Testing the Thermal Shear Instability Hypothesis for Deep Slab Seismicity

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Deep earthquakes occur 100 to 680 km below Earth’s surface within cold tectonic plates that are sinking back into the Earth’s interior. There has been one prevailing theory for the cause of these earthquakes, called transformational faulting, but this theory is not able to explain all the observations related to deep earthquakes. Recently, research has indicated that another mechanism, called thermal shear instability (TSI), or a combination of both mechanisms, may be better able to match the observations.

This research will use simulations of sinking tectonic plates (“slabs”) to determine the temperature, stresses, and rates of deformation at different

depths/locations in the slab. These conditions will then be used as the starting conditions of a second type of simulation that can model TSI. We expect that for some conditions TSI (i.e., an “earthquake”) will occur, while at other conditions it will not.

Therefore, using the output of the second type of model we can map out where in the slab TSI is a possible mechanism for deep earthquakes. If our hypothesis is correct, the shift in understanding of the mechanism causing f deep earthquakes (from one, to multiple potential mechanisms) would likely lead to new research aimed at directly linking seismic observations to the rupture properties of deep earthquakes.

Broadly speaking, these results will further our understanding of the processes and conditions that lead to earthquake rupture.

Recent modeling of, and laboratory measurements on, the conditions needed for thermal shear instability (TSI) have demonstrated that TSI may be a viable mechanism for deep earthquakes in subducting tectonic plates at depths up to around 150 km. At the same time, analysis of the magnitude-frequency distribution of deep earthquakes from 150-680 km has also been used to argue that TSI plays a role in triggering deep earthquakes, especially in warmer slabs.

This project will test the viability of TSI as a mechanism for triggering deep earthquakes within subducting tectonic plates at depths of 100-680 km. This will be done in a three-step process. First, we will run 2D visco-elasto-plastic models for multiple profiles and subduction zones with different geometry, plate ages, rates of subduction, and rates/spatial variability of deep seismicity.

The models will use a visco-elasto-plastic rheology and will be run for 0.1-1.0 my to determine a quasi-steady state spatial distribution and magnitude of elastic stresses, and total strain rate in the slab. Second, we will separately run 1D TSI models using the range of pressure, temperature, stress, and strain-rate conditions from the 2D slab models to determine at which conditions, present in the slab, TSI occurs.

The TSI models will use the same rheology as the 2D subduction models. This comparison will demonstrate where in the slab TSI is potential triggering mechanism for deep earthquakes. Finally, we will compare location-specific earthquake observations (spatial distribution, focal mechanisms, magnitudes, b-values) to the model results (spatial distribution of TSI, fault orientations, estimates of magnitudes and geometric constraints on seismicity statistics).

This comparison of the combined model results to observations will demonstrate how well our simulations capture the overall deformation of the slab at the short timescales of earthquake rupture up through the longer time-scales that determine the present-day stress-state in the slab.

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.