Brittle-ductile transition depth versus convergence rate in shallow crustal thrust faults: Considerations on seismogenic volume and impact on seismicity

Abstract

Earthquakes occur in the Earth’s crust where rocks are brittle, with magnitude increasing with the volume involved in the coseismic stage. Largest volumes are expected in convergent tectonic settings since thrust fault may be even more than 25 times larger than hypocenter depth. In general, the maximum depth of hypocenters within the crust corresponds to the brittle-ductile transition (BDT). The deepening of the BDT increases the potential seismic volume, hence raising the energy released during an earthquake. Here, by means of 2-D thermo-mechanical modelling dedicated to intraplate thrusts and thrusts within fold-and-thrust belts (shallow crust), the deepening of the BDT depth in convergent settings with variable convergence rates is investigated. Results of models characterized by shallow faults (15 ° –20 ° dip) show that BDT depth deepens by 15 km in- creasing the convergence rate from 1 to 10 cm/yr. Steeper thrust faults (25 ° –40 ° dip) show a lower degree of deepening of the BDT ( 5 km) as convergence rate is increased. Calculated BDT depths allow the calculation of maximum seismic volumes involved during thrust earthquakes. Deeper BDT depths obtained assuming higher convergence rates imply larger seismic volumes and an increase of 2 orders of magnitude of the stored potential energy, as effectively observed in nature.