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Constraining the Non-Double-Couple Components of Local Events Recorded by Dense Nodal Array

Description: 

Traditional seismic source assumptions (i.e., double-couple model) may not fully describe small-moderate magnitude events with non-negligible explosive or non-linear features (i.e., non-double-couple components, non-DC). However, in local distances, constraining these non-DC components is challenged by complex 3D velocity structures that restrain the successful simulation of seismograms. Alternatively, using polarities and amplitudes to resolve full moment tensors presents significant potential with the recent boost of the dense nodal array. This study introduces an algorithm combining different far-field observations (i.e., direct P-wave polarities, amplitudes, and S/P amplitude ratios) to characterize and visualize the non-DC components of small earthquakes. The optimal moment tensor and associated uncertainties are determined through a grid search over 6-D moment tensor space, and then projected onto lune plot for virtualization (Tape & Tape 2012). The efficiency and accuracy are tested with both synthetic data and field observations near a hydraulic-fracturing well (ToC2ME) in Western Canada. The reliability of our metric is quantified via the angular difference ω (i.e., range from 0 to 180) between two moment tensors. The range of possible solutions and mismatches increases from ω=1.6 at noise free to ω=15 when 10-times field noises are added. In addition, our metric shows strong resistance to structure heterogeneity under such dense array coverage: the optimal solutions can still be resolved with perturbations on station-based layered-velocity models as high as 40%. Our inversions for 167 M>1 events within ToC2ME agree with Zhang et al., 2019, with 93% of events displaying ω<20. Furthermore, when compared with the known hydraulic-fracturing operations, our reassessed non-DC components exhibit more interpretable physical processes. Our proposed moment tensor inversion algorithm for dense arrays could serve for many other anthropogenetic involved local-scale settings, especially where uncertainty evaluation and virtualization for non-DC couple components are needed.

Session: Induced Earthquakes: Source Characteristics, Mechanisms, Stress Field Modeling and Hazards [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Ruijia

Student Presenter: No

Invited Presentation: 

Authors