Geodetic Observations in the Northern Costa Rica Subduction Zone

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The great earthquakes and tsunamis of 2004 (Sumatra) and 2011 (Japan) were wake-up calls that our forecasting skill for these catastrophic events remains weak. Measurement of the slow motions of the Earth's surface near active faults could help to forecast the location and the size of future earthquakes. This project tests the concept by operating a network of a high precision GPS receivers in Costa Rica. This area is an active subduction zone that experiences frequent large earthquakes. The GPS units will measure ground motion for a 3-year period with a precision of a few millimeters per day. This will continue a long history of geodetic observations here that now span both the late and early stages of the earthquake cycle. The data from this project will improve understanding of the earthquake process, and could lead to improvements in our ability to forecast these potentially deadly events. UNAVCO, the NSF-funded consortium for geodesy, will provide critical maintenance support for the network. The broader impacts of the study include impacts on earthquake hazards and also help to train the next generation of scientists through support for a PhD graduate student. The existence of earthquake clusters in subduction interfaces suggests that not all accumulated strain is necessarily released in an earthquake, and can be stored over a seismic cycle for release in a subsequent, larger than average event. With the discovery of slow slip events (SSEs: earthquake-like phenomenon within the interseismic phase that release energy slowly) it is now clear that determination of a full strain accumulation budget, and improved understanding of frictional conditions on the plate interface, also require study and understanding of these interseismic strain release mechanisms. Data from the 2012 Costa Rica earthquake suggests that a pre-event SSE changed Mohr Coulomb failure stress by a trivial amount, although other failure mechanisms are possible. SSEs are also important indicators of frictional conditions. Shallow SSEs are an important subset, and while generally difficult to observe, can be clearly seen in northern Costa Rica due to proximity to the trench. They may be critical for understanding tsunami potential in this difficult-to-observe region. More generally, SSEs may outline the region of subsequent seismic rupture, this pattern has now been documented in the active margins off Costa Rica, Ecuador and parts of Japan. This project deploys geodetic infrastructure to better observe and understand these slow slip events in the Nicoya Peninsula.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.