GGSS Seminar How does deformation of a sinking tectonic plate cause earthquakes at 400-660 km beneath the Earth’s surface? by Magali Billen


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Conference room on Ghausi Hall 3102

Earthquakes near the surface of the Earth are understood to occur as the result of brittle failure through fracture and frictional sliding. However, both fracture and frictional sliding are strongly inhibited by increasing pressure causing a transition to ductile deformation at 15-50 km and a transition to fully viscous deformation at 15-80 km, depending primarily on temperature.

Therefore, it is surprising that within sinking tectonic plates (slabs) earthquakes continue to occur to depths of ~660 km (~30 GPa). The observation that these “deep” earthquakes only occur where there are slabs confirms that colder temperatures than the ambient mantle is required. Several possible mechanisms for causing these deep earthquakes have been proposed and explored in terms of their ability to match a suite of seismic observations from deep earthquakes, with one mechanism dominating scholarly attention for the past 25 years, while never really doing a great job matching observations or following the predicted dependencies on slab ageexpected for this mechanism. Over this same time, new laboratory data on rock properties have been collected with more and better observations of deep earthquakes. Recently, Prof. Billen proposed that an important factor, strain-rate dependence of the proposed deep earthquake mechanisms, had been overlooked and could reconcile observations with two of the mechanisms for deep earthquakes. I will present the data and models supporting this hypothesis, as well as, how we are going about improving the models to facilitate more direct comparison to observations.

Professor Magali Billen is a Professor in the Department of Earth and Planetary Sciences at UC Davis. Before coming to UC Davis as an Assistant Professor in 2003, she completed her PhD in Geophysics at Caltech, followed by postdocs at the University of Leeds in England and the Woods Hole Oceanographic Institution in Massachusetts. Prof. Billen and her research group conduct research on the forces that drive and resist the motion and deformation of tectonic plates. Specifically, we study the process of subduction in which one tectonic plate bends to sink beneath another tectonic plate. Utilizing finite element modeling methods, together with a variety of geological, geophysical and geochemical observations, we investigate how the material properties of rock in the Earth’s interior affects the deformation of sinking tectonic plates (slabs) and the motion of tectonic plates at the Earth’s surface. In addition, to her research interests, Prof. Billen is committed to supporting graduate student success at UC Davis, and is involved in initiatives to provide and support community developed codes for geodynamical modeling. She also has three children, two cats, and a husband from Germany, who love to travel to amazing geological places in the world.

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