Title: Recoupling the Megathrust: Evaluation of the Transition from Postseismic to Interseismic Behavior in Nicoya Costa Rica
We've just recieved word that we've been funded for a new 3-year project to evaluate the postseismic response and megathrust recoupling following the Nicoya 2012 earthquake. The project will support one graduate student (Tiegan Hobbs), 2 GPS campaigns (2015 and 2017), and 3 years of maintenance support for the Georgia Tech component of the existing seismic network. The project official starts March 1, 2015 -- right around the time we hope to start our first GPS campaign.
Abstract:
Subduction zones are responsible for over 90% of the world?s largest earthquakes, and are responsible for virtually all tsunami generation. While geoscientists understand the driving force for such earthquakes, little is known about why they occur where they do, and particularly how the subduction fault (termed the megathrust) locks up for the development of major earthquakes. These environments remain difficult to understand because they generally occur offshore, and are difficult to observe with primarily land-based geophysical instrumentation. However, the Nicoya Peninsula of Costa Rica has a unique geometry that allows for land-based observations of the seismically active interface. Additionally large earthquakes occur there approximately every 50 years, with the most recent in September 2012. Now, in the years that immediately follow this major earthquake, a unique opportunity exists to observe the recoupling of the megathrust fault and die-off of aftershocks associated with this process. The project will support the collection and modeling of ground deformation and earthquake activity in the region over a three-year period following this latest large earthquake to evaluate how the environment returns to a state of building stress for future earthquakes. The research performed in this study will improve our understanding of the relationship between behavior just after an earthquake to past, and potentially future earthquakes in a subduction zone. This work is of significant societal relevance as it identifies the utility of prior ground deformation data to develop more accurate assessments of earthquake potential in such zones.
This project will support a team to perform two time-critical field GPS campaigns and operate a local seismic network to capture the transient deformation and earthquake activity beneath Nicoya while it transitions back to interseismic behavior. The project will support a graduate student to assimilate the campaign data with ongoing continuous GPS, existing campaign GPS data collected immediately after the earthquake, and prior recorded microseismicity to image and understand the state of recoupling along the active plate interface. The team will combine GPS and seismic datasets to model the deformation as controlled by interface afterslip, viscoelastic mantle relaxation, and spatially variable interface recoupling. The team will develop numerical models using a newly constructed geometrically accurate plate interface model that identifies large structural features that correspond to seamounts and sutures in the downgoing plate. Constrained by ongoing microseismicity recorded by the local network, models will differentiate afterslip regions from mantle relaxation, clarifying the relative contribution of interface and mantle controls on deformation. Through the evaluation of campaign and continuous GPS along with the evolution of microseismicity in the years immediately following the September 2012 Nicoya Earthquake, the team will illuminate the development of megathrust coupling. The contributions from mantle relaxation, interface complexity, and adjacent slow-slip activity will be useful for understanding the development and transfer of stress along the subduction zone environment. This work will provide: 1) high-resolution images of aftershock and early interseismic microseismicity; 2) spatially-dense images of changes in ground deformation in the 5 years following the 2012 Nicoya Earthquake; 3) detailed models of interface slip and recoupling with time; 4) characterization of stress-dependent viscoelastic behavior of the mantle wedge beneath northern Costa Rica; 5) develop meaningful comparisons between the transitions from late-interseismic locking, to coseismic slip, then early-interseismic locking models; and 6) evaluate the role of subducted topography in affecting the new interseismic locking. This project, while focused on only one subduction environment, spans the late-interseismic and coseismic periods. The results from this work will be used for future evaluation of the seismic cycle not just along Nicoya, Costa Rica, but will also be applied to subduction zones globally as the community develops more and advanced images of both interseismic locking and coseismic behavior using GPS, seafloor geodesy, and other techniques.
