Integrated Ocean Drilling Program, United States

Integrated Ocean Drilling Program Expedition 334, also known as the Costa Rica Seismogenesis Project (CRISP), was designed to understand the processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. CRISP involved the only known erosional end-member of convergent margins within reach of scientific drilling. With a relatively thin sediment cover, fast convergence rate, abundant seismicity, subduction erosion, and change in subducting plate relief along strike, CRISP offered excellent opportunities to learn the causes of earthquake nucleation and rupture propagation. This project complements other deep-fault drilling (San Andreas Fault Observatory at Depth and Nankai Trough Seismogenic Zone Experiment) and investigates the first-order seismogenic processes common to most faults and those unique to erosional margins. The primary goals of Expedition 334 were 1. Characterization of lithological, physical, and frictional properties of upper plate material; 2. Estimation of subduction channel thickness and the rate of subsidence caused by subduction erosion; 3. Characterization of the fluid flow system and thermal structure of the erosive margin; and 4. Determination of the change in the stress field across the updip limit of the seismogenic zone. In order to accomplish these objectives, coring was conducted at three slope sites (Sites U1378 and U1380 on the middle slope and Site U1379 on the upper slope) and at one site on the Cocos plate (Site U1381). Expedition 334 is also the first step toward deep riser drilling through the aseismic and seismic plate boundary; both slope sites might serve as pilot holes for a potential future riser drilling project. Sites U1378 and U1379 were first characterized by logging while drilling (LWD) to document in situ physical properties, stratigraphic and structural features, and stress state, in addition to continuous core sampling to the target depth. Cores at both sites were taken to examine slope sediments and the underlying upper plate basement. The coupling of LWD data with data from sediment and basement samples provides important information about tectonic, hydrologic, and seismic features along this erosive convergent margin.