Most earthquakes occur in the Earth's shallow crust, but some are located at greater depth. These intermediate-depth earthquakes occur in the crust and upper mantle of tectonic plates that are subducting into the deep Earth at collisional plate boundaries. Beyond this, we lack an understanding of why these earthquakes occur at some places (e.g., Colombia, Afghanistan, Romania, northern Cascadia), but not at other, seemingly similar, collisional plate boundaries. We also lack a model that explains their recurrence, i.e., why they occur when they do. While not as deadly as shallow earthquakes, intermediate-depth earthquakes can occasionally be devastating. For example, the 1939 Chillan, Chile intermediate-depth earthquake killed ~28,000 people, and is by far the deadliest earthquake in that country's history. Intermediate-depth earthquakes can also be quite damaging. For example, the 2001 Nisqually, Washington earthquake caused over $2 billion in losses, despite being located on the southern fringe of metropolitan Seattle. The focus of this study is the intense "nest" of intermediate-depth earthquakes under Bucaramanga, Colombia. This area hosts the greatest concentration of intermediate-depth earthquakes in the world, and their frequent occurrence means that this presents the best opportunity in the world to study them. By using data from the state-of-the-art Colombian National Seismic Network, we will gain a deeper understanding into how these earthquakes work. Although he will be supported with funding from his own institution, this project will help build collaborations between Colombia and the US. It will also help develop capacity for earthquake monitoring and evaluation in the developing world. The understanding of intermediate-depth earthquakes that we develop should be broadly applicable to similar earthquakes worldwide, including the Pacific Northwest of the US.

This three-year project, to be carried out in collaboration with Professor German Prieto of the University of the Andes in Bogota, Colombia, will study the intense concentration of intermediate-depth earthquakes under Bucaramanga, Colombia using state-of-the-art analysis of waveform data from the Colombian National Seismic network. We will use cross-correlation-based measurements to develop: precise earthquake locations, improved 3D velocity structure, and more accurate source parameter estimates of these events. The result will be a better understanding of earthquakes in the Bucaramanga Nest in particular, and intermediate-depth earthquakes in general. Among the questions we will address are: Do these earthquakes occur due to collision of the Caribbean and Nazca Plates at depth? Do they occur on discernable fault planes or are they diffuse throughought the subducting slab? Are there Vp/Vs anomalies that might indicate a role for pore fluids in their occurence? How do the source properties of intermediate-depth earthquakes vary with earthquake size? Can we identify repeating earthquakes, and if so, is their recurrence non-Poissonian (i.e., not random in time, but governed by a physically motivated process like elastic rebound)? What controls the maximum size of intermediate-depth earthquakes under Bucaramanga? How can the results of our study be used to place useful constraints on the size and recurrence of intermediate-depth earthquakes around the world that will be useful for hazard characterization?

This project is supported by the Geophysics Program and the Colombia Program of the Office of International Science and Engineering.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Raffaella Montelli
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Stanford University
Palo Alto
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