Abstract

The effects of the 1964 Alaskan earthquake on four shallow subtidal and intertidal areas in Prince William Sound have been recorded by mass movement of pre-quake sediment and changes in the grain-size distribution of post-quake sediment because of uplift. No changes could definitely be attributed to the large waves associated with the earthquake. Limited mass movement of pre-quake silt on the Rude River delta front has resulted in local warping and rupture of the sediment. Orientation of the associated rectangular cracks appears to be directly related to the geometry of the delta. Where pre-quake sediment was predominantly silt and thus difficult to erode (Rude River delta and Cordova, uplift 6 feet), post-quake sediment accumulation resulted only from introduced material, and the increase in grain size at the surface of deposition at the time of the earthquake was rapid and large. However,where pre-quake sediment was predominantly sandy and easy to erode (Macleod and Hanning Bays, uplift 29 feet), and where introduction of post-quake sediment was limited, there was only a small increase in the grain size of post-quake sediment, despite the large increase in wave energy due to the change from a subtidal to an intertidal environment of deposition. This is thought to result from absorption of wave energy in eroding and transporting the sand; as the supply of readily available sand decreases, more energy will become available for winnowing, and the grain size will increase as deposition proceeds. Thus, here a sudden increase in energy level of the environment of deposition will be preserved in the geologic record as a gradual increase in grain size.

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