Case Studies: Alaska, Columbia

Turnagain Heights, Anchorage, Alaska

When: 1964 (Great Alaskan Earthquake)
Type: Liquefaction & Rotational Slides
Cause: Magnitude 9.2 earthquake triggered numerous mass movements. The area is underlain by rocks composed of glacially ground, clay-rich sediments. Shaking from the earthquake liquefied the clays. Numerous rotational slides were trapped deep in the clay layer, so the blocks deformed internally, moving block above.

Vargas, Venezuela, 1999

When: 14 – 16 December 1999
Type: Debris Flow / flooding
Cause: Unusually heavy rains lead to catastrophic mass wasting and flooding. Over 8,000 individual residences, and 700 apartment buildings were destroyed or damaged. One neighborhood, Los Corales, was completely buried in 3 meters of debris. Roads, telephones, electricity, etc cut off. Total economic losses were estimated to be approximately $1.79 Billion.

Cenus records for the area were not up to date, but it was estimated to have a population of ~300,000. The death toll was ~30,000, or 10% of the population. Only 1000 bodies were recovered - the rest were either buried in mud or were swept out to sea.

Below is a chart (in Spanish) showing rainfall amounts for December 1999. (source: http://pubs.usgs.gov/of/2001/ofr-01-0144/)




Why was there so much damage? Cities were built on alluvial fans, which form via debris flows. Older homes and communities were not built on fans, and suffered little damage as a result.

This area has a history of debris flows, as is common on alluvial fans. There have been an average of two slides/century since records started being kept in the late 1700s: February 1798; August 1912, January 1914; November 1938, May 1944, November 1944, August 1948, and February 1951.

Aerial view of debris-flow deposition resulting in widespread destruction on the Caraballeda fan of the Quebrada San Julián.  Avulsion of the main channel (left side of photo) resulted in deposits up to 6-m in thickness and totaling about 1.8 million cubic meters of bouldery debris.  Secondary new flood channels are visible through center of fan to the lower right of photo (Photo by Lawson Smith, US ACE). Source( http://pubs.usgs.gov/of/2001/ofr-01-0144/)

Vargas, Venezuela, 2005

When: December 2005
Type: Debris Flow / flooding
Cause: Unusually heavy rains lead to another debris flow.
Result: The numbers vary from 13 – 53 dead and 20,000+ homeless in the La Guaira region, Vargas, Venezuela. Lessons learned from the 1999 event made the government & people better prepared.