Offshore active faults, especially those in the deep sea, are very difficultto study because of the water and sedimentary cover. To characterize the nature and geometry ofoffshore active faults, a combination of methods must be employed. Generally, seismic profiling isused 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 faultbecause movements of an active fault are reflected in the bottom morphology. Submersible anddeep-tow surveys allow direct observations of deformations on the seafloor (including fracture zonesand microstructures). In the deep sea, linearly aligned cold seep communities provide indirectevidence for active faults and the spatial migration of their activities. The Western Sagami Bayfault (WSBF) in the western Sagami Bay off central Japan is an active fault that has been studied indetail using the above methods. The bottom morphology, fractured breccias directly observed andphotographed, seismic profiles, as well as distribution and migration of cold seep communitiesprovide evidence for the nature and geometry of the fault. Focal mechanism solutions of selectedearthquakes in the western Sagami Bay during the period from 1900 to 1995 show that the maximumcompression trends NW-SE and the minimum stress axis strikes NE-SW, a stress pattern indicating aleft-lateral strike-slip fault.
