February 5, 2021
3:00pm in Zoom
Cailey Condit from University of Washington
Slow earthquakes in subduction zones: constraints from the geologic, petrologic, and constituent realms

Abstract: Subduction zones host destructive megathrust earthquakes, one of the deadliest natural hazards on earth. Deformation within these zones is largely localized along the subduction plate interface, a lithologically heterogenous and chemically active fault and shear zone. At the transition between the shallow locked and deeper creeping interface, we have recently recognized a new set of slip behaviors: episodic tremor and slip (ETS) often called slow earthquakes. ETS occurs in predominately warm subduction zones below the seismogenic zone in a fluid rich environment at near lithostatic pore fluid pressures. Occurring episodically over weeks to months, these slow earthquakes may ultimately trigger megathrust events and play a key role in the slip budget of some subduction zones. However, we currently do not have a mechanistic understanding of how they occur.

In this talk I will show observations from the exhumed rock record, results from petrologic modeling, and analysis of constitutive relations to offer new constraints on the mechanisms of slow earthquakes. I will show that viscous deformation cannot accommodate slow slip strain rates along the plate interface, and that frictional deformation made possible by elevated pore fluid pressures is important. I will also demonstrate that in situ metamorphic dehydration from the subducting slab is a ready source of fluids for these high pore fluid pressures, indicating that metamorphism is an important process in the production of deep slow earthquakes.

Watch on YouTube

February 12, 2021
3:00pm in Zoom
Maya Gomes from Johns Hopkins University
How do sulfur isotopes record information about Earth’s earliest ecosystems?

Abstract: The sedimentary pyrite sulfur isotope (d34S) record is an archive of ancient microbial sulfur cycling and environmental conditions. Interpretations of pyrite d34S signatures in sediments deposited in microbial mat ecosystems are based on studies of modern microbial mat porewater sulfide d34S geochemistry. Pyrite d34S values often capture d34S signatures of porewater sulfide at the location of pyrite formation. However, microbial mats are dynamic environments in which biogeochemical cycling shifts vertically on diurnal cycles. Therefore, there is a need to study how the location of pyrite formation impacts pyrite d34S patterns in these dynamic systems. I will present diurnal porewater sulfide d34S trends and d34S values of pyrite and iron monosulfides from Middle Island Sinkhole, Lake Huron. The sediment water-interface of this sinkhole hosts a low-oxygen cyanobacterial mat ecosystem, which serves as a useful location to explore preservation of sedimentary pyrite d34S signatures in early Earth environments. I will show that, despite large (up to ~25‰) variations in d34S values over cm-depth scales linked to changes in net sulfate consumption in sediment pore waters, d34S values of pyrite are similar to porewater sulfide d34S values near the mat surface. Oxidative sulfur cycling and other microbial activity promotes pyrite formation in and immediately adjacent to the microbial mat and that iron geochemistry limits further pyrite formation with depth in the sediment. These results imply that primary d34S signatures of pyrite deposited in organic-rich, iron-poor microbial mat environments capture information about microbial sulfur cycling and environmental conditions at the mat surface and are only minimally affected by deeper sedimentary processes during early diagenesis.

February 19, 2021
3:00pm in Zoom
Carl Tape from University of Alaska, Fairbanks
Earthquakes and Earth Structure in the EarthScope Era (2014-present)

Abstract: The subsurface structure of Alaska contains clues to its formation and evolution over geologic time. Today, tectonic forces of subduction and collision are manifest in earthquake activity across the entire state. The NSF-sponsored EarthScope Seismic Array was fully deployed in 2017, enabling new high-quality geophysical data collection in large, remote regions of the state. I will discuss recent and ongoing studies of earthquakes and seismic imaging in Alaska, with a focus on crustal faulting. I will also convey how 3D seismic wavefield simulations can be used to understand the complexities of the recorded wavefield, as well as to guide improvements to tomographic models of crustal and upper mantle structure.

Watch on YouTube

March 12, 2021
3:00pm in Zoom
Forrest Horton from WHOI
March 26, 2021
3:00pm in Zoom
Rajdeep Dasgupta from Rice University
April 9, 2021
3:00pm in Zoom
Michael Manga from Berkeley
April 16, 2021
3:00pm in Zoom
Chandra Turpen from UMD
April 23, 2021
3:00pm in Zoom
April 30, 2021
3:00pm in Zoom
Aradhna Tripati from UCLA

The coordinator for the Colloquium Series is Dr. Megan Newcombe. You can contact her at newcombe@umd.edu.

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