Frictional power dissipation in a seismic ancient fault

The frictional power per unit area Q (product of frictional traction t and slip rate u in MW m-2) dissipated during earthquakes triggers fault dynamic weakening mechanisms that control rupture nucleation, propagation and arrest. Although of great relevance in earthquake mechanics, Q cannot, with rare exceptions, be determined by geophysical methods. Here we exploit theoretical, experimental and geological constraints to estimate Q dissipated on a fault patch exhumed from 7-9 km depth. According to theoretical models, in polymineralic, silicate rocks the amplitude (< 1 mm) of the grain -scale roughness of the boundary between frictional melt (pseudotachylyte) and host rock decreases with increasing Q. The dependence of grain-scale roughness with Q is due to differential melt front migration in the host rock minerals. This dependence is confirmed by friction experiments reproducing seismic slip where pseudotachylytes were produced by shearing tonalite at Q ranging from 5 to 25 MW m-2. In natural pseudotachylytes across tonalites, the grain-scale roughness broadly decreases from extensional to compressional fault domains where lower and higher Q are expected, respectively. Analysis of the natural dataset calibrated by experiments yields Q values in the range of 4-60 MW m-2 (16 MW m-2 average value). These values, estimated in small fault patches, are at the lower end of broad estimates of Q (3-300 MW m-2) obtained from frictional tractions (30-300 MPa) and fault slip rates (0.1-1 m/s) assumed as typical of upper crustal earthquakes.(c) 2023 Elsevier B.V. All rights reserved.