An earthquake with a Modified Mercalli intensity of five was felt on 26 December 1775 by a Spanish colonial expedition led by Juan Bautista de Anza in present-day Riverside County (southeastern California), at the southeastern end of the Cahuilla Valley, near the San Jacinto fault system (Figure 1). After the well-documented earthquakes felt in the Los Angeles basin during the Gaspar de Portolá expedition 28 July to 3 August 1769 (Bancroft 1884; for a summary see Ellsworth 1990; Kovach 2004), this is the second-earliest documented event in the earthquake history of southern California. The...

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