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
