Southern California Earthquake during the 1775 Anza Expedition

Abstract

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 Portola 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 1775 event was cursorily mentioned by Townley and Allen (1939) and Toppozada et al. (1981) but is not listed in the 1769–2000 catalog of magnitude 4 and greater earthquakes by the California Geological Survey (http://www.consrv.ca.gov/cgs/rghm/quakes), whereas the 5.5 cut-off magnitude in the more recent listings of historical earthquakes by Toppozada et al. (2002) and Toppozada and Branum (2002) was too high for this event to be included. The Anza expedition began at San Miguel de Horcasitas, Sonora, on 29 September 1775 and left Tubac, Sonora, on 23 October 1775. The 240 participants, among them 110 children, trekked with a stock herd along the Gila River to its junction with the Colorado River, and from there to Mision San Gabriel de los Temblores in the Los Angeles basin, which they reached 4 January 1776 (Montane Marti 1989). From San Gabriel, the colonists proceeded to Monterey, Alta California, and from there to the bay area to become the …