Description: We investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity. Westward from Osa Peninsula in Costa Rica, a major change in the character of the incoming Cocos Plate is displayed by abrupt lateral variations in seafloor depth and thermal structure. Here a Mw 6.4 thrust earthquake was followed by three aftershock clusters in June 2002. Initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the Middle America Trench off central Costa Rica. The earthquake sequence occurred while a temporary network of OBH and land stations ∼80 km to the northwest were deployed. By adding readings from permanent local stations, we obtain uncommon P wave coverage of a large subduction zone earthquake. We relocate this catalog using a nonlinear probabilistic approach within both, a 1-D and a 3-D P wave velocity models. The main shock occurred ∼25 km from the trench and probably along the plate interface at 5–10 km depth. We analyze teleseismic data to further constrain the rupture process of the main shock. The best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope, supporting the nucleation of the Osa earthquake at ∼6 km depth. The location and depth coincide with the plate boundary imaged in prestack depth-migrated reflection lines shot near the nucleation area. Aftershocks propagated downdip to the area of a 1999 Mw 6.9 sequence and partially overlapped it. The results indicate that underthrusting of the young and buoyant Cocos Ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central Costa Rica margin.

Description: The understanding of the tectonic processes shaping the Pacific margin off Costa Rica has undergone a dramatic evolution during the past 25 years. The margin, initially interpreted to be built by accretion of sediment from the ocean plate, is now interpreted as made of ophiolitic rocks that are exposed onshore, with no net accretion currently active. New seismic images indicate that upper plate tectonic erosion might be the dominant process. Erosion is accomplished in some cases through transport of large bodies from upper to lower plate by plate boundary readjustment. Subduction of seamounts locally accelerates tectonic erosion.

Description: Subduction erosion models commonly invoke concepts of high friction abrasion. We present a low friction model involving reduced strength from a disintegrating hydrofractured upper plate. Our model is constructed from features in high-resolution seafloor morphology, refraction and reflection seismic sections, and magnetic anomalies. It consists of a frontal prism and margin wedge upper plate underlain by a bend-faulted lower plate. The frontal prism maintains its modest width and taper in equilibrium with local conditions through feedback from subduction zone dip, friction along the decollement, and material strength. The prism elevates fluid pressure in the subducting plate forcing pore fluid into fractures above the plate interface thrust that hydrofract the underside of the upper plate margin wedge. Beneath the base of the slope, upper plate fracturing is aided by the up-and-down riffling over subducting topography. Progressive fracture and upward fluid migration shift the zone of least strength upward. The interplate thrust seeks the zone of least strength and shifts with the upward migrating fluid thereby detaching underlying upper plate material and transferring it to the lower plate. This process may begin at the seismogenic zone and progress updip to the landward part of the frontal prism. Beneath the lower slope fluid pressure and fracturing weaken the upper plate so that its strength decreases to the level of the subducting sediment. The model we propose can be tested with deep scientific ocean drilling.

Description: Interplate earthquakes in subduction zones are generated in the seismogenic zone, i.e. the segment of the plate boundary where unstable slip occurs. Understanding the mechanisms that control the updip and downdip limits of this zone, as well as the nature and role of asperities within it, provide significant insights into the rupture size and dynamics of the world’s largest earthquakes. The Costa Rica Seismogenesis Project (CRISP) is designed to understand the processes that control nucleation and seismic rupture propagation of large earthquakes at erosive subduction zones (Ranero et al. 2007). In 2002 a magnitude Mw=6.4 earthquake may have nucleated at the subduction thrust to be penetrated and sampled by CRISP, 40 km west of Osa Peninsula (Figure 1). However, global event localization is associated with too large errors to prove that the event actually occurred at a location and depth to be reachable by riser drilling. We have compiled a database including foreshocks, the main shock, and ~400 aftershocks, with readings from all the seismological networks that recorded the 2002 Osa sequence locally (Figure 1). This includes a temporal network of oceanbottom hydrophones (OBH) that happened to be installed close to the area (Arroyo et al. 2009). The greatly improved coverage provided by the OBH enable us to better constrain the event relocations that we are presently undertaking. Within the frame of a proposal recently submitted to DFG with IODP emphasis, detailed inspection of the data and 3-D data modelling will be carried out to yield source parameters that can be rated against structural information from seismic and drilling constraints. Moreover, teleseismic waveform inversion will provide additional constraints for the centroid depth of the 2002 Osa earthquake, allowing further study of the focal mechanism. This sequence is the latest at the Costa Rican seismogenic zone to date, in a segment of the erosional margin where seamount-covered oceanic floor is presently subducting (Figure 1). It took place trenchward from a 1999 Mw=6.9 earthquake sequence, that it is thought to have been nucleated by a seamount acting like an asperity (Bilek et al. 2003). The work proposed here aims to provide definite evidence that the planned Phase B of CRISP will be successful in drilling the seismogenic coupling zone. Furthermore, the seismological data will be interpreted jointly with thermal and drilling data from IODP Expedition 334 to refine the link between temperature and seismogenesis at erosive convergent margins.

Description: The great Chilean earthquake of 27 February 2010 occurred along a convergent margin previously surveyed with modern geophysical methods. Modern data resolve subducting ocean plate relief that probably arrested earthquake rupture. The northern limit of rupture was at the subducted extension of Juan Fernandez Ridge (JFR). JFR relief increases as the plate is faulted during bending into the trench thereby elevating the plate boundary 1-3-km. At the intersection of relief with the trench axis turbidite transport from the south is impeded producing trench turbidite fill 2-3-km thick. The southern rupture limit coincides with the subducted projection of Mocha Fracture Zone (MFZ), a 20-km wide structure with thin fractured crust and seafloor relief to 1- km. Seismically imaged beneath the frontal sediment prism is a plate interface between accreted and subducted sediments. Farther landward, where the 2010 earthquake rupture and aftershocks occurred, the upper plate is margin framework rock and the plate interface separates this metamorphic basement from subducted sediment. These observations show that subducted lower plate high-relief can affect the spatial organization of great earthquake rupture, as can the upper plate discontinuity between a frontal accreted sediment prism and the margin framework rock.

Description: Great earthquakes in subduction zones occur after stable slip in the proto-seismogenic zone transitions to the unstable slip that characterizes seismogenic zones. Subducted material input to seismogenic zones affects this transition. Material structure, lithology and physical properties change progressively during subduction, and according to current hypotheses, specific material transformations trigger the stable to unstable slip transition.Where accretion dominates a convergent margin, material input is trench sediment that is easily drill-sampled. However, where erosion dominates a margin, material input is unknown because it originates along the base of the upper plate and alters differently. The depth at which material is eroded lies beyond the sampling capabilities of past scientific ocean drilling, so the protoseismogenic zone transformed material has never been drill-sampled; nor does geophysics resolve its structure, lithology, and physical properties. The Japanese riser drill ship Chikyu in the Integrated Ocean Drilling Program (IODP) overcomes this difficulty. Preparing a site for deep drilling is a much greater task than preparing the shallower sites of past programs, so this is accomplished during workshops.