Reassessment of seismically induced, tsunamigenic submarine slope failures in Port Valdez, Alaska, USA

Abstract

The M9.2 Alaska earthquake of 1964 caused major damage to the port facilities and town of Valdez, most of it the result of submarine landslide and the consequent tsunamis. Recent bathymetric multibeam surveys, high-resolution subbottom profiles, and dated sediment cores in Port Valdez supply new information about the morphology and character of the landslide deposits. A comparison of pre- and post-earthquake bathymetry provides an estimate of the net volume of landslide debris deposited in the basin and the volume of sediment removed from the source region. Landslide features include (1) large blocks (up to 40-m high) near the location of the greatest tsunami-wave runup (∼50 m), (2) two debris lobes associated with the blocks, (3) a series of gullies, channels and talus, near the fjord-head delta and badly damaged old town of Valdez, and (4) the front of a debris lobe that flowed half-way down the fjord from the east end. A transparent unit, with a maximum thickness of 10 m, above the debris flow deposits in the deepest part of the fjord, likely represents a lower density sediment flow related to the other failure deposits. Calculations from the bathymetric difference map suggest a total net volume of displaced sediment on the order of 0.4 km3. However, an integration of the volume of debris flow deposits (mapped according to their acoustic signature) indicates a gross volume of about 1 km3, showing that the landslides incorporated significant additional sediment from the fjord floor into the debris flow as it moved. Despite the large volume of sediment failures in the eastern part of the fjord, smaller, but more coherent block failures in the western part appear to be the primary cause of the largest tsunamis that impacted the shorelines.