5:15 PM - 6:30 PM
[SCG39-P06] Shallow tremor activity in the Hyuga-nada revealed by the seafloor observation of “Science of Slow earthquakes”
Keywords:Slow earthquake, Shallow low-frequency tremor, Ocean bottom seismological observaion
In the Hyuga-nada region, the western part of Nankai Trough, after the first observation of shallow tremors in 2013, ocean bottom seismological observation in the shallow tremor focal region has been conducted continuously since 2014. In March 2017, the observation using ocean bottom seismometers and pressure gauges started as part of "Science of Slow Earthquakes". The instruments of the first observation were recovered in January-February 2019, and generally good records were obtained. During the first observation period from December 2017 to January 2018, shallow slow earthquake activity increased from off the east of Tanegashima to Hyuga-nada, and shallow tremors and VLFEs activity were recorded in our seismic network. The second observation started in August 2018, and the recovery was conducted in September 2020. During the second observation period, a MJ 6.3 earthquake (31.81°N, 131.93°E, 19km) occurred on May 10, 2019, and we succeeded in capturing the slow earthquake activity that may have been triggered by this earthquake. For these activities, we determined the location of the shallow tremors using almost the same method as Yamashita et al. [2015]. The depth of the hypocenter was fixed to the plate boundary model by Nakanishi et al. [2018], and the S-wave velocity structure was assumed to be homogeneous with 3.5 km/s. The source of the shallow tremor was determined by the ocean bottom seismological network including another project, which was conducting observations in the off east of Tanegashima Island at the same time.
As a result of the analysis, we found that the epicentral distribution of shallow tremors in the first observation is almost the same as that of the 2013 activity area up to the point where the epicentral distribution bends to the east. The distribution of the tremors coincides with the first migration episode seen in the 2013 activity. The clear migration was also confirmed, and the migration from south to north and the endpoint of the migration were almost the same as the first migration episode in the 2013 activity. The area where the migration stopped may correspond to the area where the Kyushu-Palau Ridge is subducting.
As for the second observation, the activity started a few hours after the Mj 6.3 earthquake near the northern end of the first observation activity, and the eastward and southward migration was confirmed. In the eastward migration, although the number of detectable stations was limited and the source location has not yet been determined in many events, the waveform record revealed that the activity continued until around May 25. Migration to the south was intermittent but slow, a few kilometers per day, and propagated to about 30.5°N, off the east coast of Tanegashima, over a month and a half. The end of the migration was almost the same as that of the first observation, suggesting that the southern end of the shallow tremor from Hyuga-nada to the south is around 30.5°N. Also, the direction of migration and the pattern of shallow tremor activity seem to change around 31°N. This area is considered to be a subduction area of seamounts belonging to the Kyushu-Palau Ridge; the northward migration of shallow tremors is blocked by the NW-SW oriented subducting seamounts, and the migration direction is changed to the northwest. A similar pattern was observed in the first observation and 2013 activity, suggesting that the active area of shallow tremors is strongly influenced by the subduction of the Kyushu-Palau Ridge.
Acknowledgments: This study is supported by the JSPS KAKENHI Grant Number JP16H06471, JP17K01328, and the research project for compound disaster mitigation on the great earthquakes and tsunami around the Nankai trough region, the MEXT of Japan.
As a result of the analysis, we found that the epicentral distribution of shallow tremors in the first observation is almost the same as that of the 2013 activity area up to the point where the epicentral distribution bends to the east. The distribution of the tremors coincides with the first migration episode seen in the 2013 activity. The clear migration was also confirmed, and the migration from south to north and the endpoint of the migration were almost the same as the first migration episode in the 2013 activity. The area where the migration stopped may correspond to the area where the Kyushu-Palau Ridge is subducting.
As for the second observation, the activity started a few hours after the Mj 6.3 earthquake near the northern end of the first observation activity, and the eastward and southward migration was confirmed. In the eastward migration, although the number of detectable stations was limited and the source location has not yet been determined in many events, the waveform record revealed that the activity continued until around May 25. Migration to the south was intermittent but slow, a few kilometers per day, and propagated to about 30.5°N, off the east coast of Tanegashima, over a month and a half. The end of the migration was almost the same as that of the first observation, suggesting that the southern end of the shallow tremor from Hyuga-nada to the south is around 30.5°N. Also, the direction of migration and the pattern of shallow tremor activity seem to change around 31°N. This area is considered to be a subduction area of seamounts belonging to the Kyushu-Palau Ridge; the northward migration of shallow tremors is blocked by the NW-SW oriented subducting seamounts, and the migration direction is changed to the northwest. A similar pattern was observed in the first observation and 2013 activity, suggesting that the active area of shallow tremors is strongly influenced by the subduction of the Kyushu-Palau Ridge.
Acknowledgments: This study is supported by the JSPS KAKENHI Grant Number JP16H06471, JP17K01328, and the research project for compound disaster mitigation on the great earthquakes and tsunami around the Nankai trough region, the MEXT of Japan.