Abstract
Large compressional-faulting earthquakes located relatively deep in oceanic lithosphere entering subduction zones are primarily caused by plate bending stress, but their timing, depth extent and size can be influenced by temporally-varying shear stress on the plate boundary. The 25 March 2020 MW 7.5 event in the Pacific plate seaward of Paramushir Island (northern Kuril Islands), is among the largest recorded events of this type. Its rupture extends along a large-slip region in the southwestern portion of the 1952 Kamchatka MW 9.0 rupture zone. This region has somewhat lower interplate coupling than the megathrust fault along Kamchatka to the northeast, but there could be 68 yrs of strain accumulation. The 2020 event is considered in the context of the 24 recorded major (MW≥7.0) near-trench intraplate compressional-faulting events. An updated compilation of temporally varying near-trench intraslab faulting relative to major interplate ruptures indicates that the stress cycles on the plate boundary influence both extensional and compressional near-trench faulting caused by plate bending. Particularly noteworthy are such events seaward of areas presumed to be in an advanced stage of their seismic cycle, including relatively shallow compressional events along the 1944 MW 8.1 Tonankai, Japan rupture zone, along with activity along the 1952 Kamchatka and 1922 Chile rupture zones.
Highlights
Oceanic plates flex as they converge in subduction zones, with their elastic/brittle near-surface volume experiencing shallow extensional faulting and less common deeper compressional faulting from the outer rise to beneath the overriding plate (e.g., Stauder, 1968; Chapple and Forsyth, 1979; Christensen and Ruff, 1983, 1988; Lay et al, 1989; Seno and Yamanaka, 1996; Craig et al, 2014)
The oceanic lithospheric age/thermal state and plate bending curvature that results from slab pull, in-plate normal stress and plate boundary shear stresses influence the depth extent of the seismogenic material and position of the neutral elastic bending stress surface within the flexing lithosphere, respectively
Slab pull and in-plane normal stress load on the oceanic lithosphere from the overriding plate are primarily responsible for the slab bending, but time-varying interplate shear stress arising from stick-slip of the plate boundary is thought to influence the stresses in
Summary
Oceanic plates flex as they converge in subduction zones, with their elastic/brittle near-surface volume experiencing shallow extensional faulting and less common deeper compressional faulting from the outer rise to beneath the overriding plate (e.g., Stauder, 1968; Chapple and Forsyth, 1979; Christensen and Ruff, 1983, 1988; Lay et al, 1989; Seno and Yamanaka, 1996; Craig et al, 2014). Major compressional events within the shallow slab are rare and likely require broadening of the region under compression by shallowing of the neutral surface as a result of build up of plate boundary shear stresses or transient loading by extensive extensional faulting of the shallower elastic lithosphere. For regions that experience large megathrust earthquakes, the interplate shear stress cycle can temporally modulate the faulting in the oceanic lithosphere This is indicated by near-trench extensional faulting mainly occurring after large interplate thrust events and infrequent large near-trench compressional faulting being more common before large plate boundary events (e.g., Christensen and Ruff, 1983, 1988; Lay et al, 1989). We note several regions, including the 1952 Kamchatka zone, for which outer rise compressional activity may indicate accumulating plate boundary stress associated with forthcoming major interplate thrust-faulting
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