Dam Performance - San Simeon Earthquake M6.5

Brief report submitted for U.S. Society on Dams newsletter. Full text: On December 22, 2003, a magnitude 6.5 earthquake occurred near California's central coast, 7 miles NE of San Simeon. Twelve state-regulated dams, one Army Corps of Engineers dam and seven small military dams experienced strong shaking with an estimated peak ground acceleration greater than 0.10g. The ACOE dam is a 135-ft high concrete variable arch (42 miles from the epicenter) and the rest are embankment dams, two of which are over 200 ft tall. The closest dam was 9 miles from the epicenter.

The strongest shaking at a dam site is estimated at 0.36g free-field PGA. One dam is instrumented with toe and crest accelerometers, which measured 0.11g and 0.22g respectively. Preliminary data from a new state-wide monitoring system provided "real-time" ground motion intensity maps, which were quickly published on the internet. The initial maps were updated with significant changes after analog data became available and additional review.

No information is available for the seven small military dams at this time, but all other dams affected by the earthquake performed well. Some visible damage occurred at only three of the embankment dams, and it was limited to minor longitudinal cracking along the crest. One dam, built in 1918, exhibited slightly more extensive cracking and was recommended for an updated seismic stability analysis. One dam is showing an unusual change in hydrogeology. A minor increase in seepage has occurred in conjunction with lower piezometer readings upstream, and slightly elevated readings downstream. Continued monitoring, instrument validation and investigation are in progress. Overall, the dams performed very well in this seismic event.