HR: 08:00h
AN: S31B-01 INVITED     [Abstracts]
TI: Rupture Dynamics Frozen in Exhumed Ancient Faults
AU: * Di Toro, G
EM: giulio.ditoro@unipd.it
AF: Universita' di Padova, Via Giotto 1, Padova, 35137 Italy
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: Universita' di Padova, Via Giotto 1, Padova, 35137 Italy
AB: Pseudotachylytes (solidified friction-induced melts) are the most certain fault rock indicator of seismicity on ancient faults. In this study, we show how the asymmetry in the distribution and the orientation of pseudotachylyte-bearing secondary fractures around exhumed faults can be used to reconstruct the earthquake rupture directivity, rupture velocity and fracture energy G, by comparison with the calculated asymmetrical dynamic stress field around a propagating fracture. The Gole Larghe Fault zone is an exhumed 30 Ma old dextral strike-slip fault exposed in glacier-polished outcrops in the Italian Southern Alps. The fault zone consists of an array of ca. 200 main East-West striking sub-parallel faults. In the Fault Zone, faults and secondary fractures have peculiar features in their shape, attitude and branching patterns. Most of the branching of secondary fractures consists either of tension cracks, up to several cm thick, laying perpendicular to the main fault line, or thin fractures, laying roughly at 30^o to the fault (consistently with the inferred orientation of the principal horizontal stress). Most interestingly, secondary fractures take off preferentially on one side of the fault, i.e., into the southern block. Recent theoretical studies investigated the possible patterns of fault branching that seismic rupture may leave in its trail (Poliakov et al., 2002; Rice, et al., 2004; Andrews 2005). In particular, the damage and fractures associated to an earthquake are expected to be asymmetrically distributed on either side of the fault, and to depend on the direction in which the rupture is propagating. In a specific dynamic rupture model reproducing the local stress and rock properties, we predict the asymmetrical creation of secondary fractures and tension cracks, based on the near-field, dynamic stress radiation. The observed distribution of pseudotachylyte-bearing fractures is compatible with unchanged propagation direction during repeated seismic ruptures, subsonic fracture velocities (close to Rayleigh wave velocity) and G values in the range 8.0x10⁶-6.7x10⁷ J m^-2.
DE: 7209 Earthquake dynamics (1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
DE: 8164 Stresses: crust and lithosphere
SC: Seismology [S]
MN: Fall Meeting 2005