Deep Earthquake Mechanics, Slab Deformation, and Subduction Forces

Deep Earthquake Mechanics, Slab Deformation, and Subduction Forces Schematic cross-section showing fault orientations in subducting slabs.
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Deeper than ~50 km depth in the Earth, the confining pressure is so high that it should prevent earthquakes from occurring. However, subduction zone earthquakes occur down to ~700 km depth. Based of the stability of minerals in the subducting lithosphere, mechanisms such as dehydration embrittlement and transformational faulting have been proposed for generating the earthquakes. These mechanisms have different implications for the orientation of earthquake fault planes, so they can be tested by identifying fault planes. Applying a method developed by Warren and Silver [2006] to data from all available seismic networks, we analyze the directivity of 200 large deep earthquakes in the Tonga [Warren et al, 2007a], Middle America [Warren et al, 2008], and central South America [Warren et al, 2007b] subduction zones to identify the fault planes for ~1/4 of the earthquakes. Despite large differences in temperature and lithospheric thickness, we observe similar fault-plane orientations with depth in all three subduction zones. The dominant steep, trenchward-dipping faults of the outer rise may be reactivated down to 100 km depth, while subhorizontal faults also slip from 40-100 km depth. From 100-300 km depth, all identified faults are subhorizontal. While inconsistent with the reactivation of the dominant outerrise fault orientation, this orientation could be consistent with the reactivation of the seaward-dipping faults, and recent numerical experiments [Faccenda et al, 2009] suggest that the less prominent faults may be preferentially reactivated because of the stress field in the bending slab. The similarity in the onset depth of horizontal faulting in the three subduction zones suggests that it is controlled by pressure rather than temperature or other tectonic parameters. In Tonga, which has a double seismic zone with opposite stress orientations in each plane of seismicity, deformation along these subhorizontal faults causes the slab to thin, indicating that slab pull is the primary force controlling slab seismicity at intermediate depths. While seismicity in our Middle and South America study areas stops by 250 km depth, Tonga earthquakes occur down to nearly 700 km depth. The fault-plane orientations of earthquakes >300 km depth match the patterns expected for the creation of a new system of faults: we observe both subhorizontal and subvertical fault planes consistent with a down-dip-compressional stress field. Slip along the two fault orientations causes the slab to thicken as it subducts and encounters resistance to lower mantle penetration. This resistance also results in an increase in the frequency of subhorizontal fault planes >600 km depth.
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References
</p><p>Warren, L.M., A.N. Hughes, and P.G. Silver (2007a), Earthquake mechanics and deformation in the Tonga-Kermadec subduction zone from fault-plane orientations of intermediate- and deep-focus earthquakes, J. Geophys. Res., 112.
</p><p>Warren, L.M., M.A. Langstaff, and P.G. Silver (2008), Fault-plane orientations of intermediate-depth earthquakes in the Middle America trench, J. Geophys. Res., 113, B01304.
</p><p>Warren, L.M., and P.G. Silver (2006), Measurement of differential rupture durations as constraints on the source finiteness of deep-focus earthquakes, J. Geophys. Res., 111.
</p><p>Acknowledgements: This work was supported by the National Science Foundation through grant EAR-0733170 and through Independent Research and Development time while the author worked at the Foundation, and by the Carnegie Institution of Washington through a Harry Oscar Wood Postdoctoral Fellowship.</p>

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