Although the written history of California is not long, records of earthquakes exist that affected the Spanish missions that were constructed beginning in the late 18th century. Those records ceased when the missions were secularized in 1834, and from that point until the California Gold Rush in the 1840s, records were sparse. Other sources for the occurrence of earthquakes usually came from ship captains and other explorers. The earliest known earthquake was documented in 1769 by the Spanish explorers and Catholic missionaries of the Portolá expedition as they traveled northward from San Diego along the Santa Ana River near the present site of Los Angeles. For the period 1850–2004, there was about one potentially damaging event per year on average, though many of these did not cause serio