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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS011 Poster presentation 2152 Earthquakes and the State of Stress in the Outer Rise Dr. Jascha Polet Institute for Crustal Studies University of California at Santa Barbara IASPEI Hong Kie Thio The outer rise represents a long wavelength upwarping of the oceanic plate just before it dives into the mantle. A temporal and spatial correlation may exist between interplate and intraplate (both in the outer rise as well as at greater depths within the subducting plate) earthquakes, with intraplate events possibly serving as stress gauges of the subduction earthquake cycle. To examine spatial and temporal patterns in intraplate seismicity, we have compiled a catalog of worldwide shallow intraplate earthquakes, using an automated search algorithm based on local subduction geometry and the focal mechanisms from the Global CMT catalog. With this catalog, we have 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, an observation that is difficult to explain using only a simple elastic bending model in combination with an in-slab compressional or extensional force. We also examined the orientation of the fault planes of the outer rise earthquakes with respect to the local subduction zone reference frame. To explain our results, we propose that a significant number of tensional outer rise earthquakes occur on pre-existing seafloor fabric and that compressional outer rise events are more likely to occur in response to local perturbations of the stress field caused by, for example, the subduction of bathymetric features. We will also show the results of a comparison of the mechanisms of the outer rise earthquakes with seafloor bathymetry and refraction data for specific regions. In addition to the analysis of the intraplate earthquake catalog, we also intend to present the results of an investigation of the January 13 Kuril Islands Mw=8.1 normal faulting outer rise earthquake, which occurred in close proximity to an Mw=8.3 interplate event two months earlier. We are primarily interested in determining the depth extent of its rupture process through the modeling of seismic waveforms and tsunami data. Keywords: subduction, earthquake, outer rise

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS011 Poster presentation 2153 Roles of lateral viscosity variations caused by stiff subducting slabs and weak plate margins on long-wavelength geoid anomaly Dr. Masaki Yoshida Institute for Research on Earth Evolution (IFREE) JAMSTEC IASPEI Tomoeki Nakakuki, Motoyuki Kido The observed geoid anomaly shows very broad highs over the subduction zones, especially over the circum-Pacific trench, when the longest-wavelength components (the spherical harmonic degree are 2 and 3) are subtracted (Hager, 1984). Our previous work (Yoshida, 2004) has shown that the longwavelength geoid anomaly is significantly affected by the lateral viscosity variations (LVVs) in the mantle, i.e., stiff (high viscous) subducting slabs and weak (low viscous) plate margins related to the plate-tectonic mechanism, by the use of the 2-D mantle convection model. In this study, we have examined possible effects of such LVVs on the long-wavelength geoid by using 3-D spherical shell models. In contrast with a traditional propagator matrix method by Hager, our new numerical approach can treat the mantle flow including LVVs. The finite volume method is used for the discretization of basic equations governing the instantaneous mantle flow with spatially variable viscosity. To construct more actual global density models compared with our previous models (Yoshida et al., 2001; Yoshida, 2004), we have used a model coupled with (1) the global subducting slab model based on the seismicity in the upper mantle, and (2) the S-wave global tomography model (Becker and Boschi, 2002) in the lower mantle. The radial viscosity variation is layered; the lithosphere, the upper mantle, the transition zones, the lower mantle, and the bottom boundary layer. The low viscous asthenosphere is also considered. The reference viscosity is fixed at 1021 Pa s in the upper mantle. The viscosity contrast between the lithosphere and the mantle is taken to be 104.5, which is the actual effective viscosity of the lithosphere. The viscosity of the plate margins is determined by using the global strain-rate model (Kreemer et al., 2003). First we have calculated the geoid anomaly by using a no-LVV model, in which the stiff subducting slabs and the weak plate margins are not considered. The result shows that geoid highs over the subduction zones arise only when the vertical viscosity contrast between the upper mantle and the lower mantle (RLM) is around 103. This value seems to be one order larger than the viscosity contrast suggested by the post-glacial rebound analysis (e.g., Peltier, 1998). We have next imposed the stiff subducting slabs only in the upper mantle, the viscosity of which is the same as that of the lithosphere, on the no-LVV model. The geoid anomaly shows regionally strong negative pattern over the subduction zones, especially, the Jawa and the South America trenches, even when RLM is significantly high, 104. This is because the surface deformations in such regions strongly depress due to the mechanically strong coupling between the lithosphere and stiff subducting slabs. When we have imposed weak plate margins on this model, the geoid pattern remains largely unchanged. Here we have systematically examined the effects of the viscosity contrast of subducting slabs in the range between 100 (i.e., no viscosity contrast) and 104.5 on patterns of the geoid anomaly. We have confirmed that when the viscosity contrast of the subducting slabs is around 101 to 102, the geoid anomaly over the subduction zones becomes positive pattern over such regions, if RLM is around 103. Imposing weak plate margins on this model reproduces the broadly positive anomaly which explains the observation. If RLM is lower than 102, the geoid anomaly over the subduction zones still remains broadly negative. These results indicate that the viscosity of subducting slabs is significantly weaker than that of lithosphere. When the low viscous layer under the transition zone, i.e., the second asthenosphere (Cserepes and Yuen, 2000) is included, the broadly positive anomalies over the subduction zones emerge when RLM is 101.5 to 102, which is within the range of the viscosity contrast derived from the

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