Subduction zones are where major earthquakes and tsunami generation occur but the regional human risk varies since geologic and hydrologic conditions vary along a given margin. The presence of (possibly lubricating) fluid within the 'megathrust fault' where the Pacific plate slides down beneath Costa Rica will be documented and related to the pattern of seismicity. Above the megathrust fault, many smaller faults control deformation within the sedimentary wedge. This study will provide new understanding of this margin's evolution and a more complete picture of the range of hazards will be obtained. Training of next generation marine scientists will take place via involvement of undergraduate students in the data analysis and a postdoctoral scholar will play a significant role throughout the study.
Analysis of the structure of an offshore portion of the Costa Rica margin will document fluid pathways within and at the base of the sedimentary wedge that forms the toe of the tectonic plate that overrides the megathrust fault. Advanced analysis of the 3-D multi-channel seismic reflection dataset previously collected by the R/V Langseth will be conducted. Prior study determined the geometry of the megathrust and this study will quantify physical properties within the fault zone. Amplitude versus offset results should discern whether portions of the megathrust fault are fluid bearing. The relation with seismicity patterns will address the possible role of fluids in controlling historical slip along the fault. Detailed structural modeling of the accretionary wedge, which is riddled with small faults, will be conducted to show how deformation of the overriding plate is accommodated over time. Re-analysis of a legacy ocean bottom seismic refraction profile will provide comparison to velocity structure in a neighboring part of the margin so that along-strike variability can be assessed. The findings will provide key context for interpretations of past and possible future scientific ocean drilling results.
