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S-wave reflector and crustal fluid deep beneath seismically active area in the north Ibaraki area, northeastern Japan

Takahiro Shiina, Yuta Amezawa, Takahiko Uchide, Haruo Horikawa, & Kazutoshi Imanishi

Submitted September 10, 2023, SCEC Contribution #13120, 2023 SCEC Annual Meeting Poster #026

Crustal fluid is discussed as a key factor facilitating the activity of crustal earthquakes. The distribution of the crustal fluid is investigated based on a seismic velocity structure deduced from tomographic analysis and/or an electric resistivity structure because the presence of the crustal fluid is implied in these structures. However, the imaging of the structures, especially a deeper part of the crust, is often blurred due to some regularization imposed on tomographic analyses. S-wave reflected waves are sensitive to the existence of a sharp impedance contrast, suggesting that an analysis of them could identify a boundary between a fluid-rich part and a fluid-non-rich one as the impedance contrast or the reflector. Hence the reflector also constraints the distribution of the crustal fluid, and we can get some insights on the relationship between the crustal fluid and seismicity. Following this idea, we infer the location of the S-wave reflector deep beneath the seismically active area in the north Ibaraki area, northeastern Japan, where both velocity and resistivity structures have been already retrieved. Firstly, we manually pick the arrival times of the S-wave reflected waves from seismic records of the temporal array deployed by the Geological Survey of Japan, AIST, and the nearby permanent stations. We then determine the location and geometry of the crustal reflector by a method based on the Markov chain Monte Carlo approach. Our results reveal that a reflector is located at depths of 15-25 km (lower crust) beneath the seismicity and shallowly dips (~26°) to the northwest. We find that this reflector is located above the region with low seismic wave velocity and low electrical resistivity, which indicates the volumetric distribution of fluid at the lower crust. Additionally, the location of the reflector corresponds to the bottoms of the seismogenic zones. These features possibly suggested that the fluid rich in the lower crust weakens the material and affects the bottoms of the seismogenic zones in this area.

Acknowledgment: We used Hi-net data from NIED and the JMA Unified Earthquake Catalog. This study was supported by MEXT Project for Seismology toward Research Innovation with Data of Earthquake (STAR-E) [grant number JPJ010217], and partially supported by the JST CREST [grant number JPMJCR1763].

Key Words
Crustal fluid, Later phase, Reflector

Shiina, T., Amezawa, Y., Uchide, T., Horikawa, H., & Imanishi, K. (2023, 09). S-wave reflector and crustal fluid deep beneath seismically active area in the north Ibaraki area, northeastern Japan. Poster Presentation at 2023 SCEC Annual Meeting.

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