EXAMINATION OF FOCAL DEPTH DETERMINED BY GROUND SHAKING INTENSITY IN THE INTERMOUNTAIN WESTERN U.S

The potential for damaging earthquakes in the intermountain western U.S. combined with the sparse record of larger events requires that an attempt be made to increase the earthquake data base. This study compares data available from earthquake intensities within the intermountain western U.S. to instrumental data available for the same events. Because of the requirement for the inclusion of intensity data, as well as instrumental data that includes both magnitude and depth of focus, the data set assembled is relatively small. The goal is to examine the data set to see if a relationship between intensity data and depth of focus can be demonstrated. This will hopefully lead to the use of intensity data to determine depth of focus for early to pre-instrumental earthquakes in the region, expanding the present data base for that focal parameter.

Focal depth and its relation to ground shaking intensity has previously been quantified by Bath (Bath, 1973) and more recently by Brumbaugh (2001). The latter study modified the Bath equation primarily by substituting the area within the MMV contour for the radius of the felt area. This was done so that the better observed MMV intensity data could be used to improve depth estimates. This research has led to a closer examination of a direct comparison of the MMV area to focal depth. Two small but carefully selected data sets were assembled for the comparison. The first data set was analyzed by a simple regression curve and shows a clear reelationship between magnitude (ML) and Av with a variance of r² =.9984. This result was expected and has been verified by equations developed by other researchers. The second relationship between Av and depth (h) also was analyzed by a regression plot and resulted in a curve with a variance of r² =.9761. This result was also considered to be significant and suggests a relationship between the better constrained area within Av and depth of focus. A preliminary test of the relationship was made by comparing the intensity data and instrumental data for two Arizona earthquakes. The 7-21-1959 ML 5.7 and the 1993 Mw 5.3 earthquakes had the required depth of focus and the Av available from intensity data. The depths determined from the Av areas agreed well with the instrumental depths.