Soft barrier to megathrust rupture enabled by serpentinized mantle wedge: The Chile subduction zone

How temperature and petrology control the seismogenesis of the deeper megathrust and associated hazard is poorly understood. It was previously proposed that the serpentinized mantle wedge would limit seismic rupture, but modern observations show many recent earthquakes to have ruptured far deeper than the mantle wedge corner (MWC). However, the notion of mantle wedge serpentinite impeding seismic rupture is still of great scientific value. Here, by comparing model predictions of megathrust temperature and petrology with instrumentally constrained slip distributions of megathrust earthquakes along the Chile margin, we propose a new hypothesis based on the contrasting frictional behaviours of different serpentine polymorphs widely reported in the literature. At low slip rates, lizardite (and chrysotile) exhibits rate-strengthening and hence stable-sliding, but antigorite, given high enough temperatures, exhibits rate-weakening and hence stick-slip. However, they both undergo dynamic weakening at high slip rates and thus can facilitate rupture propagation. Along most of the Chile margin, as in most other global subduction zones, the megathrust at and immediately downdip of the MWC is rich in lizardite. The lizardite-serpentinite impedes but cannot always stop seismic rupture, forming a soft barrier. Farther downdip, the megathrust is rich in antigorite. Antigorite patches and their dense collections can facilitate rupture propagation and even initiation, and some of the stronger patches can radiate abundant short-period seismic energy. This MWC “soft-barrier” hypothesis explains the different modes of downdip slip distribution observed along Chile: bimodal slip straddling the MWC, slip confined downdip or updip of the MWC, and slip predominantly updip of the MWC with a deep extension crossing the MWC. The hypothesis is qualitatively compatible with global observations of megathrust seismicity and embraces the exception of very warm subduction zones.