Title: Mineral dehydrations at subduction zones conditions – experimental evidence of tectonic tremors and high Vp/Vs ratio

Abstract:
Since their discovery (Obara 2002), the mechanics driving the generation of tectonic tremors has been debated. In hot subduction zones, tectonic tremors generally occur in relation with zones where large Vp/Vs ratios have been observed, in such a way that the presence of fluids (and fluid diffusion) is often invoked as an important aspect of their generation mechanisms. Yet, and to this day, tremors have never reproduced in the laboratory at subduction zones P–T conditions, in such way that their origin remains amongst the most elusive seismic phenomena observed in subduction zones.
Here, we employ a new High Pressure – High Temperature (HPT) experimental device designed to replicate the conditions typical of tremor-generating environments. Utilizing mixtures of antigorite and olivine as an analogue for the water-rich lithologies prevalent in subduction zones, we conducted experiments under hydrostatic (no deformation) pressure conditions (from 1.5 to 3.0 GPa) and temperatures between 750 and 800 °C, i.e. following a P-T path typical of hot subduction zones. During these experiments, we monitor the microseismicity (Acoustic Emissions- AEs), and observe a clear transition between earthquake-like AEs (below 1GPa and 500°C) and microseismic signals that are reminiscent of natural tremors. These tremor-like AEs follow both the Gutenberg-Richter relationship and a linear scaling between moment and duration, while their frequency x size ratio scales with that of natural tremors. Importantly, these signals, triggered upon mineral dehydration, are also continuously observed in the absence of a fluid phase, which is evidence that these tremor-like seismic waves are emitted by the solid phase deforming viscously, rather than by the percolating fluid phase.
Using the same apparatus, we also performed experiments where the evolution of P-wave velocities was monitored during pure antigorite dehydration. In these experiments, no tremor-like signal were recorded, while dehydration was accompanied by an important decrease in Vp. The softening of elastic properties upon dehydration can be related to fracturing processes at grain scale generated by water release, which supports the idea that dehydration stress transfer may be a reasonable model for both tremor and intermediate depth earthquake triggering. Finally, combining our experimental data and thermodynamic databases, we compute the elastic properties of dehydrating mineral assemblages and predict that, at subduction zone conditions, mineral dehydration signature must indeed that of a low Vp, but a large Vp/Vs ratio, which is compatible with seismological observations performed on subduction zones around the world.