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 + 1.41M − 0.00526 * Δh − 2.63 * log Δh, where Δh 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.

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

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