity of first motion of the P-wave, and the maximum amplitude of the vertical component for 83 events listed in the JMA catalog. Second, we picked the P to S converted phase onsets and calculated initial station corrections to correct the large travel time delay due to soft and thick sediment at the seafloor. The calculation of hypocenter location was performed by hypoMH (Hirata and Matsu'ura, 1987). We set the Vp/Vs ratio as 1.9 at all depths, which was obtained by making Wadati diagrams. A 1-D velocity model was constructed based on the 2-D P-wave velocity structure, which was obtained by a seismic survey (Arai et al., 2023). To refine the hypocenter determination, the station correction terms were iteratively modified until travel time residuals changed sufficiently small, and 81 events were finally obtained.
The RMS values of the travel time residuals for the P and S phases decreased from 0.311 s to 0.089 s and from 1.059 s to 0.342 s, respectively. Most of the earthquakes have small location errors of 0.5 km in the horizontal direction and 0.4 km in the depth direction, which shows accurate hypocenter distribution compared to the JMA catalog. Hypocenter locations of this study moved horizontally about 10 km to the northwest and became shallower than those of the JMA catalog. Fifteen reliable focal mechanisms obtained using HASH (Hardebeck and Shearer, 2002, 2003) showed high-angle normal faulting.
Comparing analysis results with the P-wave velocity structure of Arai et al. (2023) suggests that most of the earthquakes occurred within the Philippine Sea Plate. The spatio-temporal distribution indicates that the hypocenters migrated from about 2 km below the plate boundary to near the plate boundary. It is possible that intraplate earthquakes occur on the down-dip side of slow earthquakes in Hyuga-nada by the same mechanism as in the Hikurangi subduction zone, where many intraplate earthquakes occur (Warren-Smith et al., 2019; Nishikawa et al., 2021). In the future, we will conduct relative hypocenter determination and verify the results of the analysis including migration, as well as analyze shallow tremors and verify the mechanism of ordinary and slow earthquakes in Hyuga-nada.
Acknowledgements
This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its “The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research)” and “Research Project for Disaster Prevention on the great Earthquakes along the Nankai trough”. We thank the crews of the T/S Nagasakimaru (Faculty of Fisheries, Nagasaki University) and the R/V Kaiyomaru No. 3 (Kaiyo Engineering Co.) for their skillful work and these voyage participants from Kyoto University, Kyushu University, Kagoshima University, Tokyo University of Marine Science and Technology, and Tokyo University. Dr. R. Arai of the Japan Agency for Marine-Earth Science and Technology kindly provided the seismic survey line data of Hyuga-nada in 2020.