5:15 PM - 6:45 PM
[SSS08-P13] Frequency characteristics of the earthquake beneath eastern margin of the Sea of Japan: Investigation via corrected frequency index analysis
Keywords:Seismicity, Low frequency earthquake, Frequency Index
Recent comprehensive studies have revealed that low-frequency earthquakes in inland areas (inland LFE) occur not only around volcanoes as previously thought but also in broader regions including tectonic boundaries and aftershock areas of large earthquakes. On the other hand, seismic activity along the eastern margin of the Japan Sea has been observed along strain concentration zones, with numerous aftershocks occurring, such as the 1993 southwest-off Hokkaido earthquake (M7.8) and 1983 Sea of Japan earthquake (M7.7). However, previous studies paid little attention to the frequency characteristics of earthquakes in these regions.
In this study, we investigated the spatial distribution of frequency characteristics of earthquakes occurring along the eastern margin of the Japan Sea by refining the Frequency Index (FI) used for detecting volcanic low-frequency earthquakes. We used earthquakes occurred between April 2004 and December 2022, occurring along the eastern margin of the Japan Sea, and seismic stations in Honshu island on the Japan Sea side within 200 km from the epicenter. The FI is originally defined as the logarithm of the ratio of amplitude spectra between high and low frequency bands. However, due to the utilization of a greater number of earthquakes and stations compared to previous studies, the distance dependency and magnitude dependency of the FI were significant in our case. To reduce the effect of these dependencies, a corrected FI (cFI) was newly introduced. The cFI was calculated as the difference between the observed (uncorrected) FI and the theoretical FI calculated based on the source spectrum expected from circular crack model, geometrical and intrinsic attenuations under the assumption of the homogeneous media. Three necessary parameters, Q, S-wave velocity, and stress drop, are selected to as 700, 3500 m/s and 10 MPa, respectively, as a combination that best explains the observed FI values. We note that there is a strong trade-off among these parameters, and they cannot be determined uniquely. We calculated the averaged cFI for a certain earthquake by averaging cFIs at stations satisfying the condition of analysis.
The results show distinct regional variation in the cFI. Particularly in the four regions – west off Hokkaido, around the Oga Peninsula, off Niigata, and around the Noto Peninsula – cFI were significantly lower than other regions, suggesting low-frequency seismic wave radiation compared to their magnitudes. Note that inland LFEs were found to exit in inland areas surrounding all four regions. However, except for the Oga Peninsula region, the depth of inland LFEs and offshore earthquake with lower cFI are different, indicating no spatial continuity in the occurrence of LFEs. In contrast, around the Oga Peninsula, distribution of inland LFEs and low-cFI earthquakes are continuous. Besides, the Noto Peninsular region has experienced earthquake swarms since around 2018, and the geofluids has been suggested to contribute to their generation mechanism. We also examined the occurrence of the low-frequency earthquakes by cFI values. The earthquakes with the cFI more than three standard deviations smaller than the mean were regarded as the LFE. We found that the LFEs occurred during the 2007 Noto Peninsula earthquake and around January 2021, just before the activation of the earthquake swarm in this area. The LFEs may be associated with fluid movement which triggered the swarm.
In this study, the quantitative examination of the frequency characteristics of the earthquake over wider range of epicentral distance and a broader magnitude range by introducing the cFI. Further study to applying the cFI to various earthquakes including inland earthquakes and those in volcanic area, it is expected to provide more detailed frequency characteristics of earthquakes.
In this study, we investigated the spatial distribution of frequency characteristics of earthquakes occurring along the eastern margin of the Japan Sea by refining the Frequency Index (FI) used for detecting volcanic low-frequency earthquakes. We used earthquakes occurred between April 2004 and December 2022, occurring along the eastern margin of the Japan Sea, and seismic stations in Honshu island on the Japan Sea side within 200 km from the epicenter. The FI is originally defined as the logarithm of the ratio of amplitude spectra between high and low frequency bands. However, due to the utilization of a greater number of earthquakes and stations compared to previous studies, the distance dependency and magnitude dependency of the FI were significant in our case. To reduce the effect of these dependencies, a corrected FI (cFI) was newly introduced. The cFI was calculated as the difference between the observed (uncorrected) FI and the theoretical FI calculated based on the source spectrum expected from circular crack model, geometrical and intrinsic attenuations under the assumption of the homogeneous media. Three necessary parameters, Q, S-wave velocity, and stress drop, are selected to as 700, 3500 m/s and 10 MPa, respectively, as a combination that best explains the observed FI values. We note that there is a strong trade-off among these parameters, and they cannot be determined uniquely. We calculated the averaged cFI for a certain earthquake by averaging cFIs at stations satisfying the condition of analysis.
The results show distinct regional variation in the cFI. Particularly in the four regions – west off Hokkaido, around the Oga Peninsula, off Niigata, and around the Noto Peninsula – cFI were significantly lower than other regions, suggesting low-frequency seismic wave radiation compared to their magnitudes. Note that inland LFEs were found to exit in inland areas surrounding all four regions. However, except for the Oga Peninsula region, the depth of inland LFEs and offshore earthquake with lower cFI are different, indicating no spatial continuity in the occurrence of LFEs. In contrast, around the Oga Peninsula, distribution of inland LFEs and low-cFI earthquakes are continuous. Besides, the Noto Peninsular region has experienced earthquake swarms since around 2018, and the geofluids has been suggested to contribute to their generation mechanism. We also examined the occurrence of the low-frequency earthquakes by cFI values. The earthquakes with the cFI more than three standard deviations smaller than the mean were regarded as the LFE. We found that the LFEs occurred during the 2007 Noto Peninsula earthquake and around January 2021, just before the activation of the earthquake swarm in this area. The LFEs may be associated with fluid movement which triggered the swarm.
In this study, the quantitative examination of the frequency characteristics of the earthquake over wider range of epicentral distance and a broader magnitude range by introducing the cFI. Further study to applying the cFI to various earthquakes including inland earthquakes and those in volcanic area, it is expected to provide more detailed frequency characteristics of earthquakes.