A comprehensive investigation of an offshore active fault in the Western Sagami Bay, central Japan

Offshore active faults, especially those in the deep sea, are very difficult to study because of the water and sedimentary cover. To characterize the nature and geometry of offshore active faults, a combination of methods must be employed. Generally, seismic profiling is used to map these faults, but often only fault-related folds rather than fracture planes are imaged. Multi-beam swath bathymetry provides information on the structure and growth history of a fault because movements of an active fault are reflected in the bottom morphology. Submersible and deep-tow surveys allow direct observations of deformations on the seafloor (including fracture zones and microstructures). In the deep sea, linearly aligned cold seep communities provide indirect evidence for active faults and the spatial migration of their activities. The Western Sagami Bay fault (WSBF) in the western Sagami Bay off central Japan is an active fault that has been studied in detail using the above methods. The bottom morphology, fractured breccias directly observed and photographed, seismic profiles, as well as distribution and migration of cold seep communities provide evidence for the nature and geometry of the fault. Focal mechanism solutions of selected earthquakes in the western Sagami Bay during the period from 1900 to 1995 show that the maximum compression trends NW-SE and the minimum stress axis strikes NE-SW, a stress pattern indicating a left-lateral strike-slip fault.

Offshore active faults, especially those in the deep sea, are very difficult to study because of the water and sedimentary cover. To characterize the nature and geometry of offshore active faults, a combination of methods must be employed. Generally, seismic profiling is used to map these faults, but often only fault-related folds rather than fracture planes are imaged. Multi-beam swath bathymetry provides information on the structure and growth history of a fault because movements of an active fault are reflected in the bottom morphology. Submersible and deep-tow surveys allow direct observations of deformations on the seafloor (including fracture zones and microstructures). In the deep sea, linearly aligned cold seep communities provide indirect evidence for active faults and the spatial migration of their activities.