HR: 13:55h
AN: T13E-02 INVITED     [Abstracts]
TI: Natural and Experimental Evidence of Melt Lubrication of Faults During Earthquakes
AU: * Di Toro, G
EM: giulio.ditoro@unipd.it
AF: Dipartimento di Geologia, Paleontologia e Geofisica, Via Giotto 1, Padova, 35137 Italy
AU: Hirose, T
EM: hirose@kueps.kyoto-u.ac.jp
AF: Department of Geology and Mineralogy, Kyoto University, Kyoto, 606-8502 Japan
AU: Nielsen, S
EM: snielsen@na.infn.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Roma, 00143 Italy
AU: Pennacchioni, G
EM: giorgio.pennacchioni@unipd.it
AF: Dipartimento di Geologia, Paleontologia e Geofisica, Via Giotto 1, Padova, 35137 Italy
AU: Shimamoto, T
EM: shima@ kueps.kyoto-u.ac.jp
AF: Department of Geology and Mineralogy, Kyoto University, Kyoto, 606-8502 Japan
AB: How large is the dynamic frictional resistance (τ_f) of faults during earthquakes? Interpretation of seismological data suggest a variety of coseismic processes that can be explained by low τ_f and several fault dynamic weakening mechanisms have been proposed. Among these, melt lubrication, as solidified friction melts (pseudotachylytes) decorate some exhumed faults. But a silica-rich melt might act as a coseismic fault sealant instead, impeding rupture propagation. Experiments that reproduce in-situ conditions of the earthquake may yield a definitive answer to the dispute. Here we obtain the first coherent estimates of low τ_f in the presence of granitic melt, from both exhumed faults and high-speed rock friction experiments. For the field estimate of τ_f, we assume that all frictional work on the fault surface (average τ_f times slip) was converted to heat, in proportion to the volume of pseudotachylyte observed. A reliable estimate of τ_f can only be performed on exceptional fault exposures, because coseismic fault slip and pseudotachylyte volume ought to be well constrained. Exhumed pseudotachylyte-bearing faults from glacier-polished outcrops in the Adamello Massif (Italian Southern Alps) indicate a τ_f of ~12.9-41.8 MPa at 10 km in depth. High-speed (1.4 m/s)rotary-shear friction experiments performed at normal stress between 3.9-15.6 MPa on rock samples stem from the same fault indicate weak, linear dependence of τ_f on normal stress (effective friction coefficient ~0.06-0.2) in the presence of melt. Texture of artificially-produced and natural pseudotachylytes is very similar and experimental τ_f values extrapolated to a depth of 10 km lay on the same strength curve as the field estimates, well below the Byerlee frictional strength. We conclude that, at intermediate crustal depths, low fault strength and melt-lubrication can indeed occur during seismic faulting.
DE: 7209 Earthquake dynamics (1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: Fall Meeting 2005