Abstract
Earthquake faulting at ~600 km depth remains puzzling. Here we present a new kinematic interpretation of two Mw7.6 earthquakes of November 24, 2015. In contrast to teleseismic analysis of this doublet, we use regional seismic data providing robust two-point source models, further validated by regional back-projection and rupture-stop analysis. The doublet represents segmented rupture of a ∼30-year gap in a narrow, deep fault zone, fully consistent with the stress field derived from neighbouring 1976–2015 earthquakes. Seismic observations are interpreted using a geodynamic model of regional subduction, incorporating realistic rheology and major phase transitions, yielding a model slab that is nearly vertical in the deep-earthquake zone but stagnant below 660 km, consistent with tomographic imaging. Geodynamically modelled stresses match the seismically inferred stress field, where the steeply down-dip orientation of compressive stress axes at ∼600 km arises from combined viscous and buoyant forces resisting slab penetration into the lower mantle and deformation associated with slab buckling and stagnation. Observed fault-rupture geometry, demonstrated likelihood of seismic triggering, and high model temperatures in young subducted lithosphere, together favour nanometric crystallisation (and associated grain-boundary sliding) attending high-pressure dehydration as a likely seismogenic mechanism, unless a segment of much older lithosphere is present at depth.
Highlights
Earthquake faulting at ~600 km depth remains puzzling
20 deep-focus events of Mw > 6 (GCMT7,8 catalogue 1976–2015) form a narrow belt delineating a deep fault zone, ∼400 km long and striking ∼160° (Fig. 1a), with depths confined to a 30-km narrow range (590–620 km). Why such well-expressed, spatially limited seismicity, and how does it relate to ongoing Nazca plate subduction beneath South America at 7–8 cm/year9–13? The so-called thermal parameter[14] of ∼1300–2000 km ranks this subduction as ‘warm’, but the slab geometry[15] is only weakly constrained by seismicity, because the depths 200–600 km are aseismic
We derive a stress field that is successfully explained by a dynamical model of ∼100-My slab evolution consistent with tomography
Summary
Earthquake faulting at ~600 km depth remains puzzling. Here we present a new kinematic interpretation of two Mw7.6 earthquakes of November 24, 2015. Observed fault-rupture geometry, demonstrated likelihood of seismic triggering, and high model temperatures in young subducted lithosphere, together favour nanometric crystallisation (and associated grain-boundary sliding) attending high-pressure dehydration as a likely seismogenic mechanism, unless a segment of much older lithosphere is present at depth. 20 deep-focus events of Mw > 6 (GCMT7,8 catalogue 1976–2015) form a narrow belt delineating a deep fault zone, ∼400 km long and striking ∼160° (Fig. 1a), with depths confined (in 70% of cases) to a 30-km narrow range (590–620 km). Why such well-expressed, spatially limited seismicity, and how does it relate to ongoing Nazca plate subduction beneath South America at 7–8 cm/year9–13? 2015, clearly aligned with the aforementioned 160°-striking fault zone, such paucity of aftershocks being rather typical for warm slabs[14]
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