Seismogenesis of megathrust earthquakes at the Costa Rican erosive continental margin
Final Report Abstract
We investigated 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 several hundreds of aftershocks 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 re-locate this catalogue using a nonlinear probabilistic approach within 1-D and 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 analyse 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 down-dip 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. The Mw=6.4 2002 Osa earthquake ruptured at a depth range where the fault zone can be reach by riser drilling.
Publications
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(2013) The 2002 Osa (Costa Rica) earthquake sequence revisited: will CRISP deep drilling reach the seismogenic coupling zone? ICDP/IODP Kolloquium Freiberg 2013, 25.-27.03.2013, Freiberg
Arroyo, I. G., I. Grevemeyer, J. Behrmann, and E. R. Flueh
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(2014), Interplate seismicity at the CRISP drilling site: The 2002 Mw 6.4 Osa Earthquake at the southeastern end of the Middle America Trench, Geochem. Geophys. Geosyst., 15, 3035–3050
Arroyo, I. G., I. Grevemeyer, C. R. Ranero, and R. von Huene
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(2014), The seismogenic zone in the Central Costa Rican Pacific margin: high-quality hypocentres from an amphibious network, Int. J. Earth Sci., 103, 1747–1764
Arroyo, I. G., S. Husen, and E. R. Flueh
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(2015), Constraining source locations of shallow subduction megathrust earthquakes in 1-D and 3-D velocity models – A case study of the 2002 Mw=6.4 Osa earthquake, Costa Rica, AGU Fall Meeting 2015, 14.-18.12.2015, San Francisco, USA
Arroyo, I. G., and I. Grevemeyer
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(2015), Density structure and geometry of the Costa Rican subduction zone from 3-D gravity modeling and local earthquake data, Solid Earth, 6, 1169–1183
Lücke, O. H., and I. G. Arroyo
