Magnetotelluric Investigation of Subduction Zone Hydrology and Megathrust Slip Behavior at the Nicoya Peninsula

Georgia Tech Research Corporation, Atlanta GA

Investigators

Abstract

The west coast of Central America is a convergent plate boundary that regularly experiences large subduction zone earthquakes. In Nicoya Peninsula, Costa Rica, both large damaging earthquakes with significant ground shaking as well aseismic slow-slip events (SSEs) with little to no ground shaking have been observed. Determining why some parts of the plate boundary experience megathrust rupture, while others experience SSEs is important in seismic hazard assessment. It is hypothesized that the style of fault slip may be driven by variations in subsurface fluid content, since fluid-rich zones can lubricate faults while fluid-poor zones promote stick-slip behavior. However, it is unclear precisely what combination of factors dictate the fault slip behavior or whether the hydrology and fluid content are alone responsible. To address these critical unknowns, this project will collect electromagnetic imaging data to constrain the subsurface hydrology and the distribution of fluids beneath the Nicoya Peninsula. The new data will be collected in collaboration with Costa Rican partners at the National University of Costa Rica and the Instituto Costarricense de Electricidad. The data will help distinguish the conditions that enable damaging earthquakes from conditions which promote aseismic slip. This project will support an early-career investigator and a postdoctoral researcher, who will lead the field work and data analysis. Several student volunteers from the U.S. and Costa Rica will be recruited for the fieldwork. Student participants will attend a short-course during the fieldwork where they will: review subduction zone processes and the geology of Nicoya Peninsula specifically, learn about the goals of the project, and receive training to process and analyze the data they helped to collect with freely available open-source software packages. The physical control(s) on the style of fault rupture is fundamental for evaluating seismic hazards, particularly at subduction zones. The Nicoya Peninsula, Costa Rica, is ideal for studying how fluids modulate earthquake behavior due to well-documented variations in seismicity, style of megathrust slip, and plate coupling, in addition to the relatively shallow depth to the plate interface given the survey area’s proximity to the trench. This project will deploy an array of magnetotelluric (MT) instruments at approximately 80 sites to image the three-dimensional electrical resistivity structure beneath the peninsula and constrain the distribution of fluids and thus the geohydrology of the seismogenic zone. The data from this project will be integrated with seismicity and fault slip observations, which are well-resolved by long-term seismic and geodetic instrument networks. The combined data will determine, for example, if electrically conductive regions correlate with zones of SSEs and resistive regions correlate with geodetically-locked zones prone to damaging earthquakes. The MT results will also be combined with geochemical data from surface seeps previously sampled in the survey area in order to determine the sources and migration pathways of fluids emitted at the seeps and how this may impact long term volatile cycling in subduction zone forearcs. 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.