This comes from an account of the Spanish exploration of California under Gaspar de Portola, the first governor of California.

As the expedition entered Orange County on July 28, 1769, they experienced a series of violent earthquakes that changed the course of the Santa Ana River. They prayed and watched the native Indians do the same. As a result the account notes that:

Father Crespi later wrote that at that very moment a violent earthquake struck and the river was thus proclaimed Rio de Los Dulcime Nombre de Jesus de Los Temblores.

An earthquake Web site cataloging earthquakes through history notes:

The earthquake history of California serendipitously begins with the first overland expedition through the State in 1769. In response to the perceived threat posed by Russian expansion into the northern Pacific and growing British presence in the northwestern Pacific, Spain embarked on the colonization of present-day California through the establishment of a series of Franciscan missions, supported by military garrisons at San Diego and Monterey. In the summer of 1769, Gaspar de Portola led the first expedition from San Diego to establish a land route to Monterey.

On July 28, while camped along the Santa Ana River, about 50 km southeast of Los Angeles, a sharp earthquake was felt that “*** lasted about half as long as an Ave Maria.”

From the diaries of three members of the expedition, we know that earthquakes were felt on nearly a daily basis through August 3, as the party traveled northwestward to near San Gabriel and then westward across Los Angeles to the Pacific. The diary of Fray Juan Crespi (Bolton, 1927) mentions no fewer than a dozen aftershocks, some described as violent. After August 4, no further earthquakes were mentioned as the expedition traveled into the San Fernando Valley and exited to the north.