TESTING OF THE SITE AMPLIFICATION HYPOTHESIS ON EARTHQUAKE DAMAGE THAT OCCURRED IN THE SAN FERNANDO VALLEY DURING THE 1994 NORTHRIDGE EARTHQUAKE WHICH IS RELATED TO THE UNDERLYING GEOLOGIC STRUCTURE

Based upon site amplification models that were published in 1996 for the 1994 Northridge Earthquake, scientists at UCLA proposed a fault and fold model to illustrate why earthquake damage that occurred in the relatively level area located in and around the City of Santa Monica. A similar model was proposed by the USGS for the earthquake damage that occurred in the San Fernando Valley in and around the CSUN campus in Northridge, and they tied the previous 1971 San Fernando, 1987 Whittier Narrows, 1991 Sierra Madre, and 1994 Northridge earthquakes were used to estimate site response in the urban Los Angeles, California, area. Later the USGS research scientists proposed a more complex model for a similar pattern of earthquake damage that occurred in the Sherman Oaks area in the San Fernando Valley. We reviewed numerous referred geological and geophysical studies of the 1994 Northridge earthquake that indicated that surface site effects are related to the structural focusing of earthquake energy at the surface of the earth from the underlying geologic structure.

Based upon available data and published maps of earthquake damaged to buildings, highways, bridges and utilities we checked the underlying geologic conditions using the existing published geologic cross sections for these same areas. We next tested this earthquake damage pattern with similar damage patterns from the 1971 San Fernando Earthquake to study the underlying geologic structure. It appeared that in all cases the underlying structural geology was either related to the synclinal folding of sediments and basement rock due to faulting, or to various models of faulting, segmentation, multiple faults, dense rock, and synclinal folding that caused site-specific amplification.

In addition, we tested the available data from several previous California earthquakes starting with the two 1812 earthquakes, 1925, 1927, 1933, 1952 and the 1983 Coalinga, and 1987 Whittier earthquakes and encountered the same results.