← Back to Sessions

FocMecDR: A Cross-Correlation-Based Double-Ratio Earthquake Focal Mechanism Inversion Algorithm

Description: 

Focal mechanisms of earthquakes play significant roles in earthquake seismology. Waveform-based methods work well for relatively large earthquakes (e.g., M>3) but may fail for small ones. As a result, first-motion polarities and P/S amplitude ratios have been widely adopted to invert focal mechanisms for small events. Whereas the identification of polarities is challenging, for example, phases with low signal-to-noise ratios and slow-starting arrivals. On the other hand, P/S amplitude ratios usually vary by a factor of 2 or more, which requires empirical station correction for individual stations. Recent work shows that polarities and P/S amplitude ratios can be inverted for a cluster of earthquakes using cross-correlation and matrix inversion (e.g., Shelly et al., 2016; Shelly et al., 2022), providing inputs for the traditional focal mechanism inversion software (e.g., HASH). Though this method significantly improves the accuracy of the focal mechanism solutions and the number of solvable events, but it still has the drawbacks of traditional methods, for instance, station corrections are required.

In this work, I propose a cross-correlated-based double-ratio earthquake focal mechanism inversion algorithm to automatically invert relative focal mechanisms for clustered earthquakes. Like the concept of double-difference in earthquake location, here I use an earthquake with known focal mechanism as a reference and invert focal mechanisms for the nearby target events through minimizing the difference between the ratio of P/S ratio for the observed data and theoretical values, which can be computed given a pair of focal mechanisms, as well as matching their relative polarities, which can be inferred from the normalized CC coefficient (from -1 to 1) with a cutoff threshold. The assumption is that the Green’s functions for the clustered events are the same (slight arrival differences can be handled by time shift via CC) and the differences of P/S ratios are attributed to the focal mechanism change. Field benchmark in the Ridgecrest region shows promising results for accurate relative focal mechanism inversion.

Session: Earthquake Source Parameters: Theory, Observations and Interpretations [Poster]

Type: Poster

Date: 4/18/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Miao Zhang

Student Presenter: No

Invited Presentation: 

Authors