Japan Geoscience Union Meeting 2021

Presentation information

[J] Poster

S (Solid Earth Sciences ) » S-SS Seismology

[S-SS08] Fault Rheology and Earthquake Physics

Fri. Jun 4, 2021 5:15 PM - 6:30 PM Ch.14

convener:Shunya Kaneki(Disaster Prevention Research Institute, Kyoto University), Makiko Ohtani(Earthquake Research Institute, the University of Tokyo), Keishi Okazaki(Japan Agency for Marine-Earth Science and Technology), Keisuke Yoshida(Tohoku University)

5:15 PM - 6:30 PM

[SSS08-P16] Frictional properties on the subducting Pacific Plate off the coast of Ibaraki and Chiba prefectures in Japan

*Tsukasa Masuda1, Takuji Yamada2 (1.Ibaraki University, 2.Faculty of Science, Graduate School of Science and Technology, Ibaraki University)

Keywords:Frictional properties, Pacific Plate, Stress drop

The Pacific Plate is subducting beneath the Okhotsk Plate at east off the coast of Ibaraki and Chiba prefectures in Japan, which causes the occurrence of many small earthquakes constantly. We analyzed the values of stress drop of 576 earthquakes (4.2≦Mw≦5.0) that took place there on the plate interface from 2003 to 2019 and investigated the distribution of the stress drop on the Pacific Plate.
Following the method of Yamada et al. (2021), we first selected earthquakes with Mw3.5 that were the closest to individual analyzed earthquakes as the empirical Greenʼs functions (EGFs). We then divide waveforms of analyzed earthquakes by individual EGF waveforms in the frequency domain and retrieve the spectral ratio of source effects for each earthquake pair. Next, we assumed that the earthquake spectra follow an omega-square model (Boatwright, 1978) and analyzed corner frequencies of earthquakes. Finally, the values of stress drop were estimated from the corner frequencies by a circular fault model of Madariaga (1976). As this procedure provides values of stress drop for individual stations and components, we estimated values of stress drop by calculating values of their log average of individual stations and components.
We found earthquakes in the source area of the 2011 Tohoku earthquake had higher values of stress drop. This is consistent with previous studies suggesting that the spatial pattern of stress drop provides the relative distribution of shear strength in space (Yamada et al., 2017; 2021). In addition, a temporal decrease in stress drop was observed in a couple of areas adjacent to ones with a large slip during the 2011 Tohoku earthquake, suggesting a decrease in shear strength. Possible causes include effects of aftershock activity and the viscoelastic response of the mantle.

-- Reference --
Boatwright (1978), Bull Seismol Soc Am 68:1117-1131.
Iinuma T, Hino R, Kido M, Inazu D, Osada Y, Ito Y, Hozono M, Tsushima H, Suzuki S, Fujimoto H, Miura S (2012), J Geophys Res 117:B7. https ://doi.org/10.1029/2012JB009186
Madariaga (1976), Bull Seismol Soc Am 66:639-666.
Yamada T, Duan M, and Kawahara J (2021), Earth Planets Space 73:18. https://doi.org/10.1186/s40623-020-01326-8
Yamada T, Saito Y, Tanioka Y, Kawahara J (2017), Prog Earth Planet Sci 4:38. https://doi.org/10.1186/s40645-017-0152-7