Seismic Evidences for Complex Faulting Through the Gulf of Guayaquil, Ecuador

Description

Along the Ecuadorian margin, the North Andean Sliver is moving in the northeastward direction due tothe oblique subduction of the Nazca plate. The opening of the gulf of Guayaquil is a consequence of thismotion. Two principal models compete to explain the opening. One proposes an opening achievedessentially with strike slip motion along a single major fault through the gulf, the other with a combinationof strike slip and normal faulting on both sides of the gulf. The consequences in term of seismic hazardare very different. A single strike slip fault model could imply a long fault segment capable of generatinglarge magnitude events. In contrast, a multi segments composite fault system will give conditions forproducing small to medium size earthquakes. The southern Ecuador subduction zone is characterizedby the absence of large historical earthquake. Data from the historical and instrumental seismicity formagnitude above 4 show the fore arc has a high level of seismic activity within and around the gulf thatconnects to the crustal seismic activity of the volcanic arc. In contrast, the fore arc elsewhere showsvery little or no seismic activity between the interplate zone and the volcanic arc. Regional and globalCMTS data show mostly diffuse strike slip mechanisms that do not line up on a single fault system. Wepresent new earthquake data from the recently upgraded national seismic network of Ecuador. Theyprovide the first image of SW-NE trending crustal faults stretching in the central part of the gulf andrunning eastward between Puna island and the southern coast of the gulf. It appears to be a complexmulti segments fault system. The observed seismic alignments do not cross the Puna island as it waspostulated in previous regional tectonic studies. Others seismic clusters are observed parallel to thenorthern coast of the gulf, indicating active structures eventually accommodating the North Southopening of the gulf through normal faulting.

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