Estimating locations and magnitudes of earthquakes in southern California from modified Mercalli intensities

Modified Mercalli intensity (MMI) assignments, instrumental moment magnitudes M, and epicenter locations of thirteen 5.6 ??? M ??? 7.1 "training-set" events in southern California were used to obtain the attenuation relation MMI = 1.64 + 1.41M - 0.00526 * ??h - 2.63 * log ??h, where ??h is the hypocentral distance in kilometers and M is moment magnitude. Intensity magnitudes MI and locations for five 5.9 ??? M ??? 7.3 independent test events were consistent with the instrumental source parameters. Fourteen "historical" earthquakes between 1890 and 1927 were then analyzed. Of particular interest are the MI 7.2 9 February 1890 and MI 6.6 28 May 1892 earthquakes, which were previously assumed to have occurred near the southern San Jacinto fault; a more likely location is in the Eastern California Shear Zone (ECSZ). These events, and the 1992 M 7.3 Landers and 1999 M 7.1 Hector Mine events, suggest that the ECSZ has been seismically active since at least the end of the nineteenth century. The earthquake catalog completeness level in the ECSZ is ???M 6.5 at least until the early twentieth century.