HR: 1330h
AN: S42C-0184    [PDF]
TI: Field Evidences for Fault Surface Lubrication by Friction-Induced Melts During Coseismic Slip
AU: * Di Toro, G
EM: giulio.ditoro@unipd.it
AF: Universita' di Padova, Via Giotto 1, Padova, 35137 Italy
AU: Teza, G
EM: giteza@everyday.com
AF: SINTECO Robotica Automazioni, Villanova, Longarone, 32013 Italy
AB: Lubrication by friction-induced melts has been proposed as a mechanism for fault weakening during earthquakes. Field evidence for melt lubrication of the fault surfaces is lacking, however, although the andesitic/basaltic composition (i.e. low-viscosity melts) of the matrix of many tectonic pseudotachylytes (solidified friction-induced melts) suggests low dynamic shear strength during coseismic slip. The Gole Larghe Fault Zone is an exhumed seismic source crosscutting the Adamello tonalites (Italian Southern Alps) and is exposed in a glacially polished area. In the fault zone, displacement is partitioned in more than 100 subparallel faults. Fault rocks are an association of pseudotachylytes and cataclasites produced at 6-8 km depth and 250-300 $^{o}$C. Given the large extent of the outcrop and the large number of structural markers within the tonalites, it has been possible to reproduce the 2D profile (i.e. intersection of the fault surface with the outcrop surface) of 25 different pseudotachylyte-bearing faults and to relate the fault profile with the displacement accommodated by each fault. Two lines of field evidence suggest that shear strength is low during coseismic slip: 1) A maximum shear stress of only 20 MPa has been estimated from the amount of mechanical work converted to heat during coseismic slip to produce the measured volume of pseudotachylyte in single-jerk faults. Although a shear strength of 20 MPa is a minimum estimate since part of the displacement has been accommodated without production of pseudotachylytes, such strengths are very small considering the depth where seismic faulting has occurred. 2) The fractal dimension (measured with a box-counting method) of the 2D profile of pseudotachylyte-bearing faults increases from 1.0 to a constant value of 1.15 with increasing displacement. In experimentally generated friction melts, the fractal dimension of molten surfaces increases from 1.0 to 1.1 when, with increasing slip, a steady state value for the friction coefficient of 0.5 is achieved. The increase of fractal dimension with displacement from field studies could be evidence of steady state friction during coseismic slip in the presence of melt. Thus, field evidence suggests that melts might lubricate fault surfaces during coseismic slip.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 8045 Role of fluids
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting