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

Abstract

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 + 1.41 M − 0.00526 * Δ h − 2.63 * log Δ h , where Δ 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.