Modern Californian Earthquake Catalogs, Their Comparison

In an effort to create a combined earthquake catalog for Southern California, we analyze several earthquake catalogs compiled in California since the 1970s in order to determine the accuracy of their magnitude determination and earthquake focal mechanisms. The Caltech, Harvard CMT, and USGS moment tensor catalogs, two catalogs (northern and southern Californian) of focal mechanisms solutions, based on the HYPO71 program, as well as two catalogs of regional moment tensor inversion (from UC Berkeley and Caltech) are compared. We investigate the differences in magnitude estimates in these catalogs and focal mechanism orientation uncertainty for earthquakes with magnitude 4.7 and higher. We define the focal mechanism discrepancy by determining the smallest 3-D rotation angle needed to transform one double-couple mechanism into another. Only several tens of earthquakes can usually be correlated in different catalogs, thus the measurement reliability is relatively low. However, using pairwise differences between the catalogs we evaluate the magnitude uncertainties (standard deviations) as about 0.19, 0.08, 0.09, 0.22, 0.19, 0.08, and 0.09 for the listed catalogs, respectively. The average rotation angle is approximately 18, 18, 38, 33, 18, 20 degrees, for the last six catalogs in the list, respectively. We note that the focal mechanism accuracy for the first motion observations (HYPO71 solutions) is low, since the quoted angles are comparable with the average angle obtained for the random rotation of a double-couple source: it is 75.2 +/- 20.8 degrees.