HR: 14:40h
AN: S42G-05    [PDF]
TI: Effects of Crustal Densification in Warm Slabs on In-slab Earthquakes and Episodic Tremors and Slips
AU: * Wang, K
EM: wang@pgc.nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 W Saanich Rd, Sidney, BC, V8L 4B2 Canada
AU: * Wang, K
EM: wang@pgc.nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, V8W 3P6 Canada
AB: During subduction, dehydration may facilitate earthquake rupture in both the slab crust and slab mantle. The up to 15% rock densification that accompanies the metabasalt-eclogite transformation is expected to have several mechanical consequences. In warm slabs such as Cascadia and Nankai, this transformation and mantle serpentine breakdown begin at rather shallow depths (30 - 50 km). The pervasively hydrated upper crust transforms to eclogite under equilibrium conditions, but the transformation of the anhydrous parts of the lower crust is kinetically delayed to greater depths. Therefore, densification begins in a thin layer along the top of the slab. Volume reduction gives rise to an equivalent stretching force in the thin layer in all slab-parallel directions, activating existing faults and developing new fractures. Analogous to a weak layer sandwiched between, and bonded to, two strong layers under stretching, fracture spacing in the weak layer scales with the layer thickness. The theory predicts that the densified thin layer must be ~{!0~}shattered~{!1~}. The shattered upper crust may have numerous small earthquakes but does not favor large ruptures. In contrast, the much more uniform lower crust and mantle can host larger ruptures, although seismic ruptures occur only in the limited hydrated parts. This explains the observation that relatively few earthquakes deeper inside the slab tend to have larger magnitudes than those just below the slab surface. For example, three recent damaging events (1999 Oaxaca, Mexico; 2001 Geiyo, Nankai; 2001 Nisqually, Cascadia) in warm slabs all occurred in the lower crust or mantle. The densification is generally a steady state process: An increasingly thinner slab moves into an increasingly thinner subduction "slot" continuously, with the downdip width of transition from normal to thinned crust scaling linearly with the subduction rate. However, at the fracture scale, the process is highly nonlinear, and there must be small fluctuations due to mechanical and kinetic irregularities. Readjustments of high-fluid-pressure rocks in and above the shattered subducting crust in responses to such densification fluctuations may be responsible for episodic lower-frequency tremors recorded at the Cascadia and Nankai subduction zones. Transient modifications of slab-surface topography and roughness in the same process may trigger silent slips along the plate interface.
DE: 7209 Earthquake dynamics and mechanics
DE: 7230 Seismicity and seismotectonics
DE: 8010 Fractures and faults
DE: 8150 Plate boundary--general (3040)
DE: 8164 Stresses--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting