Horizontal principal stress orientation in the Costa Rica Seismogenesis Project (CRISP) transect from borehole breakouts

Abstract

AbstractThe Costa Rica Seismogenesis Project (CRISP) drilled the Pacific margin of the Middle America Trench just north of where the Cocos Ridge enters the subduction zone, resulting in basal erosion of the upper plate. Here we report the orientations of the maximum horizontal principal stress (SHmax) from borehole breakouts detected by logging‐while‐drilling and wireline downhole measurements. All SHmax directions were estimated in the sediment cover of the margin, above the deeper rocks of the deformed margin wedge. We observe three overall SHmax orientations: NNE‐SSW (25° azimuth) in the deepest interval drilled at the upper slope Site U1379; ENE‐WSW (82°) in the rest of Site U1379 and in Site U1413, also drilled in the upper slope; and NNW‐SSE (157°) in the mid‐slope Site U1378. Our preferred interpretation is that the deepest interval of Site U1379 records the stress conditions in the underlying margin wedge, as SHmax is parallel to the direction of the Cocos‐Caribbean plate convergence and of the compressional axes of plate boundary fault earthquakes. The variable SHmax directions observed elsewhere are likely due to the effect of a network of normal faults that subdivide the sediment cover into a number of independently deforming blocks. In addition, the observed SHmax directions may be influenced by the subducting Cocos Ridge, which acts as an indenter causing oblique deformation, and by the transition to seismogenic subduction along the plate boundary fault.