Slab Thinning Controls the Distribution of Large Deep Intraslab
Earthquakes in the Western Pacific Subduction Zones
Abstract
The nature of deep earthquakes with depths greater than 70 km is
enigmatic because brittle failure at this high-temperature and the
high-pressure regime should be inhibited. Three main hypotheses have
been proposed to explain what causes deep earthquakes within the
subducting slabs, dehydration embrittlement, phase transformational
faulting, and thermal runaway instability. However, the existing
seismological constraints can’t yet definitively distinguish between
these hypotheses because the fine 3D slab structures are not well
constrained in terms of slab upper interface, thickness, and internal
fine layering. To better image the slabs in the Western Pacific
subduction zones, this study employs a full waveform inversion (FWI)
that minimizes waveform shape misfit between the synthetics and the
observed waveforms from a large dataset, with 142 earthquakes recorded
by about 2,400 broadband stations in East Asia. A 3-D initial model that
combines two previous FWI models in East Asia (i.e., FWEA18 and
EARA2014) are iteratively updated by minimizing the misfit measured from
both body waves (8–40 s) and surface waves (30–120 s). Compared to the
previous models, the new FWI model (EARA2020) shows much stronger wave
speed perturbations within the imaged slabs with respect to the ambient
mantle, with maximum perturbation of 8% for Vp and 13% for Vs.
Furthermore, the slab thickness derived from EARA2020 exhibits
significant downdip and along-strike variations at depths greater than
100 km. The large intra-slab deep earthquakes (Mw>6.0)
appear to occur where significant slab thinning happens. This
observation suggests that the significant deformation (or strain
accumulation) of the slab is likely the first-order factor that controls
the distribution of large deep earthquakes within the slab regardless of
their triggering mechanism.