# THMC instabilities and the Multiphysics of Earthquakes

## TL;DR

This paper explores how coupling Thermo-Hydro-Mechano-Chemical processes can lead to volumetric instabilities that potentially trigger earthquake shear instabilities through resonance phenomena in a reaction-diffusion framework.

## Contribution

It introduces a novel approach analyzing THMC coupling and cross-diffusivities as a mechanism for earthquake precursor instabilities.

## Key findings

- Cross-diffusivities induce slow pressure waves.
- Resonance in long-wavelength limit triggers seismic events.
- Volumetric instabilities may precede shear instabilities.

## Abstract

Theoretical approaches to earthquake instabilities propose shear dominated instabilities as a source mechanism. Here we take a fresh look at the role of possible volumetric instabilities preceding a shear instability. We investigate the phenomena that may prepare earthquake instabilities using coupling of Thermo-Hydro-Mechano-Chemical (THMC) reaction-diffusion equations in a THMC diffusion matrix. We show that the off-diagonal cross-diffusivities can give rise to cross-diffusion slow (no inertia) pressure waves and propose that a resonance phenomenon of the long-wavelength limit causes constructive interference and triggers the seismic moment release.