Bulletin of the Seismological Society of America
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Bulletin of the Seismological Society of America; December 2002; v. 92; no. 8; p. 3180-3189; DOI: 10.1785/0120010171
© 2002 Seismological Society of America
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Article

Anomalous Concentrations of Seismically Triggered Rock Falls in Pacoima Canyon: Are They Caused by Highly Susceptible Slopes or Local Amplification of Seismic Shaking?

Edwin L. Harp and Randall W. Jibson

USGS
Box 25046, MS 966
Denver Federal Center
Denver, CO 80225
(E.L.H.)

Manuscript received 21 May 2001.

Anomalously high concentrations of rock falls were triggered in Pacoima Canyon (Los Angeles, California) during the 1994 Northridge earthquake. Similar concentrations were also documented from the 1971 San Fernando earthquake. Using an engineering rock-mass classification that evaluates the susceptibility of rock slopes to seismic failure based on the fracture properties of a rock mass (in terms of a numerical "Q-value" that describes rock quality), the rock slopes in Pacoima Canyon were compared with rock slopes in surrounding areas where topography and lithology are similar, but rock-fall concentrations from the earthquakes were much lower. A statistical comparison of Q-values from five sites surrounding Pacoima Canyon indicates that seismic susceptibilities are similar to those within Pacoima Canyon; differences in the characteristics of rock slopes between these sites are not sufficient to account for the relatively high concentrations of rock falls within Pacoima Canyon as compared to low concentrations elsewhere. By eliminating susceptibility differences as a cause, the most likely explanation for the differences in rock-fall concentrations is anomalously high shaking levels in Pacoima Canyon, possibly resulting from topographic amplification within the canyon.






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