Classifying Offshore Faults for Hazard Assessment: The Advantage of Working in the Marine Environment

The Israeli continental slope is dissected by numerous salt-related, thin-skinned, normal faults, forming tens meter high seabed scarps. However, this faulted zone is currently being crossed by pipelines transporting gas from offshore fields. This situation highlights the need to assess the hazard posed by these faults. Unfortunately, geohazards mitigation methodology in the marine environment lags tens of years after the well-established methodology onshore. One possibility is to follow the onshore practice. In that case, a Holocene horizon needs to be detected in the sub-seabed; then, for each fault, the question of whether this horizon is displaced or not needs to be answered. This requires high-resolution seismic surveys and numerous coring that cannot be carried out for large regions. Here we suggest a different approach. Instead of imitating the onshore practice, we take advantage of the unique opportunities available in the marine environment, where seismic data is superior in quality and quantity. Instead of investing huge efforts (multiple coring to a dated horizon) in finding whether or not each specific fault in the study area meets a pre-defined criterion of activeness, we map the subsurface and determine the levels of fault hazard, based on the amount of recent displacement and the size of the fault plane. Instead of answering a yes-and-no question (active or not active), we classify faults into three hazard levels, highlighting green and red zones for master planning. A particularly red zone is the upper slope south of the Dor disturbance, where a series of big listric faults with large displacements rupture the seabed. Noteworthy, the sedimentation rate in this area is four times faster than the displacement rate, yet fault scarps are still a few tens of meters high. We suggest that this indicates seismic rupture rather than creep.