An Empirical Interpretation of the Effects of Topography on Ground Motion of the San Fernando, California, Earthquake, 9 February 1971.
ARMY ENGINEER WATERWAYS EXPERIMENT STATION VICKSBURG MISS
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This study attempts to determine site effects on earthquake ground motion and the correlation between acceleration andor velocity generated during the San Fernando Earthquake and topography of the San Gabriel Mountain range. It was found that the contours of peak acceleration and peak velocity generally follow the topography of the San Gabriel Mountain range. The topographical effects on the ground motion could be interpreted in a simple manner as a function of elevation and direction of wave transmission path. The elevation and direction become the dominant factors in the distribution of the ground motion in the near field. A simple, practical method for calculating the bedrock motion using the ground motion-elevation gradient has been applied in the area south of Kagel Mountain and north of Santa Monica Mountain, in the San Fernando Valley. This method is validated using after-shock data. This ground motion-elevation gradient method was applied to an area where the topography has its highest elevation at the epicentral region and decreases in elevation to the surrounding locations in the near field within 30 km. In any case, when the epicenter occurs at an elevation lower than the elevation of the surrounding area, this gradient method may not be applicable and must be tested for this alternate condition.
- Geology, Geochemistry and Mineralogy