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Focal Mechanisms of Microseismicity at the Decatur, Illinois, CCS Site Inverted From Multiple Borehole Seismic Arrays

Description: 

Underground fluid injection can trigger earthquakes by activating hidden fault systems favorable to diffusive pore pressures with local stress fields. Tracking down microearthquakes during passive seismic monitoring in fluid injection sites enables imaging fault systems and estimating their seismic productivity. Along with the hypocentral distribution of microseismicity in the fluid injection field, focal mechanisms of earthquakes directly provide the geometries of fault systems and instabilities with respect to the regional stress field. While borehole sensors near injection wells effectively detect microseismicity by suppressing ambient noise near the surface, the co-location of sensors at different levels limits the analysis of the radiation patterns from different faulting types. In this study, we examine fault mechanisms of microseismicity at the Decatur CO₂ injection site by using recordings at two borehole seismic arrays. The first P-motions of two borehole arrays still do not suffice to determine the focal mechanisms. The coherence measurement of the S/P amplitude ratios at sensors within individual arrays further constrains the uncertainty of focal mechanisms. This contradicts the unstable S/P amplitude measurements associated with site-specific calibrations at surface sensors. We validate the solutions obtained from first motion polarity and S/P amplitude ratios by comparing solutions of a few events for which surface and borehole sensors observe the first P-wave motions. The obtained focal mechanism solutions generally match the distribution of earthquakes and the local maximum stress direction. The subtle variation of focal mechanisms in each earthquake cluster represents either the heterogeneity of the fault system or the local-scale perturbation of the stress field. Our observation highlights the importance of installing multiple borehole arrays to investigate focal mechanisms of small earthquakes to estimate small-scale fault systems.

Session: Seismic Monitoring, Modelling and Management Needed for Geothermal Energy and Geologic Carbon Storage - I

Type: Oral

Date: 5/2/2024

Presentation Time: 08:45 AM (local time)

Presenting Author: Jeong-Ung

Student Presenter: No

Invited Presentation: 

Authors