Microseismicity Appears to Outline Highly Coupled Regions on the Central Chile Megathrust

We compiled a novel microseismicity catalog for the Central Chile megathrust (29°-35°S), comprising 8,750 earthquakes between April 2014 and December 2018. These events describe a pattern of three trenchward open half-ellipses, consisting of a continuous, coast-parallel seismicity band at 30-45 km depth, and narrow elongated seismicity clusters that protrude to the shallow megathrust and separate largely aseismic regions along strike. To test whether these shapes could outline highly coupled regions ("asperities") on the megathrust, we invert GPS displacement data for interplate locking. The best-fit locking model does not show good correspondence to seismicity, possibly due to lacking resolution. When we prescribe high locking inside the half-ellipses, however, we obtain models with similar data fits that are preferred according to the Bayesian Information Criterion (BIC). We thus propose that seismicity on the Central Chile megathrust may outline three adjacent highly coupled regions, two of them located between the rupture areas of the 2010 Maule and the 2015 Illapel earthquakes, a segment of the Chilean margin that may be in a late interseismic stage of the seismic cycle.

Weakening Mechanisms in a Basalt-Hosted Subduction Megathrust Fault Segment, Southern Alaska

Basaltic and gabbroic rocks that define the seafloor have been suggested to act as sources of rheological heterogeneity during subduction, with the capacity to enhance or dampen seismicity. Despite this, relatively little is known from the rock record regarding the progression and conditions of mafic oceanic crust deformation during subduction, particularly in the shallow megathrust region of the seismogenic zone. We describe subduction-related deformation structures and characterize deformation conditions from an exhumed, basalt-hosted megathrust in the Chugach accretionary complex of south-central Alaska. Rocks in the Chugach preserve a record of seafloor mineralogical changes from pre-subduction, hydrothermal circulation that produced sheet silicates with a lower frictional strength than intact basalt. Pre-subduction alteration also served to introduce hydrous phases that can expel water during deformation and raise the pore fluid pressure. Once strain localized within basalts onto a megathrust fault plane at lithostatic pore fluid pressures, the basalt weakened further through a combination of cataclasis, dilatational shear fracturing, and slip on chlorite-rich shear bands. This process occurred in a narrower fault zone, and at higher maximum differential stress and greater pore fluid pressure fluctuations than recorded in some sediment-hosted megathrusts at similar pressure and temperature conditions. Our data indicate that when the lower plate contains basalt bathymetric features, basalt dismembers during subduction into a chlorite-rich fault gouge that surrounds lenses or slices of intact, less-altered basalt.