Understanding how fault conditions impact earthquake rupture has been identified as a fundamental challenge of the seismological community, especially along shallow subduction megathrust faults where giant earthquakes can produce devastating shaking and tsunami. The study addresses this issue by characterizing earthquake source characteristics for comparison to specific tectonic and geologic variations within a small geographic area along the Middle America Trench, an area that has produced past large tsunami and earthquakes and where unique onshore and offshore seismic recordings exist for a large catalog of earthquakes.
The researchers hypothesize that earthquake source characteristics such as apparent stress are related to material conditions along faults and hence will differ within the subduction zone. Along the subduction megathrust, there are numerous potential culprits for altering the fault conditions. Subducted seamounts, ridges, fracture zones, plate-bending faults, and variable sediment packets riding on the subducting plate can provide significant variations in the megathrust geometry. Variable pore fluid pressure can affect the strength conditions along the fault by changing the effective normal stress. The ratio of compressional to shear wave speeds (Vp/Vs) is sensitive to changes in fault conditions, such as the presence of fluids in pores and cracks, temperature and composition, but not to slab geometry. Thus, to determine how these conditions affect earthquake rupture, both high quality data on fault conditions as well as extensive earthquake catalogs are need, both of which are available along this study area. They will be: 1) determining earthquake source spectra to provide estimates of the rupture time and stress drop, which are two parameters commonly used in global studies to describe earthquake rupture properties; and 2) calculating near source Vp/Vs using waveform cross-correlation derived differential times following an innovative technique. The Researchers will compare source parameters with the wealth of geophysical data already available in Middle America, as well as with the new estimates of Vp/Vs to help determine whether interplate fault segments have unusual fluid pressures or material composition. This will allow them to advance ideas about the role of fluids in earthquake rupture.
The researchers will be studying a region that has a history of large (M>7) earthquakes and tsunami earthquakes. Their results should improve understanding of local seismic and tsunami hazard and also bear on broader understanding of seismic and tsunami hazard along other subduction margins. The project will also support the participation of 2 graduate students, continue international collaborations, and produce results that will be disseminated widely at national meetings, international publications, and through online marine geophysical databases and PI supported webpages.
