Harvard Earth and Planetary Sciences Visiting Scholar 2019

We welcome Prof. Magali Billen as the Harvard EPS Visiting Scholar from November 4-6, 2019. Professor Billen is a geophysicist who uses numerical tools to understand subduction zone dynamics and a wide variety of related phenomena. Additional information about Dr. Billen’s research can be found at her research website.

Selected through graduate student nomination and voting, the EPS Visiting Scholar is invited to join the department for an extended stay to give a colloquium, lead seminars, and meet with the members of the department. All members of the EPS community are welcome to attend the events listed below.

EPS Colloquium: "Deep Slab Seismicity Limited by Rate of Slab Deformation in the Transition Zone"

Date: Monday, November 4, 12-1pm. Lunch provided.
Location: Haller Hall, Geology Museum 102
Abstract: Deep earthquakes (> 300 km depth) within subducting lithosphere (slabs) have remained enigmatic because of the similarities with shallow earthquakes and the high pressures at which they occur. Previous attempts to explain the global depth distribution of deep earthquakes in terms of the thermal conditions at which possible triggering mechanisms are viable, fail to explain the variability in seismicity within and between slabs. I will show that simulations of subduction with non-linear rheology and compositionally-dependent phase transitions, exhibit strongly variable strain-rate magnitude in space and time, which is similar to observed strain-rate depth profiles from seismicity. Taking strain-rate as a first-order control on where deep earthquakes occurs explains why there are large gaps in seismicity (low strain-rate), variable peaks in seismicity (high strain-rate bending regions) and an abrupt cessation of seismicity below 660 km. Therefore, independent of the triggering mechanism, the distribution of deep earthquakes is determined by the same primary physical factors (cold temperature and high strain-rate) as shallow earthquakes. Building on this new perspective of the factors controlling deep seismicity should allow researchers to more directly link the required conditions (e.g., pressure, temperature, strain-rate) for viable triggering mechanisms with seismic observations of deep earthquakes (e.g., stress drop, radiation e ciency, b-values) and to better constrain the rheology of the lithosphere and mantle.

Workshop: Rheological Calculations in Python

Date: Tuesday, November 5, 10-11am
Location: Geo Museum 204

Discussion: Computational Infrastructure for Geodynamics”

Date: Tuesday, November 5, 2-3pm. Refreshments provided.
Location: Geo Museum 103a

Lecture: "Episodic Plate Motion and Thermal Structure in Subduction Zones Caused by Slab Folding in the Transition Zone"

Date: Wednesday, November 6, 12-1pm
Location: Geology Museum 204 (McKinstry Room)
Abstract: Although most present-day subduction zones are in trench retreat, plate reconstructions and geological observations show that individual margins experience episodes of advancing, retreating or stationary trench motion with time-variable subduction rates. However, most laboratory and numerical simulations predict steady plate velocities and sustained trench retreat unless the slab experiences folding in the transition zone. Using 2D dynamical models of subduction with a mobile trench and overriding plate, we find that rapid sinking of the slab during folding causes a reduction in asthenosphere viscosity through the non-linear rheology, which allows the overriding plate to move in the opposite direction of the asthenosphere. This decoupling of the direction of plate and asthenosphere flow allows for episodes of rapid trench advance after each slab folding event. By analyzing the interaction between slab deformation (sinking direction and speed), stress-induced changes in asthenosphere viscosity, asthenosphere flow and plate motions, we show that there are three modes of slab-flow-plate interaction: 1) coupled trench retreat during rapid vertical sinking, 2) coupled trench advance during prograde sinking of the slab, and 3) decoupled, rapid trench advance during folding with prograde motion of the shallow slab and retrograde motion of the deep slab. These results show that non-linear viscosity plays an important role in determining the force balance controlling trench motion and conversely that trench motion can be used as a constraint on the asthenosphere viscosity underlying the overriding plate. In addition, cooling by several hundreds of degrees during episodes of fast subduction could lead to a reduction in slab dehydration and fluid-induced melting in the mantle wedge. Such cold episodes would also likely lead to time-variability in the water content and related geochemical tracers in erupted lavas, as well as the amount of water being transported by slabs into the deep mantle.

For more information please contact the student organizers at right.