A recent deep earthquake doublet in light of long-term evolution of Nazca subduction

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.

North Korea's 2017 Test and its Nontectonic Aftershock

“…Seismic waveforms are inverted for full MT using software ISOLA (Sokos et al, ; Zahradník et al, ). Full MT is calculated by least‐squares fitting of band‐pass filtered displacement waveforms, accompanied by grid‐searching of the centroid depth and time.…”

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“…Those types of events, a pair of similarly sized earthquakes which occur very close in time and location, are often referred to as "doublets" (Hayes et al, 2017). Both normal faulting events nucleated within the the subducted Nazca Plate on the same fault (Ruiz et al, 2017;Ye et al, 2016;Zahradník et al, 2017). Ruiz et al (2017) show in their study, using teleseismic and regional data, that the overall resemblance between the two events (similarity in geometry, rupture velocity, stress drop and radiated energy) suggest that they share a similar brittle rupture process and nucleated under very similar conditions.…”

A New High-Pressure Phase Transition in Natural Gedrite

“…[48] suggested that water vapor released from dehydration of hydrous minerals, such as amphiboles, would reduce friction and make brittle fractures possible at greater depths, a process known as dehydration embrittlement. Dehydration embrittlement has been proposed to be an essential mechanism for faulting processes for intermediate and possibly deep earthquakes [42,47,[49][50][51][52][53][54]. With the metastable preservation of amphiboles, dehydration reactions would be expected to occur at higher pressures than previously assumed, and may lead to dehydration embrittlement at greater depths.…”