Winter 2020

Tuesdays at 3:30 PM
Nat Sci Annex 101

January 7, 2020

Speaker: Harriet Lau, UC Berkeley

Title: Tidal Tomography: What an often-neglected phenomenon known as Earth tides can tell us about buoyancy in the deepest part of the mantle


Host: Mathis Hain

January 14, 2020

Speaker: David Dralle, CSU Sacramento

Title: The Salmonid and The Subsurface: The Importance of Rock Type for Understanding and Sustaining Stream Ecosystems

Abstract: How does the structure of the critical zone — especially below shallow soils and into saprolite and weathered rock — control water storage and release to California forests and streams? In this talk, I will address this question with a synthesis of observational and modeling results from three intensively studied California catchments. Findings reveal bottom-up, lithologic controls on weathering profiles, subsurface water storage capacity, and summer baseflow drainage from the critical zone. Results highlight how, in addition to plant physiology and climate, the subsurface is a key regulator of ecosystem productivity, stream temperature, water availability, and forest response to drought.


Host: Margaret Zimmer

January 21, 2020

Speaker: Demian Saffer, Penn State

Title: Recurring and triggered slow slip events near the trench along the Nankai subduction thrust

Abstract: Slow slip events (SSE), non-volcanic tremor, and very low-frequency earthquakes (VLFE) are well documented down-dip of the seismogenic zone of major faults, yet similar observations for the shallowest reaches of subduction megathrusts are rare. We document a family of repeating strain events in the Nankai subduction zone, updip of rupture zone of great (M8) earthquakes, using data from two borehole observatories which penetrate the hanging wall accretionary prism: IODP Site C0002, located 36 km landward of the trench; and Site C0010, located 25 km landward.

After filtering oceanographic noise using a local hydrostatic reference at each site, the pressure records over a 6 year period from 2010-2016 reveal 8 strain transients that are synchronous at the two holes. Of these, 6 arise spontaneously, and occur at ~1 yr intervals with durations of ~7-21 days. All are positive in sign at C0010, with consistent magnitudes of ~0.3-0.9 kPa; at Site C0002 three are negative in sign and two are positive, with magnitudes of ~0.3-0.7 kPa. The remaining 2 events are larger (1.7-2.7 kPa), negative in sign at both sites, and immediately follow: (1) the 2011 M9 Tohoku earthquake; and (2) the 2016 M7 Kumamoto earthquake. In some cases, the pressure transients are accompanied by swarms of low-frequency tremor and VLFE activity that appear to migrate trenchward. We interpret the pressure signals to reflect volumetric strain in response to SSEs. The data are well fit by slip of ~1-4 cm on a patch at the plate interface that extends 20-40 km in the dip direction, and is centered beneath Site C0002 (spontaneous events) or slightly updip (triggered events). A key implication is that the SSE accommodate ~30-50% of plate convergence across the outer ~40 km of the forearc. This coincides with a region of the shallow-most megathrust characterized previously by: (1) elevated pore fluid pressure; (2) transitional frictional behavior that promotes the nucleation of unstable slip at low sliding velocities; and (3) low stress magnitudes as constrained by analysis of wellbore failures. The repeating nature of the events, taken together with apparent triggering by regional earthquakes and the inference of low in situ stress magnitudes, indicates that the outermost reaches of the megathrust are highly sensitive to perturbation and are perched near a state of failure.

Host: Emily Brodsky

January 28, 2020

Speaker: Sonia Tikoo-Schantz, Stanford

Title: Magnetism with an impact: Chicxulub crater


Host: Ian Garrick-Bethell

February 4, 2020

Speaker: Gregor Steinbrügge, UT Austin/Stanford

Title: The structure and dynamics of Europa’s ice shell

Abstract: Jupiter’s moon Europa is a fascinating world and a prime candidate for life within our Solar System. I will focus on the outer ice shell of the satellite where the dissipated tidal energy sustains a subsurface ocean. If the ice shell allows for exchange processes, exogenic material deposited on the surface would be an important factor for habitability. If the exchange of material with the ocean is reciprocal, the shell can also serve as a window to the interior of the moon. I will discuss the recent progress in understanding the structure and dynamics of Europa’s ice shell and give an overview on how NASA’s upcoming Europa Clipper mission can contribute to the exploration of this unique icy satellite by radar sounding.


Host: Slawek Tulaczyk

February 11, 2020

Speaker: Don DePaolo, UC Berkeley


Host: Jim Zachos

February 18, 2020

Speaker: Xinting Yu, UCSC


Host: Xi Zhang

February 25, 2020

Speaker: Gabrielle Wong-Parodi, Stanford


Host: Mathis Hain

March 3, 2020

Speaker: Adi Torfstein, Hebrew U of Jerusalem

Host: Terry Blackburn


March 10, 2020

Speaker: Elisa Mantelli, Princeton

Title: The origin of fast ice flow in continental ice sheets. A tale of fluid flow instabilities


Host: Slawek Tulaczyk