The erosive convergent plate margin offshore Costa Rica represents a seismic zone that produces high-magnitude earthquakes and tsunamis. The overriding Caribbean Plate is tectonically eroded by the subducting Cocos Plate, resulting in strong and widespread deformation of the upper plate. Differences in composition, compaction or deformation of the clay-rich marine sediments can have a significant impact on the strength of the continental plate, as well as the frictional behavior of the rocks entering the subduction channel.
Core samples from IODP Expeditions 334 and 344 of the Costa Rica Seismogenesis Project (CRISP) recovered from a depth range of 5–125 mbsf were experimentally deformed in triaxial tests under isotropic consolidated and undrained conditions at confining pressures of 400–900 kPa, room temperature, axial displacement rates of 0.0025–0.01 mm/min and up to ~50% axial compressive strain. Deviatoric stresses range between 127 and 418 kPa, pore pressure between 615 and 822 kPa. The samples exhibit internal friction angles of 8-42° and cohesion values of about 29-34 kPa.
The stress-strain records show exclusively structurally weak behavior and a higher consolidation of the upper plate compared to the incoming plate sediments. The sediments from prism toe and middle slope of the Caribbean Plate display systematically higher peak stresses despite a similar overburden. This together with the consolidation state indicate a loss of overburden, for example due to slumping at the continental slope. The mechanical properties might be crucial for fracturing and localized brittle deformation of the continental forearc during tectonic erosion.