Neotectonics of Graciosa island (Azores): a contribution to seismic hazard assessment of a volcanic area in a complex geodynamic setting

摘要

Graciosa is a mid-Pleistocene to Holocene volcanic island that lies in a?complex plate boundary between the North American, Eurasian, and?Nubian plates. Large fault scarps displace the oldest (Middle Pleistocene)?volcanic units, but in the younger areas recent volcanism?(Holocene to Upper Pleistocene) conceals the surface expression of?faulting, limiting neotectonic observations. The large displacement accumulated?by the older volcanic units when compared with the younger?formations suggests a variability of deformation rates and the possibility?of alternating periods of higher and lower tectonic deformation?rates; this would increase the recurrence interval of surface rupturing?earthquakes. Nevertheless, in historical times a few destructive earthquakes?affected the island attesting for its seismic hazard. Regarding?the structural data, two main fault systems, incompatible with a single?stress field, were identified at Graciosa Island. Thus, it is proposed?that the region is affected by two alternating stress fields. The stress?field #1 corresponds to the regional stress regime proposed by several?authors for the interplate shear zone that constitutes the Azorean segment?of the Eurasia-Nubia plate boundary. It is suggested that the?stress field #2 will act when the area under the influence of the regional?stress field #1 narrows as a result of variations in the differential?spreading rates north and south of Azores. The islands closer to the?edge of the sheared region will temporarily come under the influence of?a different (external) stress field (stress field #2). Such data support?the concept that, in the Azores, the Eurasia-Nubia boundary corresponds?to a complex and wide deformation zone, variable in time.