Pseudotachylytes and earthquake source mechanics

Destructive earthquakes nucleate at depth (10 to 15 km), therefore monitoring active faults at the Earth’s surface, or interpreting seismic waves, yields only limited information on earthquake mechanics. Tectonic pseudotachylytes (solidified friction-induced melts) decorate some exhumed ancient faults and remain, up to now, the only fault rocks recognized as the unambiguous signature of seismic slip. It follows that pseudotachylyte- bearing fault networks might retain a wealth of information on seismic faulting and earthquake mechanics. In this chapter, we will show that in the case of large exposures of pseudotachylyte-bearing faults, as the glacier-polished outcrops in the Adamello massif (Southern Alps, Italy), we might constrain several earthquake source parameters by linking field studies with microstructural observations, high-velocity rock friction experiments, modeling of the shear heating and melt flow, and dynamic rupture models. In particular, it is possible to estimate the rupture directivity and the fault dynamic shear resistance. We conclude that the structural analysis of exhumed pseudotachylyte-bearing faults is a powerful tool for the reconstruction of the earthquake source mechanics, complementary to seismological investigations.