Publication Date:
2015-10-24
Description:
In the northwestern Pacific, the Pacific plate subducts to the west at the Japan, Kuril, and Kamchatka trenches. Throughout most of the subduction zone, the subducting slab is planar and dipping at an angle of 30 ∘ –60 ∘ , with the exception of a fold in the southern Kuril segment. To investigate how the slab deforms in response to the applied forces and which mechanism generates the earthquakes, we analyze the rupture properties of 111 large ( M W ≥5.7) intermediate-depth and deep-focus earthquakes (60–656 km depth) from 1990–2014 in the Japan-Kuril-Kamchatka subduction zone. For each earthquake, we use rupture directivity to estimate rupture direction and rupture speed and to distinguish the fault plane from the auxiliary plane of the focal mechanism. 76% of the earthquakes with sufficient station coverage are well-modeled by unilateral rupture propagation. The estimated rupture speeds range from zero to supershear. The estimated rupture directions allow identification of the fault plane as the more horizontal nodal plane for 30 earthquakes, while an additional 11 earthquakes rupture towards the intersection of the nodal planes, so the fault plane cannot be identified. Combining our newly-identified fault planes with previously-identified fault planes in the region, we observe that, in planar slab segments, most earthquakes slip along a dominant fault orientation. For a steeply-dipping slab, this orientation is subhorizontal. In more sharply bent slab segments, such as the Kuril fold, deformation is accomodated along more variable fault orientations, including subvertical faults. The correlation of slab geometry with fault orientation suggests that the local stress field controls fault orientations.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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