Seismic energy partitioning inferred from pseudotachylyte-bearing faults (Gole Larghe Fault, Adamello batholith, Italy)

Abstract

Fracture energy EG (the energy used for expanding a rupture surface area) is the fraction of work during seismic faulting that is required for creation of (i) new surfaces in the slip zone, and (ii) damage zone in the wall rocks. Partitioning of the earthquake energy between EG and frictional heat EH, determines the features of the rupture propagation and the mechanical behavior of the fault. The cataclastic microstructures associated with pseudotachylyte (solidified clast-laden friction-induced melt produced during coseismic slip) veins might contain information about the partitioning. In this preliminary study we used microstructural observations on pseudotachylytes from the Gole Larghe Fault zone (Southern Alps, Italy) to determine both EH and EG.

The EH for unit fault surface area is estimated from pseudotachylyte vein thickness 2w. The energy required to produce friction melt is EH = [(1- f) H+ cP(Tm-Thr)]r 2w where f is the volume ratio of lithic clasts within the pseudotachylyte, H is the latent heat of fusion, cP is the specific heat at constant pressure, (Tm-Thr) is the difference between initial melt temperature and host rock temperature and r is the density.

The EG is estimated by multiplying the newly created grain surface per unit of fault area by the specific surface energy (J m-2). In fact the studied pseudotachylyte vein contains plagioclase clasts displaying a characteristic internal fragmentation not observed in the host rock. This indicates a direct association between newly created grain surfaces and the seismic rupture process via pseudotachylyte production.

It follows that pseudotachylytes might yield information on the energy partitioning between EG and EH.