5:15 PM - 6:45 PM
[SCG40-P20] Seismic activity around plate boundary near westernmost Nankai trough revealed by ocean bottom seismometer observation
The Nankai trough region has numerous historically devastating earthquakes. However, a earthquake greater than magnitude 8 has not been recorded in the Hyuga-nada region covering the westernmost area of the Nankai trough. A lack of a large earthquake may be attributed to a coupling between the subducting Philippine Sea plate and a landward plate. It is widely known that low-frequency tremors and very low frequency earthquakes frequently occur in the region. Accurate distribution of normal earthquakes is one of important information for understanding the plate subduction in the study region. To gain insights into the plate subduction, several Long Term Ocean Bottom Seismometer (LTOBS) networks were deployed to observe seismic activities. This study focuses on the analyses of four datasets among the observations performed in 2015-2016, 2017-2018, 2022 and 2022-2023, respectively.
The first step for data processing is to extract three minutes of the waveform including events from collected raw data. Since the raw data were continuous time series waveforms, establishing a reference for event detection was essential. The Japan Meteorological Agency (JMA) provided an earthquake catalog based on terrestrial observations, which includes date and time information. We identified origin times from the JMA catalog, and set a reference three seconds prior to origin time for extracting the seismic waveforms. After cutting out the waveform, we manually picked up P-wave and S-wave arrivals, an arrival of SP wave converted from S-wave, polarity of the first arrival, and maximum amplitude. P- and S-wave arrivals and maximum amplitude were employed in a location program using absolute travel times and a 1-D velocity structure. During this process, station corrections were applied to compensate for structural heterogeneity beneath the LTOBSs, and initial station corrections were calculated using difference between S-wave arrival, SP wave arrival, and P-wave velocity from the velocity model. S-wave velocity in sediment layer is assumed. Subsequently, we relocated the events by a program which determines hypocenters using a double-difference technique, to enhance the location accuracy. Furthermore, data on first arrival polarity were inputted into a computer program to estimate focal mechanisms.
The located results of the datasets revealed that most of the events occurred within 15km to 25km in depths. It is interpreted that most of the earthquakes occurred within the Philippine Sea Plate. Although some earthquakes had thrust-fault type focal mechanisms, the dominant focal mechanisms were normal-fault and/or strike-slip types. The activity region of the earthquakes is inferred to be in tensile stress field.
After the JMA catalog and our relocated results were investigated, we found that some events concentrated in spatially small regions and occurred during limited time periods. The earthquake swarms occurred during December 18th to 23rd in 2017, January 20th to 18th in 2018 and on September 7th in 2022. The swarms were also positioned within the subducting plate. The events in the swarms had predominantly reverse and/or normal faulting focal solutions.
The first step for data processing is to extract three minutes of the waveform including events from collected raw data. Since the raw data were continuous time series waveforms, establishing a reference for event detection was essential. The Japan Meteorological Agency (JMA) provided an earthquake catalog based on terrestrial observations, which includes date and time information. We identified origin times from the JMA catalog, and set a reference three seconds prior to origin time for extracting the seismic waveforms. After cutting out the waveform, we manually picked up P-wave and S-wave arrivals, an arrival of SP wave converted from S-wave, polarity of the first arrival, and maximum amplitude. P- and S-wave arrivals and maximum amplitude were employed in a location program using absolute travel times and a 1-D velocity structure. During this process, station corrections were applied to compensate for structural heterogeneity beneath the LTOBSs, and initial station corrections were calculated using difference between S-wave arrival, SP wave arrival, and P-wave velocity from the velocity model. S-wave velocity in sediment layer is assumed. Subsequently, we relocated the events by a program which determines hypocenters using a double-difference technique, to enhance the location accuracy. Furthermore, data on first arrival polarity were inputted into a computer program to estimate focal mechanisms.
The located results of the datasets revealed that most of the events occurred within 15km to 25km in depths. It is interpreted that most of the earthquakes occurred within the Philippine Sea Plate. Although some earthquakes had thrust-fault type focal mechanisms, the dominant focal mechanisms were normal-fault and/or strike-slip types. The activity region of the earthquakes is inferred to be in tensile stress field.
After the JMA catalog and our relocated results were investigated, we found that some events concentrated in spatially small regions and occurred during limited time periods. The earthquake swarms occurred during December 18th to 23rd in 2017, January 20th to 18th in 2018 and on September 7th in 2022. The swarms were also positioned within the subducting plate. The events in the swarms had predominantly reverse and/or normal faulting focal solutions.