CAREER: Integrated Study of Shallow Subduction Seismogenesis and Locking Along the Middle America Trench

Georgia Tech Research Corporation, Atlanta GA

Investigators

Abstract

The processes that control shallow interface coupling, and near-trench earthquake rupture, in both the largest megathrust and tsunami earthquakes, are poorly understood. The amount of seismic coupling that builds up to sustain these events is highly variable. Thus, we cannot simply define the magnitude and rate of recurrence of earthquakes. Additionally, coupling changes dramatically along subduction zones, with rupture of locked patches having diverse effects that are both scientifically important and societally relevant. In rare events, such as the Java 2006 earthquake, the shallowest portion of the interface fails, causing dramatically increased tsunami potential. These events, called ?tsunami earthquakes?, are enigmatic because the processes that control the shallow seismogenic locking are poorly understood. This study will illuminate regions of locking along the megathrust region by examining the relationship between observed locking, past large earthquakes, current small earthquakes, and variations along the subduction interface. The Northern Costa Rica and Nicaraguan region was chosen because of the wealth of data, recent occurrence of a rare tsunami earthquake, and the unusual proximity of land to the seismogenic interface. The project will be a five-year effort that integrates seismic, geodetic and modeling results, utilizing the diverse geophysical tools for which Newman has expertise. To do this he will improve the regional GPS velocity field with a new campaign in 2010; He will combine existing earthquake datasets, seismic tomography and profiles, along with new events to develop a continuous plate interface model. He will use the seismicity to perform a state-of-the-art analysis of the spatial and temporal variability in the seismicity-rate distribution, and determine its validity as a proxy for locking. If confirmed, the tool will be used to identify such characteristic changes along the seismogenic interface offshore Nicaragua, in the zone of a recent large tsunami earthquake, and where GPS has no resolution. Intellectual Merit: Through analysis of existing and ongoing recordings in Costa Rica, the project will: 1) Develop a new seismicity constrained subduction interface model, for use in further modeling, and interpretation; 2) Explore interseismic changes in earthquake rates throughout the seismogenic zone, for use in understanding regional and time-dependent changes in activity, stress, and observed aseismic slip; 3) Examine the history of large interface earthquakes to determine the regional variability of coupling; and 4) Develop a new regional GPS velocity field for interseismic convergence, and identify any transitory long-period temporal character of convergence. This research will build on in an ongoing 12 station seismic array in Costa Rica that includes four Georgia Tech-owned broad-band seismometers, and will be done in collaboration with an ongoing continuous GPS network in the region. The results will be modeled using the seismically defined interface, and locking results will be compared with seismicity rate mapping to further determine the spatial and temporal variability in the region. If validated as a unique proxy for interface stresses, seismicity-rate (b-value) mapping will be useful for effectively identifying locked zones in offshore regions where geodetic techniques are currently very expensive and less utilized. Broader Impact: This project will improve our understanding of the mechanical controls on shallow subduction seismogenesis and tsunami hazard, having direct societal benefit. Two Georgia Tech graduate students will be supported by this project, where they will be trained to perform research and disseminate results through papers and presentations. The project will support scientific training and infrastructure in Costa Rica. The PI is a beginning investigator, and the project will help develop a dynamic geophysics research group at Georgia Tech. Knowledge gained through the project will be incorporated in courses at Georgia Tech. The program will develop a Georgia educational seismic network that will be used with learning modules to enhance the earth science curriculum in Georgia schools. The schools reached by this program will include a diverse socioeconomic group of students, with the initial target school being a charter school near downtown Atlanta that has primarily low income and minority students.