Resolving puzzles of the phase-transformation-based mechanism of the deep-focus earthquake

TL;DR

This paper introduces a novel model for deep-focus earthquakes that explains rapid phase transformations and seismic strain rates through a self-amplifying feedback mechanism involving plasticity, heating, and phase change in olivine.

Contribution

It presents an analytical 3D solution for coupled deformation, phase transformation, and heating, resolving key puzzles about earthquake initiation and metastable olivine behavior.

Findings

Predicts conditions for transformation-induced plasticity and self-blown-up TRIP-PT-heating.

Shows temperature in shear bands can exceed unstable stationary temperature, triggering earthquakes.

Highlights the importance of strain-induced phase transformation over pressure/stress-induced mechanisms.

Abstract

Deep-focus earthquakes that occur at 350-660 km, where pressures 12-23 GPa and temperature 1800-2000 K, are generally assumed to be caused by olivine-spinel phase transformation (PT). However, there are many existing puzzles: (a) What are the mechanisms for jump from geological 10^-17-10^-15 1/s to seismic 10-10^3 1/s strain rates? Is it possible without PT? (b) How does metastable olivine, which does not completely transform to spinel at high temperature and deeply in the region of stability of spinel for over the million years, suddenly transforms during seconds and generates seismic strain rates 10-10^3 1/s? (c) How to connect deviatorically dominated seismic signals with volume-change dominated PT strain during PT? Here we introduce a combination of several novel concepts that allow us to resolve the above puzzles quantitatively. We treat the PT in olivine like plastic strain-induced (instead of pressure/stress-induced) and find an analytical 3D solution for coupled deformation-PT-heating processes in a shear band. This solution predicts conditions for severe (singular) transformation-induced plasticity (TRIP) and self-blown-up TRIP-PT-heating process due to positive thermomechanochemical feedback between TRIP and strain-induced PT. In nature, this process leads to temperature in a band exceeding the unstable stationary temperature, above which the self-blown-up shear-heating process in the shear band occurs after finishing the PT. Without PT and TRIP, significant temperature and strain rate increase is impossible. Due to the much smaller band thickness in the laboratory, heating within the band does not occur, and plastic flow after the PT is very limited. Our findings change the main concepts in studying the initiation of the deep-focus earthquakes and PTs during plastic flow in geophysics in general.