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: 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: The Costa Rica Seismogenesis Project (CRISP) is designed to explore the processes involved in the nucleation of large interplate earthquakes in erosional subduction zones. On 16 June 2002 a magnitude Mw=6.4 earthquake and its aftershocks may have nucleated at the subduction thrust to be penetrated and sampled by CRISP, ~40 km west of Osa Peninsula. Global event locations present uncertainties too large to prove that the event actually occurred at a location and depth reachable by riser drilling. We have compiled a database including foreshocks, the main shock, and ~400 aftershocks, with phase arrival times from all the seismological networks that recorded the 2002 Osa sequence locally. This includes a temporal network of ocean-bottom hydrophones (OBH) that happened to be installed close to the area at the time of the earthquake. The coverage increase provided by the OBH network allow us to better constrain the event relocations, and to further analyze the seismicity in the vicinity of Osa for the six months during which they were deployed. We derived a minimum 1D model and used probabilistic earthquake relocation. Moreover, we undertook teleseismic waveform inversion to provide additional constraints for the centroid depth of the 2002 Osa earthquake. The latter, together with the maximum likelihood hypocenter, places the main shock origin at 5 to 10 km depth, ~30 km landward from the trench. Along the Costa Rican seismogenic zone, the 2002 Osa sequence is the most recent. It nucleated in the SE region of the forearc where this erosional margin is underthrust by a seamount covered ocean plate. A Mw=6.9 earthquake sequence occurred in 1999, co-located with a subducted ridge and associated seamounts. The Osa mainshock and first hours of aftershocks began in the CRISP area, ~30 km seaward of the 1999 sequence. In the following two weeks, subsequent aftershocks migrated into the 1999 aftershock area and also clustered in an area updip from it. The Osa updip seismicity apparently occurred where interplate temperatures are ~100°C or less. In this study, we present the relocation of the 2002 Osa earthquake sequence and background seismicity using different techniques and a moment tensor inversion for the mainshock, and discuss the corresponding uncertainties, in an effort to provide further evidence that the planned Phase B of CRISP will be successful in drilling the seismogenic coupling zone.

Description: Three destructive earthquakes along the Alaska subduction zone sourced transoceanic tsunamis during the past 70 years. Since it is reasoned that past rupture areas might again source tsunamis in the future, we studied potential asperities and barriers in the subduction zone by examining Quaternary Gulf of Alaska plate history, geophysical data, and morphology. We relate the aftershock areas to subducting lower plate relief and dissimilar materials in the seismogenic zone in the 1964 Kodiak and adjacent 1938 Semidi Islands earthquake segments. In the 1946 Unimak earthquake segment, the exposed lower plate seafloor lacks major relief that might organize great earthquake rupture. However, the upper plate contains a deep transverse-trending basin and basement ridges associated with the Eocene continental Alaska convergent margin transition to the Aleutian island arc. These upper plate features are sufficiently large to have affected rupture propagation. In addition, massive slope failure in the Unimak area may explain the local 42-m-high 1946 tsunami runup. Although Quaternary geologic and tectonic processes included accretion to form a frontal prism, the study of seismic images, samples, and continental slope physiography shows a previous history of tectonic erosion. Implied asperities and barriers in the seismogenic zone could organize future great earthquake rupture.

Description: The Costa Rica Seismogenesis Project (CRISP) is designed to explore the processes involved in the nucleation of large interplate earthquakes in erosional subduction zones. On 16 June 2002 a magnitude Mw=6.4 earthquake and its aftershocks may have nucleated at the subduction thrust to be penetrated and sampled by CRISP, ~40 km west of Osa Peninsula. Global event locations present uncertainties too large to prove that the event actually occurred at a location and depth reachable by riser drilling. We have compiled a database including foreshocks, the main shock, and ~400 aftershocks, with phase arrival times from all the seismological networks that recorded the 2002 Osa sequence locally. This includes a temporal network of ocean-bottom hydrophones (OBH) that happened to be installed close to the area at the time of the earthquake. The coverage increase provided by the OBH network allow us to better constrain the event relocations, and to further analyze the seismicity in the vicinity of Osa for the six months during which they were deployed. Moreover, we undertook teleseismic waveform inversion to provide additional constraints for the centroid depth of the 2002 Osa earthquake, allowing further study of the focal mechanism. Along the Costa Rican seismogenic zone, the 2002 Osa sequence is the most recent. It nucleated in the SE region of the forearc where this erosional margin is underthrust by a seamount covered ocean plate. A Mw=6.9 earthquake sequence occurred in 1999, co-located with a subducted ridge and associated seamounts. The Osa mainshock and first hours of aftershocks began in the CRISP area, ~30 km seaward of the 1999 sequence. In the following two weeks, subsequent aftershocks migrated into the 1999 aftershock area and also clustered in an area updip from it. The Osa updip seismicity apparently occurred where interplate temperatures are ~100°C or less. In this study, we present the relocation of the 2002 Osa earthquake sequence and background seismicity using different techniques and a moment tensor inversion for the mainshock, and discuss the corresponding uncertainties, in an effort to provide further evidence that the planned Phase B of CRISP will be successful in drilling the seismogenic coupling zone.

Description: Three destructive earthquakes along the Alaska subduction zone sourced transoceanic tsunamis during the past 70 years. Since it is reasoned that past rupture areas might again source tsunamis in the future, we studied potential asperities and barriers in the subduction zone by examining Quaternary Gulf of Alaska plate history, geophysical data, and morphology. We relate the aftershock areas to subducting lower plate relief and dissimilar materials in the seismogenic zone in the 1964 Kodiak and adjacent 1938 Semidi Islands earthquake segments. In the 1946 Unimak earthquake segment, the exposed lower plate seafloor lacks major relief that might organize great earthquake rupture. However, the upper plate contains a deep transverse-trending basin and basement ridges associated with the Eocene continental Alaska convergent margin transition to the Aleutian island arc. These upper plate features are sufficiently large to have affected rupture propagation. In addition, massive slope failure in the Unimak area may explain the local 42-m-high 1946 tsunami runup. Although Quaternary geologic and tectonic processes included accretion to form a frontal prism, the study of seismic images, samples, and continental slope physiography shows a previous history of tectonic erosion. Implied asperities and barriers in the seismogenic zone could organize future great earthquake rupture.