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[SSS09-08] Possible cause of seismic intensity of 5 upper observed only at Shirosato-machi Ogachi station during the 2022 MJMA4.9 Southern Ibaraki earthquake
Keywords:Cause of large seismic intensity, Site amplification, High-frequency seismic wave, Filter for seismic intensity calculation, Southern Ibaraki earthquake
On November 9th, 2022, an MJMA4.9 earthquake occurred in the southern Ibaraki at a depth of 51 km. The largest seismic intensity of 5 upper was observed only at Shirosato-machi Ogachi seismic intensity meter maintained by Ibaraki prefecture, which is 36 km away to the northeast of the epicenter. The second largest seismic intensity was 4, two scale smaller than the largest. After installation of the Shirosato-machi Ogachi station in 2018, there were three MJMA5.0 earthquakes in 2018, 2020, and 2021 in the southern Ibaraki with a similar focal depth to the 2022 event. Unlike the 2022 event, the seismic intensities recorded at Shirosato-machi Ogachi station due to these three earthquakes were not significantly larger than the other stations. In this study, we have examined a reason why the largest seismic intensity of 5 upper was observed only at Shirosato-machi Ogachi station during the 2022 event.
Both horizontal components of acceleration waveform at Shirosato-machi Ogachi exceed 500 Gal and have monotonic wave trains which have a peak at 7-8 Hz in the Fourier amplitude spectra. Even after applying the filter for seismic intensity calculation, which emphasizes the lower frequency content around 0.5 Hz, amplitudes of the filtered waveforms still remain 100-200 Gal since the 7-8 Hz components have very large amplitude.
Microtremor survey using very small array was conducted besides the seismic intensity meter just after the 2022 event. A subsurface structure model estimated from the inversion of the microtremor array data has a 5-m surface layer with VS < 200 m/s on a VS = 400 m/s layer. Transfer function from the top of VS = 400 m/s layer to the ground has a peak at 8 Hz, which is consistent with the peak frequency of the observed horizontal waveforms during the 2022 event. Therefore, the large amplification due to shallow subsurface structure around the seismic intensity meter is one reason for the large amplitude monotonic ground motion rich in 7-8 Hz components resulting in the largest seismic intensity.
Since the Shirosato-machi Ogachi station did not experience particularly large seismic intensity during the previous three MJMA5.0 earthquakes, the subsurface structure is not the only reason for the large seismic intensity in the 2022 event. Spatial distributions of the acceleration responses at periods of 0.1 s and 0.15 s using K-NET and KiK-net data show that the stations located in the region from the central to the northern Ibaraki and the central Tochigi have a larger amplitude for the 2022 event than the other three earthquakes. For periods 0.5 s and 1.0 s, such larger amplitudes for the region were not observed for the 2022 event. Therefore, it is concluded that the incident seismic waves were rich in the high-frequency content (6-10 Hz) and the 7-8 Hz components were amplified by the shallow subsurface layer just below the Shirosato-machi Ogachi station largely enough to produce the seismic intensity of 5 upper for the 2022 event.
Acknowledgements: We used strong motion waveform data at Shirosato-machi Ogachi station provided by Ibaraki prefecture via SK-net website.
Both horizontal components of acceleration waveform at Shirosato-machi Ogachi exceed 500 Gal and have monotonic wave trains which have a peak at 7-8 Hz in the Fourier amplitude spectra. Even after applying the filter for seismic intensity calculation, which emphasizes the lower frequency content around 0.5 Hz, amplitudes of the filtered waveforms still remain 100-200 Gal since the 7-8 Hz components have very large amplitude.
Microtremor survey using very small array was conducted besides the seismic intensity meter just after the 2022 event. A subsurface structure model estimated from the inversion of the microtremor array data has a 5-m surface layer with VS < 200 m/s on a VS = 400 m/s layer. Transfer function from the top of VS = 400 m/s layer to the ground has a peak at 8 Hz, which is consistent with the peak frequency of the observed horizontal waveforms during the 2022 event. Therefore, the large amplification due to shallow subsurface structure around the seismic intensity meter is one reason for the large amplitude monotonic ground motion rich in 7-8 Hz components resulting in the largest seismic intensity.
Since the Shirosato-machi Ogachi station did not experience particularly large seismic intensity during the previous three MJMA5.0 earthquakes, the subsurface structure is not the only reason for the large seismic intensity in the 2022 event. Spatial distributions of the acceleration responses at periods of 0.1 s and 0.15 s using K-NET and KiK-net data show that the stations located in the region from the central to the northern Ibaraki and the central Tochigi have a larger amplitude for the 2022 event than the other three earthquakes. For periods 0.5 s and 1.0 s, such larger amplitudes for the region were not observed for the 2022 event. Therefore, it is concluded that the incident seismic waves were rich in the high-frequency content (6-10 Hz) and the 7-8 Hz components were amplified by the shallow subsurface layer just below the Shirosato-machi Ogachi station largely enough to produce the seismic intensity of 5 upper for the 2022 event.
Acknowledgements: We used strong motion waveform data at Shirosato-machi Ogachi station provided by Ibaraki prefecture via SK-net website.