Large-scale Basement-involved Landslides, California Continental Borderland Article by Kammerling, M. J., M. R. Legg Pure and Applied Geophysics. 160 (10-11): Presented by Samuel W. Franklin, Geography major, GEOG 370, February 24, 2008

Determining Possibility of a Landslide  Problem: Large seafloor relief, shallow metamorphic basement, and seismic activity in the California Continental Borderland combine to produce major submarine slides capable of generating local tsunamis.  Hypothesis: Measurements and analysis of slopes will determine the likelihood of a large-scale underwater slide.

Methods & Data Collection Slope Stability  Site: Off shore of Southern California and Baja California, Mexico  Used seafloor mapping by USGS to find possible problematic areas.  Used submersibles to get actual slopes, overhangs and rock type.  Quantified slope stability using Newmark’s dynamic slope stability analysis technique.  Then compared areas with large slopes to areas affected by seismic activity.  Used area of possible slide size vs. speed to determine the varying intensity of tsunami.

Results  In the Borderland Area researched there is an average relative seismic slope stability of moderate to unstable (scale used: Very Stable, High, Moderate, Low, Unstable).  Because major active faults lie along these large escarpments the potential for large scale, submarine landslides exists.  There are two specific areas found to be in danger and have been the cause of tsunami damage in the past.

Conclusion  Low stability slopes could be prone to catastrophic failure (large landslides) and tsunami generation.  Seismic activity greater than 6.5 on the Mercalli scale will result in a tsunami  Criticisms: Focused highly on probability. And there has not been a definitively destructive tsunami in seventy years.