CORRUGATIONS ALONG THE SHALLOW COSTA RICAN SUBDUCTION MEGATHRUST

The topography of fault surfaces has been shown to influence many aspects of fault and earthquake mechanics. Despite this influence, the meter-scale topography of a subduction megathrust has not been constrained. A 2011 3D seismic reflection survey of the subduction zone offshore southern Costa Rica has imaged the subduction zone interface at high resolution. We have identified a previously unrecognized series of distinct, parallel, elongate corrugations, or striae, along the subduction interface, oriented subperpendicular to the trench. These corrugations extend across the full ~11 km width of the seismic volume and down dip from the trench up to ~7 kilometers. The corrugations are hundreds of meters wide and meters to tens of meters high, making them comparable in scale and structure to megascale glacial lineations, imaged along former ice-sediment interfaces of deglaciated terrain, and corrugated onshore and offshore low-angle detachment faults, imaged along slow to moderate spreading centers and observed variably in extensional terranes. We interpret the Costa Rica corrugations as large-scale slip lineations that form due to slip along the subduction megathrust. The corrugations are oriented along their long axes ~16˚ clockwise to the plate motion vector, paralleling regional earthquake slip vectors and constraining dextral, trench-parallel slip to 22-24 mm/yr, higher than previous estimates. We also observe a clockwise increase of ~5˚ in slip direction across the ~11 km width of the 3D volume from the northwestern to southeastern end. This increase parallels the southeastward clockwise rotation of slip seen along the Costa Rica margin that is attributed to Cocos Ridge subduction. Because these corrugations are observed across the entire width of the 3D volume, one of two such volumes in the world, we find it possible that analogous corrugations exist along portions of subduction megathrusts globally.