Nonvolcanic tremor locations and mechanisms in Guerrero, Mexico, from energy-based and particle motion polarization analysis

Abstract

We introduce the Tremor Energy and Polarization (TREP) method, which jointly determines the source location and focal mechanism of sustained nonvolcanic tremor (NVT) signals. The method minimizes a compound cost function by means of a grid search over a three‐dimensional hypocentral lattice. Inverted metrics are derived from three NVT observables: (1) the energy spatial distribution, (2) the energy spatial derivatives, and (3) the azimuthal direction of the particle motion polarization ellipsoid. To assess the tremor sources, TREP assumes double‐couple point dislocations with frequency‐dependent quality factors (Q) in a layered medium. Performance and resolution of the method is thoroughly assessed via synthetic inversion tests with random noise, where the "observed" data correspond to an NVT‐like finite difference (FD) model we introduce. The FD tremor source is composed of hundreds of quasi‐dynamic penny‐shaped cracks governed by a time‐weakening friction law. In agreement with previous works, epicentral locations of 26 NVTs in Guerrero are separated in two main groups, one between 200 and 230 km from the trench, and another at about 170 km. However, unlike earlier investigations, most NVT hypocenters concentrate at 43 km depth near the plate interface and have subparallel rake angles to the Cocos plate convergence direction. These locations have uncertainties of ~5 km in the three components and are consistent with independent results for low‐frequency earthquakes in the region, supporting their common origin related to slip transients in the plate interface. Our results also suggest the occurrence of NVT sources within the slab, ~5 km below the interface.

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