Seismicity studies, based on both historic data and data accummulated from the operation of a high resolution seismic network, provide some important input into quantitative seismic risk studies for areas of offshore petroleum development in Lower Cook Inlet, Shelikof Strait and off Kodiak Island. The greatest seismic risk is associated with the shallow seismic thrust zone of the subducting Pacific plate. There is a high probability that a great (M > 7.8) earthquake will occur in the Shumagin gap within the lifetime of any potential petroleum development in that area. The relative sharpness of the transarc boundaries of the aftershock zones of great earthquakes in the Aleutians suggests that the shallow thrust zone is segmented into separate tectonic blocks which release accumulated strain independently. There is geologic and seismic evidence that one such boundary traverses the arc near the southern-edge of Kodiak and separates the 1964 aftershock zone from the Shumagin gap. Additional segmentation may exist within the 1964 aftershock zone. Seismic risk appears lower in the other offshore areas that do not overlie the shallow subduction thrust zone, but is still primarily associated with the subduction process. The shallow, crustral seismic activity (less than 50 km hypocentral depth) outside the subduction thrust zone does not correlate with the major known fault systems of the area, and is of a more diffuse nature.
Ideally, a seismic risk study should provide the design engineer with the exact time histories of ground motions to which a given structure would be exposed during its lifetime. It would then be a rather straightforward, though not necessarily easy matter, to design a structure safe and within economic reason. In principle, such an approach requires the prediction of earthquakes in time and space and the description of (a) the motion generated in the source area as a consequence of the earthquake dislocation process, (b) the modification of the source motion during its propagation towards the site, (c) the further modification by surficial geology and soil layers beneath the site, and (d) any soil-structure interaction. Many details of these processes are still poorly understood and the state of the art of seismic risk analysis allows only specification of the level of certain ground motion parameters for a selected probability of exceedence, i.e., the analysis has to incorporate the uncertainties associated with the dominant contributing factors. The largest uncertainties are associated with the determination of the recurrence rate of large earthquakes in time and space and with the definition of the source parameters.
This study attempts to remove some of these uncertainties by developing a detailed seismotectonic model of a portion of the eastern Aleutian arc. This model should help to better define the seismic sources and the dominant source mechanisms. The study is part of the Outer Continental Shelf Environmental Assessment Program (OCSEAP) conducted by BLM/NOAA prior to offshore lease sales.
