Shallow seismogenic zone detected from an offshore‐onshore temporary seismic network in the Esmeraldas area (northern Ecuador)

For a given site, many factors control the seismic risk. Earthquake magnitude, hypocentral distance, rupture mechanism, site effects and site vulnerability are among the most important. This article deals with one of these factors: the depth of the seismogenic zone, in the northern Ecuadorian subduction system, beneath a highly vulnerable site, the city of Esmeraldas and its industrial complex, the Ecuadorian oil refinery and shipping terminal. To address this problem, we analyzed data from a three weeks passive seismological experiment, conducted in the spring of 1998, using 13 Ocean Bottom Seismometers and 10 portable land‐stations. A preliminary interpretation of wide‐angle data obtained in the fall of 2000, in the Manta area, 100 km South of the study area, unambiguously indicates the presence of a velocity inversion in the Ecuadorian margin velocity structure. This velocity inversion is characterized by a shadow‐zone of ∼1 s on the record‐sections, and is interpreted as the result of a velocity contrast between the upper plate structure and the sedimentary and basaltic layer II of the subducted oceanic Nazca plate. One‐dimensional velocity models are deduced from these wide‐angle data and are used for earthquake location in the Esmeraldas area. This highly improved the hypocentral parameter determinations. The updip limit of the seismogenic zone is found at a depth of ∼12 km, 35 km eastward of the trench, and the depth of the seismogenic zone below the Esmeraldas city is found at ∼20 km. This shallow depth of the seismogenic zone dramatically increases the seismic hazard of the area.