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Estimating Locations and Magnitudes of Earthquakes in Southern California from Modified Mercalli Intensities

¹ U.S. Geological Survey
Earthquakes Hazards Team
Menlo Park, California 94025-3591

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 M_I 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 M_I 7.2 9 February 1890 and M_I 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.

Online material: Figures and table of empirical MMI site corrections.