Mechanisms of Mantle Wedge and Intraslab Earthquakes Beneath Southwestern Colombia From Analysis of Earthquake Source Properties

Saint Louis University, Saint Louis MO

Investigators

Abstract

The occurrence of earthquakes below 50 kilometers depth, and as deep as 700 kilometers depth, is unexpected because of the high pressure and temperature at those depths. Such deep earthquakes, which represent ~25% of all earthquakes globally, occur in subduction zones and occasionally cause damage at the surface, so better understanding of why they occur will help with seismic hazard analysis and preparation. It will also help us to better understand the variety of conditions under which earthquakes can occur. This work will provide new constraints on the possible mechanisms of such earthquakes by studying the properties of a unique cluster of earthquakes that occur beneath northwestern South America. This project benefits society because of its focus on earthquake occurrence and hazard and because it enhances STEM education by training a postdoctoral fellow in seismology. The Cauca cluster of intermediate-depth earthquakes, which is located beneath Colombia between at depths of 50-200 kilometers, has ~58 earthquakes per year with local magnitude ML greater than or equal to 2.5. The earthquakes are located both in a planar seismic zone in the subducting slab and in two distinct columns, or fingers, extending 30-40 kilometers normal to and above the slab and into the mantle wedge. Using seismic data from 2010 to the present from the Colombian National Seismic Network, the Cauca cluster earthquakes will be characterized by precisely relocating the earthquakes, inverting waveforms to calculate focal mechanisms for many of the larger earthquakes, and estimating static and dynamic source properties for the earthquakes. These analyses will provide the first-ever characterization of mantle wedge earthquakes and a comparison of the source properties of earthquakes in two settings, thus enabling the testing of different hypotheses --- such as dehydration embrittlement, shear instability, hydrofracture, and collapse of fluid propagation pathways --- for why these earthquakes occur. The new constraints on the mechanisms of intermediate-depth earthquakes and fluid migration pathways through the mantle wedge will be applicable to all subduction zones This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.