9:45 AM - 10:00 AM
[SSS06-04] A simple way to distinguish rather shallow PAC-intraplate events from interplate ones around Japan
Keywords:judge source type with the intensity distribution, difference between interplate and intraplate events, the useful GMPE of intensities in Japan, Off Fukushima earthquake on Feb. 13, 2021, Ground Motion Prediction Equation
On Feb. 13, 2021, a shallow* MJMA7.3 (Mrapid was 7.1) event occurred off Fukushima prefecture. The hypocenter depth (rapid 60km-> 55km) was very close to the assumed PAC plate surface. Since S-net has been used for hypocenter determinations by JMA, its aftershock distribution within an hour after helped us to assume that the event was intra-slab event.
However, there is the much easier and faster way to judge the source type of such a shallow event in the east off Tohoku district by the intensity distributions in Japanese islands available within a few minutes. In Fig. 1, the expected intensity distributions by the useful GMPE (Matsu’ura et al, 2020) are shown of the same epicenter and M (37.7°N, 141.8°E, Mw7.1) events with three source types (crustal, interplate, and intraplate), respectively. Their GMPE is classified by the source type, and robust for the estimation errors of M and depth.
In Fig. 2, differences between expected intensities for some PAC-intraplate, PAC-interplate, and very shallow crustal earthquakes are compared on epicentral distance. In Fig. 3, the observed intensities of the 2021 Feb. 13 event observed at K-NET and KiK-net stations are shown with some expected intensities on hypocentral distances.
Solid marks show the adjusted intensities to exclude differences between sites on the abnormal intensity effect of PAC slab, and on the amplification factors by the surface soil. We can obtain a hypocenter, M and intensities within a few minutes after an event occur. Then the GMPE can expect intensities for various source types, and we can compare them with observed intensities. The source type can be judged, without F-net mechanism solution nor the aftershock distribution.
Acknowledgements
We used K-NET, KiK-net data by National Research Institute for Earth Science and Disaster Resilience. This study was done with funding from the Ministry of Education, Culture, Sports, Science and Technology, Japan for the Headquarters of Earthquake Research Promotion of the Japanese government.
Reference: Matsu’ura, R. S., H. Tanaka, M. Furumura, T. Takahama, andA. Noda (2020). A New Ground-Motion Prediction Equation of Japanese Instrumental Seismic Intensities Reflecting Source Type Characteristics in Japan, Bull. Seismol. Soc. Am. 110, 2661–2692, doi: 10.1785/0120180337
* In the depth classification by Utsu (1979), "very shallow" is a crustal event, "intermediate" is an intermediate intra-slab event deeper than 60km, and shallow indicates 60km or shallower interplate or intraplate in the eastern off-shore areas of Japan. He knew that it is hard to distinguish "Intra" from "Inter" for old earthquakes.
However, there is the much easier and faster way to judge the source type of such a shallow event in the east off Tohoku district by the intensity distributions in Japanese islands available within a few minutes. In Fig. 1, the expected intensity distributions by the useful GMPE (Matsu’ura et al, 2020) are shown of the same epicenter and M (37.7°N, 141.8°E, Mw7.1) events with three source types (crustal, interplate, and intraplate), respectively. Their GMPE is classified by the source type, and robust for the estimation errors of M and depth.
In Fig. 2, differences between expected intensities for some PAC-intraplate, PAC-interplate, and very shallow crustal earthquakes are compared on epicentral distance. In Fig. 3, the observed intensities of the 2021 Feb. 13 event observed at K-NET and KiK-net stations are shown with some expected intensities on hypocentral distances.
Solid marks show the adjusted intensities to exclude differences between sites on the abnormal intensity effect of PAC slab, and on the amplification factors by the surface soil. We can obtain a hypocenter, M and intensities within a few minutes after an event occur. Then the GMPE can expect intensities for various source types, and we can compare them with observed intensities. The source type can be judged, without F-net mechanism solution nor the aftershock distribution.
Acknowledgements
We used K-NET, KiK-net data by National Research Institute for Earth Science and Disaster Resilience. This study was done with funding from the Ministry of Education, Culture, Sports, Science and Technology, Japan for the Headquarters of Earthquake Research Promotion of the Japanese government.
Reference: Matsu’ura, R. S., H. Tanaka, M. Furumura, T. Takahama, andA. Noda (2020). A New Ground-Motion Prediction Equation of Japanese Instrumental Seismic Intensities Reflecting Source Type Characteristics in Japan, Bull. Seismol. Soc. Am. 110, 2661–2692, doi: 10.1785/0120180337
* In the depth classification by Utsu (1979), "very shallow" is a crustal event, "intermediate" is an intermediate intra-slab event deeper than 60km, and shallow indicates 60km or shallower interplate or intraplate in the eastern off-shore areas of Japan. He knew that it is hard to distinguish "Intra" from "Inter" for old earthquakes.