David P. Finlayson

New Imaging of Submarine Landslides from the 1964 Earthquake Near Whittier, Alaska, and a Comparison to Failures in Other Alaskan Fjords

The 1964 Alaska M w 9.2 earthquake triggered numerous submarine slope failures in fjords of southern Alaska. These failures generated local tsunamis, such as at Whittier, where they inundated the town within 4 min of the beginning of shaking. Run-up was up to 32 m, with 13 casualties. We collected new multibeam bathymetry and high-resolution sparker seismic data in Passage Canal, and we examined bathymetry changes before and after the earthquake. The data reveal the debris flow deposit from the 1964 landslides, which covers the western 5 km of the fjord bottom. Individual blocks in the flow are up to 145-m wide and 25-m tall. Bathymetry changes show the mass transfer deposits originated from the fjord head and Whittier Creek deltas and had a volume of about 42 million m3. The 1964 deposit has an average thickness of ∼5.4 m. Beyond the debris flow, the failures likely deposited a ∼4.6-m thick megaturbidite in a distal basin. We have studied the 1964 submarine landslides in three fjords. All involved failure of the fjord-head delta. All failures eroded basin-floor sediments and incorporated them as they travelled. All the failures deposited blocks, but their size and travel distances varied greatly. We find a correlation between maximum block size and maximum tsunami run-up regardless of the volume of the slides. Lastly, the fjord’s margins were influenced by increased supply of glacial sediments during the little ice age, which along with a long interseismic interval (∼900 years) may have caused the 1964 earthquake to produce particularly numerous and large submarine landslides.

Bathymetry and digital elevation models of Coyote Creek and Alviso Slough, South San Francisco Bay, California

A GIS polygon shapefile outlining the region where we experienced IMU problems during the October 2012 survey. During post-processing of the October 2012 survey tie line analyses revealed a depth-bias in lines collected on the western intertidal flats on October 19th. The cause of this bias is uncertain; however it may be the result of an imprecise IMU position calibration on that day. These intertidal flat survey lines collected on October 19th were shoaled by 8 cm to minimize offsets between tie lines. After the shift was applied the mean difference of tie line intersections for this survey was 2 cm (SD = 4). Note that Guadalupe Slough was also surveyed on October 19th, but data within the slough did not appear to show the same offset as soundings within the flats, and as a result, were not adjusted. Although our best effort was made to correct these data, the accuracy of elevations within these polygons is less reliable than for the remainder of these datasets.

Bathymetry, acoustic backscatter, and seafloor character of Farallon Escarpment and Rittenburg Bank, northern California

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