Collaborative Research: Reconciling the geophysical observations of slow slip and tremor with geologic and rheological constraints

Subduction zones represent a boundary between Earth's tectonic plates where one slides under the other. Subduction zones host the largest earthquakes on Earth, and recent research has revealed more complex deformation events that take place over periods of months or years (referred to here as slow slip and tremor or SST). SST events do not have the sudden destructive power of earthquakes but they can influence the occurrence of large earthquakes.

Despite the near ubiquity of SST in modern subduction zones, the mechanisms that drive SST remain poorly understood, in part because direct observations of processes occurring deep in the Earth's crust are not possible. Rocks that have been subducted to SST depths and brought back up to the surface provide a window into processes happening deep in subduction zones.

This project will: (1) investigate the types of rocks that potentially hosted SST by analyzing the minerals that make up the rocks, including the ages, chemistry, and deformation and (2) compare the rock record to geophysical imaging of modern subduction zones by modeling the geophysical signature of these rocks based on the observations. Collectively, the results of this work will reveal the distribution of potential SST sources as preserved in the rock record and how they correlate to modern subduction zones.

This project involves extensive international collaborations and training of graduate and undergraduate students as well as a postdoctoral scholar. Additionally, as part of this work yearly workshops will be developed in seismic properties, rheology/deformation and geochronology/geochemistry tailored in subduction zone science for the participants of the project.

These workshops will result in scientific/methodologic resources and workflows for future research on rock record evidence of SST. These materials will become publicly available as resources for the subduction zone research community.

At the base of the seismogenic zone stored elastic energy may be released gradually in slow slip events along with low frequency earthquakes and non-volcanic tremor that contribute significantly to the seismic cycle. This project addresses long-standing questions on whether slow slip events can be hosted in metasedimentary or meta-mafic rocks and the potential spatial distribution of SST sources.

Geophysical studies find that SST often coincides with sheared and underthrusted metasedimentary rocks or with the uppermost oceanic crust of the downgoing slab. However, field observations integrated with experimental constraints suggest that metasedimentary rocks are not good candidates for SST sources. Three subduction complexes were selected that span deep SST depths and offer excellent exposures of underplated metasedimentary rocks intercalated with meta-mafic and meta-ultramafic lithologies.

The project will investigate evidence of SST in these complexes (e.g., vein networks, geochemical signatures of alteration) using a field-based approach coupled with geochronology/geochemistry, structural geology, microstructural analysis, and rheology. By using the results from the rock record to calculate seismic properties coupled with the geophysical observations of SST in modern subduction zones, this work will test (a) whether SST may be preferentially accommodated by metasedimentary or meta-mafic lithologies, and (b) how structural heterogeneity along the subduction interface affects the spatial distribution of SST.

The outcomes of this work will have implications for the geologic conditions and slip behavior at the base of the seismogenic zone and will therefore better inform both SST observations in active subduction zones and geodynamic models of the seismic cycle and associated earthquake hazards.

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.