HR: 1340h
AN: T33B-0544    [Abstracts]
TI: The faulting process of outer rise earthquakes
AU: * Polet, J
EM: polet@crustal.ucsb.edu
AF: Institute for Crustal Studies, UCSB, University of California, Santa Barbara 1140 Girvetz Hall, Santa Barbara, Ca 93106 United States
AB: Updip from the interface between subducting and overriding plate, the outer rise comprises an upwarping of the oceanic lithosphere just before it descends into the trench. Previous work on outer rise seismicity suggested that correlations may exist between interplate and intraplate seismicity, both spatially as well as temporally, with intraplate earthquakes possibly serving as stress gauges for the large-scale deformation involved in subduction zones. We have compiled a catalog of worldwide outer rise seismicity, using an automated search algorithm based on local subduction and source mechanism geometry on the Harvard CMT catalog, and confirmed the preferential occurrence of normal faulting outer rise events after large interplate thrust events, in particular after tsunami earthquakes. Compressional outer rise events also appear more frequently after interface events, contradicting earlier studies that suggested these events occurred preferentially precursory to interface events. An inelastic analysis of lithospheric stress distributions predicts similar seismic behavior as indicated by our catalog. Using this new outer rise seismicity catalog, we also investigated the spatial occurrence of outer rise events and found a correlation of heightened outer rise earthquake activity with stronger plate curvature, greater plate age and a larger dip of the subducting plate. We further examined the orientation of the fault planes of the outer rise source mechanisms with respect to the local trench reference frame. Two observations were made for the tensional outer rise events: 1. the poles of the fault planes dipping away from the trench are more tightly clustered (possibly indicating that these planes more frequently are the 'true' fault plane) 2. an asymmetry exists in the fault plane geometry, with the dip of the trenchward dipping planes generally greater (on average 15 degrees larger) than that of the planes dipping away from the trench We cannot preclude that this second observation is due to the possibility that many tensional outer rise events occur where the subducting plate already has a dip of 7 degrees. However, a subset of only those tensional outer rise events at a distance of at least 50 km from the trench shows the same result. Errors in earthquake location would have to consistently move these mechanisms about 25 km away from the trench to explain the fault plane asymmetry. The results of a similar analysis for the compressional outer rise events show a much more scattered pattern of fault plane poles. Since compressional outer rise earthquakes are usually located at greater depths than their tensional counterparts, and thus occur where frictional strength is higher, we would expect that their fault are more likely to be newly created according to prevailing stress conditions due to the bending of the subducting plate. Tensional events on the other hand would be more likely to occur on pre-existing weak zones and thus display a stronger scatter of fault plane poles in the trench reference frame. However, our results do not meet this expectation. To explain the more scattered nature of the poles of the compressional outer rise events in the trench reference frame, we propose that these earthquakes may occur more frequently in response to small-scale perturbations of the stress field through, for example, the subduction of bathymetric features. As a next step, we plan to investigate the source characteristics of the large (Mw>6) outer rise earthquakes in more detail. We will determine the radiated energy and examine the energy budget of these events to shed more light on the fracture process at work.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8123 Dynamics: seismotectonics
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005