GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 Estmate of seismic fracture surface energy from pseudotachylyte-bearing faults S. Aldrighet 1 , G. Di Toro 1,2 , G. Pennacchioni 1 1 Dipartmento di Geoscienze, Università degli Studi di Padova, Padua, Italy 2 Sezione di Roma 1, Isttuto Nazionale di Geofsica e Vulcanologia, Rome, Italy Earthquakes are the result of propagaton (at km s -1 ) of a rupture and associated slip (at m s -1 ) along a fault (Scholz, 2019). The total energy involved in a seismic event is unknown, but qualitatvely most of the energy is dissipated by rock fracturing and frictonal heat. Seismic fracture energy G (J m -2 ) is the energy dissipated in the rupture propagaton and can be estmated by the inversion of seismic waves (Abercrombie and Rice, 2005; Tint et al., 2005; Cocco et al., 2023). However, the physical signifcance of G remains elusive. G may include the contributons of both rock fracturing (energy to form new rock surfaces U S , J m -2 ) and fault frictonal heatng ( Q , J m -2 ). Here we determine both U S and Q in natural and experimental pseudotachylyte-bearing faults following the approach used by Pitarello et al. (2008). Pseudotachylytes are solidifed frictonal melts produced during seismic slip. In these rocks: (i) U S is proportonal to the surface of the new fragments produced in both the slip zone and in the wall rocks; and (ii) Q is proportonal to the volume of frictonal melt. The selected natural pseudotachylytes are from the East-West-striking, dextral, strike-slip Gole Larghe Fault Zone (Adamello, Italy; Di Toro and Pennacchioni, 2004). To estmate U S we used Electron Back-Scater Electrons (EBSD), High Resoluton Mid Angle Back-Scatered Electrons (HRMABSD) and Cathodoluminescence-Field Emission-Scanning Electron Microscopy (CL-FESEM). In partcular, CL-FESEM imaging reveals a microfracture network in the wall rocks that cannot be detected with other techniques used (Fig. 1). In the pseudotachylyte-bearing fault, the microstructural analysis reveals (i) a high degree of fragmentaton of the wall rock adjacent to the pseudotachylyte fault vein (formed along the slip surface), with clast size down to < 90 nm in diameter, and (ii) a systematc diference in fracture density and orientaton of the microfractures on the two opposite wall rock sides of the fault (Fig. 1). In fact, in the northern wall rock the fracture density is low and the microfractures are oriented preferentally East-West, while in the southern wall rock the fracture density is high and oriented preferentally North-South (Fig. 1). This asymmetric distributon of microfracture damage is consistent with the stress perturbaton at the crack tp associated with the propagaton of the seismic rupture (Di Toro et al., 2005). This suggests that the microfractures developed during coseismic slip and, therefore, can be used to estmate U S .