GEOLOGIC SETTING AND REGIONAL SEISMICITY OF THE ERUPTING AUGUSTINE VOLCANO, LOWER COOK INLET, ALASKA: THE INFLUENCE OF BEDROCK GEOLOGY ON VOLCANIC PROCESSES

Strike-slip faults in the bedrock below a volcano can profoundly influence the volcano's development (e.g. Tibaldi et al., 2005), in part by promoting landslides from particular sectors of the volcano's flank (e.g. Lagmay et al., 2000; Norini and Lagmay, 2005). We describe a strike-slip fault below Augustine Volcano in lower Cook Inlet, Alaska. This volcano entered a new eruptive phase in January 2006. Seismicity associated with this volcanism tends to be shallow (<1-2 km) and concentrated directly below the volcanic vent. In contrast, the regional seismicity of lower Cook Inlet is diffuse and reveals few patterns, except for one steeply northwest-dipping (60˚-70˚) earthquake cluster located 25 km northeast of the volcano. This cluster is defined by small-magnitude (M~2-3) events and extends in depth from 5 km to 20 km. A grid of oil-industry seismic-reflection data show that this cluster underlies a structural zone made up of northwest-dipping reverse faults and faulted anticlines. Focal mechanisms for an M3.3 (7 km deep) and an M3.7 (15 km deep) event indicate left-lateral-oblique displacement within the structural zone. We propose that the zone is underlain at depth by a left-lateral strike-slip fault that cuts deeply (~20 km) into the crust, as indicated by the earthquake cluster. This thick-skinned style of deformation is similar to the style found in other areas of the Cook Inlet basin (e.g. Bruhn and Haeussler, 2006).

The strike-slip fault, trends southwestward toward Augustine Volcano, where the fault intersects the west-striking Augustine-Seldovia arch. This arch is a fundamental feature of the Cook Inlet basin because it forms the south flank of the thick (7 km) accumulation of Cenozoic rocks under the north part of the basin. South of the arch, Cenozoic rocks are thin (~1 km) or absent. The strike-slip fault does not appear in Mesozoic rocks exposed on the northern Alaska Peninsula, so if the fault ends offshore and near the volcano, then crustal extension near the fault's termination could have determined the volcano's location. Furthermore, the intersection between the arch and the strike-slip fault could have caused complex crustal deformation that played a role in the volcano's development by, for example, enabling magma migration and controlling where zones of weakness developed within the volcano's flank.