Arroyo, Ivonne G.

Behrmann, Jan

Flueh, Ernst R.

Grevemeyer, Ingo

English

OceanRep

The 2002 Osa (Costa Rica) earthquake sequence revisited: Will CRISP deep drilling reach the seismogenic coupling zone?I.G. Arroyo1, I. Grevemeyer1, J. Behrmann1, E.R. Flueh11GEOMAR Helmholtz Zentrum für Ozeanforschung Kiel, Wischhofstraße 1-3. 24148 Kiel, Germany (iarroyo@geomar.de)On March 11, 2011 the Magnitude 9.0 Tohoku earthquake hit NE Japan offnorthern Honshu. In contrast to the evidence from seismology and GPS-derived slip models, maximum (> 50 m) coseismic slip occurred near the trench axis, an area in forearcs generally believed to be characterized by stable sliding rather than seismic coupling and locking. This concept holds at least for accretionary subduction zones. However, after the Tohoku earthquake it must be challenged for erosive margins. Offshore of Central America, IODP may help to answer some of the questions, as the Costa Rica Seismogenesis Project (CRISP) is designed to explore the processes involved in the nucleation of large interplate earthquakes at tectonically erosive subduction zones. CRISP is a three-stage drilling project. CRISP-A was drilled in 2011 and CRISP-B successfully extended key holes to greater depth in late 2012. However, will CRISP-C ever be able to reach the seismogenic zone? The June 16, 2002 magnitude Mw=6.4 earthquake and its aftershocks may point towards an answer.Unfortunately, global event locations present uncertainties too large to prove that the event actually occurred at a location and depth accessible by riser drilling. We have compiled a database including foreshocks, the mainshock, 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 provided by the marine network enabled us to better constrain hypocentral parameters. Moreover, teleseismic waveform inversion provided additional constraints on the centroid depth of the 2002 Osa earthquake, allowing us to study the focal mechanism.Along the central portion of the Costa Rican subduction zone, the 2002 Osa sequence is the most recent large offshore event. It nucleated in the SE region of the forearc where this erosional margin is underthrust by seamounts covering the incoming oceanic plate. A Mw=6.9 earthquake sequence occurred in 1999, co-located with a subducted ridge and associated seamounts. The Osa main shock and first hours of aftershocks nucleated in the CRISP drilling area, ~30 km seaward of the 1999 sequence. In the following two weeks, aftershocks migrated into the 1999 aftershock area and also clustered in an area updip from the 1999 main shock. The updip seismicity in the area of CRIP drilling near the Osa peninsula, however, apparently occurred where interplate temperatures are ~100°C or less. These temperature estimates are based on surface heat flow and thermal modelling. CRISP-A drilling confirmed the low temperatures, thus challenging previous concepts of seismogenesis at erosive margins.Deutsche Forschungsgemeinschaft funded project CRISP-EQ started in October 2012 to yield precise hypocentral parameters for the Osa mainshock and its aftershocks. We have updated location precision replacing a 1-D reference model with a so called 2.5-D velocitiy model that incoroporates strong lateral changes in the structure of the active continental margin of Costa Rica. The resulting hypocentre occurs to the NW of the epicentre obtained from the 1-D model, and several kilometres seaward with respect to global location estimates. While the global Centroid Moment Tension inversion procedure of Lamont Earth Observatory of Columbia University found a centroid depth (i.e., location of main moment release) of 16 km, our estimates now suggest a nucleation and centroid depth of 6 km. Thus, our results support the notion that CRISP-C will be able to reach the seismogenic zone. In the future, we plan to derive a full 3-D velocity field for SE Costa Rica and its Pacific margin from a unique dataset of several temporary deployments of OBS and land stations, that will provide much lower error bounds for the location of the Osa seismic sequence. Further, along with constraints from CRISP-A and B drilling we will survey the properties controlling earthquake nucleation and rupture propagation at erosive continetal margins.Bitte benutzen Sie die Formatvorlage 'Figure-Text' für Ihre Abbildungsunterschriften.Please use the style sheet 'Figure-Text' for figure captions.

The 2002 Osa (Costa Rica) earthquake sequence revisited: Will CRISP deep drilling reach the seismogenic coupling zone?

http://oceanrep.geomar.de/21172/1/Arroyo_posterIODP_abstract.doc

The 2002 Osa (Costa Rica) earthquake sequence revisited: Will CRISP deep drilling reach the seismogenic coupling zone?

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