Exploring Thermal Shear Runaway as a triggering process for Intermediate-Depth Earthquakes: Overview of the Northern Chilean seismic nest.

Abstract:

The intermediate depth earthquakes of high moment magnitude (Mw ≥ 8) in Chile have had a relative greater impact in terms of damage, injuries and deaths, than thrust type ones with similar magnitude (e.g. 1939, 1950, 1965, 1997, 2003, and 2005). Some of them have been studied in details, showing paucity of aftershocks, down-dip tensional focal mechanisms, high stress-drop and subhorizontal rupture. At present, their physical mechanism remains unclear because ambient temperatures and pressures are expected to lead to ductile, rather than brittle deformation. We examine source characteristics of more than 100 intraslab intermediate depth earthquakes using local and regional waveforms data obtained from broadband and accelerometers stations of IPOC network in northern Chile. With this high quality database, we estimated the total radiated energy from the energy flux carried by P and S waves integrating this flux in time and space, and evaluated their seismic moment directly from both spectral amplitude and near-field waveform inversion methods. We estimated the three parameters E_a, τ_a and M₀ because their estimates entail no model dependence. Interestingly, the seismic nest studied using near-field re-location and only data from stations close to the source (D<250km) appears to not be homogeneous in terms of depths, displaying unusual seismic gaps along the Wadati-Benioff zone. Moreover, as confirmed by other studies of intermediate-depth earthquakes in subduction zones, very high stress drop ( >> 10MPa) and low radiation efficiency in this seismic nest were found. These unusual seismic parameter values can be interpreted as the expression of the loose of a big quantity of the emitted energy by heating processes during the rupture. Although it remains difficult to conclude about the processes of seismic nucleation, we present here results that seem to support a thermal weakening behavior of the fault zones and the existence of thermal stress processes like thermal shear runaway as a preferred mechanism for intermediate earthquake triggering. Despite the non-exhaustive aspect of this study, data presented here lead to the necessity of new systematic near-field studies to obtain valuable conclusions and constrain more accurately the physics of rupture mechanisms of these intermediate-depth seismic event.