In this project we are combining two geophysical methods for imaging Earth's interior (seismic and electromagnetic) to improve our understanding of the geologic history, and present structure of the Earth in the Pacific Northwest United States. These studies will improve our understanding of the very dynamic Earth in this region, providing information that will ultimately help to mitigate geologic hazards such as earthquakes and volcanic eruptions, and inform efforts to develop natural resources (e.g., geothermal energy). The core dataset for this project is provided by the NSF-funded Earthscope Transportable Array, which provides co-located spatially uniform grids of high quality seismic and electromagnetic (EM) sites. Initial comparison of images obtained from the seismic and EM Earthscope data separately reveal striking similarities, which suggest a common large-scale geometric distribution of different, but plausibly related, material properties. Through this project we are developing more sophisticated methods for integrating the two datasets, with the goal of providing new constraints on physical state and composition of Earth's interior in this region. The methods developed are expected to be more widely applicable, both to similar studies of Earth structure and geologic history in other regions, and for smaller scale applications of joint geophysical imaging to resource exploration or environmental problems.
Bulk rock conductivities at crustal and upper mantle depths are strongly influenced by even small amounts of an interconnected conductive phase, making EM methods a powerful tool for mapping fluid distributions and constraining magmatic processes. However, interpretation of EM data in isolation is often ambiguous, as there can be multiple possible causes for high conductivity, and resolution is intrinsically limited by the diffusive nature of EM fields in a conductor. By integrating EM and seismic data non-uniqueness in both can be reduced, providing more powerful and useful constraints on composition and physical state of Earth's interior. With regard to methodology, our approach will span a spectrum, from simple inter-comparison of "unbiased" models obtained by inverting each type of data separately, through testing of hypotheses of common structure, to formal joint inversion. The collaboration is strengthening connections between seismic and EM researchers, and through training of young researchers in complimentary geophysical techniques, promises to enhance future interpretation of Earthscope and other geophysical datasets in the coming years.
