About 50 kilometers below Earth’s surface, earthquakes occur primarily along subducting slabs at convergent plate boundaries. Although previous studies have demonstrated that these seismic events often have source properties that differ from shallower earthquakes, the physical processes responsible for deep earthquakes are still poorly understood.
To elucidate the mechanisms that control faulting at depths below 50 kilometers, Poli and Prieto have cataloged and studied the source parameters and developed detailed energy budgets for 415 moderate and large (magnitude greater than 5.8) earthquakes that occurred over the past 16 years at or below intermediate depths, between 50 and 350 kilometers. The results indicate that deep earthquakes have larger fracture energies than shallow events and that their fracture energies increase with greater amounts of fault slip, a finding that suggests the mechanism of rupture for deep earthquakes differs from what has been observed for shallow events. The team also observed an increase in radiation efficiency—the amount of work that is mechanically dissipated—with depth, which indicates that the rupture mechanism may likewise vary between intermediate and deep earthquakes.
So what could be responsible for this different mechanism of rupture? Given the large catalog, the researchers were also able to examine how rupture parameters differ within the same subduction zone. Their analysis shows that these properties can vary along the fault and that the observed differences are likely controlled by the shape and age of the subducting slab, as well as the occurrence of volcanic regions.
Collectively, these results comprise the most complete summary of deep earthquake source properties to date. The authors note that the results should help constrain which mechanisms control the nucleation and propagation of deep seismic events. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2016JB013521, 2016)
—Terri Cook, Freelance Writer