HR: 0800h
AN: DI51A-0282    [Abstracts]
TI: Spontaneous thermal runaway as an earthquake mechanism at elevated pressure: insights from petrological and numerical studies
AU: John, T
EM: timm.john@fys.uio.no
AF: Physics of Geological Processes, University of Oslo PO BOX 1048 Blindern, Oslo, 0316, Norway
AU: * Rupke, L H
EM: l.h.rupke@fys.uio.no
AF: Physics of Geological Processes, University of Oslo PO BOX 1048 Blindern, Oslo, 0316, Norway
AU: Medvedev, S
EM: sergei.medvedev@fys.uio.no
AF: Physics of Geological Processes, University of Oslo PO BOX 1048 Blindern, Oslo, 0316, Norway
AU: Podladchikov, Y
EM: iouri.podladtchikov@matnat.uio.no
AF: Physics of Geological Processes, University of Oslo PO BOX 1048 Blindern, Oslo, 0316, Norway
AU: Andersen, T B
EM: t.b.andersen@geo.uio.no
AF: Physics of Geological Processes, University of Oslo PO BOX 1048 Blindern, Oslo, 0316, Norway
AB: Convergent margins are characterized by strong seismic activity with earthquakes occurring at depths of up to 700km. Shallow earthquakes (<60km) are explainable by the brittle failure of rocks. At greater depths, the increased ambient pressure should inhibit brittle failure and two main hypotheses have been proposed to explain intermediate-depth (60-300km) seismicity: (1) dehydration embrittlement and (2) melt shear instabilities.
During dehydration embrittlement elevated fluid pore pressures counteract the lithostatic pressure and thereby lower the effective pressure to values at which tectonic stresses can lead to seismic failure. The fundamental unknown in this scenario is the pore pressure. Ductile shear instabilities are a different failure mode of rocks. Rapid deformation rates lead to frictional heating and melt lubrication resulting in self-accelerating deformation at seismogenic strain rates. The fundamental unknowns in this failure mode are the conditions at which a perturbation of the system will self-amplify instead of decaying and thereby lead to extreme localization of deformation and frictional heating. Pseudotachylytes are the only certain geological evidence for paleo- earthquakes and more and more eclogite-facies pseudotachylytes localities have now been discovered. They were found in rocks of the deeply exhumed continental roots and in exhumed fragments of subducted slabs. Their existence indicate that frictional melting is possible and may even be required for earthquakes under pressure-temperature conditions reasonable for depths >60 km.
We present the results of a joint petrological, analytical and numerical study of thermal runaway as an intermediate earthquake mechanism. Field evidence from the Krakenes Gabbro in Western Norway show coexisting narrow eclogite-facies shear zones and pseudotachylytes. Using an analytical and numerical model for thermal-runway (Braeck and Podladchikov, 2007), we explore under which conditions (e.g. differential stress, P--T, and rheology) these shear zones/pseudotachylytes may have formed and why they coexist along strike. Our modeling results demonstrate that thermal runaway mechanism is applicable for intermediate-depth and deep earthquakes in subduction zones. In fact, at depths greater than 60 km thermal-runaway occurs at lower differential stresses than brittle failure and involves realistic stress drops.

Braeck, S. and Podladchikov, Y. Y., 2007. Spontaneous Thermal Runaway as an Ultimate Failure Mechanism of Materials. Physical Review Letters. DOI: 10.1103/PhysRevLett.98.095504.
DE: 3225 Numerical approximations and analysis (4260)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 7209 Earthquake dynamics (1242)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting