Relative Focal Mechanism Inversion Using Relative Polarities and S/P Double Ratios

TL;DR

FocMecDR is a new inversion method that uses relative polarities and S/P amplitude double ratios to determine small earthquake focal mechanisms more reliably, especially when traditional methods face challenges.

Contribution

The paper introduces FocMecDR, a novel relative focal-mechanism inversion technique that improves accuracy by combining relative polarities with S/P amplitude double ratios.

Findings

Synthetic tests confirm high accuracy of FocMecDR.

Field data from Ridgecrest earthquakes validate the method's reliability.

Application reveals a uniform stress field during earthquake nucleation.

Abstract

Focal mechanisms of small earthquakes are critical for characterizing faults and regional stress. P-wave polarities and S/P amplitude ratios (e.g., in HASH) are widely used to determine focal mechanisms for small earthquakes, but first-motion picking can be difficult for emergent onsets, and S/P ratios are often highly inconsistent because of imperfect velocity models and unknown site responses. Here, I present FocMecDR, a relative focal-mechanism inversion method that uses relative polarities and double ratios of S/P amplitudes. Analogous to double-difference earthquake location, FocMecDR uses a reference event with a known focal mechanism to solve focal mechanisms for nearby earthquakes by minimizing the misfit between observed and theoretical S/P amplitude double ratios while enforcing cross-correlation-based relative polarities. Extensive synthetic tests demonstrate the effectiveness and accuracy of the method. Field verification using an antisimilar earthquake pair and aftershocks from the northwestern end of the 2019 Ridgecrest rupture zone further confirms its reliability. Application to the foreshocks preceding the 2019 Ridgecrest mainshock reveals a highly uniform stress field during nucleation, suggesting a possible framework for evaluating fault-zone stress and rupture readiness before large earthquakes.