Shear-wave Splitting Observed in the Geysers Geothermal Field to Monitor the Spatiotemporal Crustal Conditions
Description:
S waves propagate in an anisotropic medium separate orthogonally to each other, called shear wave splitting. Mineral properties or crust cracks are considered as the heterogeneous structure to split shear waves. The characteristics of the heterogeneous structure are estimated from the leading shear wave polarization direction (LSPD) and the travel time difference (DT). Some studies pointed out local heterogeneity, for example, faults and dense cracks, affect the direction of the LSPD. In a geothermal field, crack status is affected by the increase and decrease of water as a result of the power plant operation. If we could utilize shear wave splitting observed from microearthquakes in the geothermal fields, it would be useful for spatiotemporal reservoir monitoring.
The Geysers geothermal field locates in northern San Francisco. In that region, rich seismic network is provided by The Northern California Earthquake Data Center (NCEDC). I select five three component stations, LCK, FFA, ESM, ACR, and DRH. These stations have a small incident angle from the source and less impact from reflection and surface waves. LCK, FFA, ESM, ACR locate the eastern side of the swarm activity, and DRH locates the western side. The sampling frequency is 500 Hz. There are 30 earthquakes observed from 2020 to 2023, with magnitudes between 3.0 and 4.0, latitudes between 38.5 and 39.0, and longitudes between -123 and -122. DSPD and DT are estimated from the Cross-correlation method (e.g., Shin and Meyer, 1990), with a 1-second time window.
The stations LCK, FFA, and ESM show the predominant polarization direction for the northeast, while ACR does not show a strong polarization direction. The characteristics of the polarization direction are consistent with the shear wave splitting research in this region (Elkibbi and Rial, 2005). Though the LSPD estimates showed relatively large variation, there are no clear temporal changes for three years. DRH shows a similar predominant polarization direction while it is located in a distant area from the current active swarm area.
Session: Seismic Monitoring, Modelling and Management Needed for Geothermal Energy and Geologic Carbon Storage [Poster Session]
Type: Poster
Date: 5/2/2024
Presentation Time: 08:00 AM (local time)
Presenting Author: Nana
Student Presenter: No
Invited Presentation:
Authors
Nana Yoshimitsu Presenting Author Corresponding Author yoshimitsu.nana.6i@kyoto-u.ac.jp Kyoto University |
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Shear-wave Splitting Observed in the Geysers Geothermal Field to Monitor the Spatiotemporal Crustal Conditions
Session
Seismic Monitoring, Modelling and Management Needed for Geothermal Energy and Geologic Carbon Storage