Intraslab Stresses in the Cascadia Subduction Zone from Inversion of Earthquake Focal Mechanisms

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At the Cascadia subduction zone, intraslab earthquakes occur mostly in the northern part of the margin and its southern end, the Mendocino triple junction (MTJ). We determine intraslab stress orientations by inverting earthquake focal me- chanisms and develop working hypotheses to explain the inferred intraslab stresses and observed seismicity. Our inversion results show that the subducting Juan de Fuca (JDF) slab in northern Cascadia is primarily under compression normal to the slab surface and tension in the downdip direction, most likely controlled by the net slab pull. An exception is a northernmost shallow region near the Nootka fault zone where the state of stress is dominated by nearly east-west tension. We hypothesize that the shear force on the Nootka fault zone and margin-parallel mantle resistance to slab motion induce the east-west tension in this region. Near the MTJ, stresses in the JDF plate are dominated by north-south compression down to about 20 km depth, consistent with a strong push by the Pacific plate from south of the Mendocino trans- form fault, but the deeper part of the slab shows downdip-tension, similar to northern Cascadia. Deviatoric stresses in the JDF slab appear to be very low, resulting in very low intraslab seismicity. In comparison with northern Cascadia, the stresses in most of southern Cascadia are even lower, resulting in nearly no intraslab seismicity.