We investigated two trench sites to determine Holocene paleoseismicity on the flanks of Agua Tibia Mountain along the Elsinore fault, southern California. Our investigation revealed that a minimum of four earthquakes produced surface rupture on this southern reach of the Temecula segment of the fault in the past 4.5 ka. We also recognized evidence for an earthquake that predated 4.5 ka, possibly by only a short period of time. Radiocarbon and historical data constrain the timing of the most recent earthquake to between A.D. 1655 and the onset of construction of the Pala Mission complex, in about A.D. 1810.
Four of the recognized events occurred between 2.7 ka and just before 4.5 ka, whereas the fifth occurred less than 340 years ago. These observations suggest that either earthquake occurrence has been irregular at this site or that the paleoseismic record at Agua Tibia Mountain is incomplete. If the latter is correct and if the record between 2.7 and 4.5 ka is complete, then our preferred interpretation suggests an average return time for surface-rupturing events of 550 to 600 years. The best estimates of event times for the three oldest dated events are 2.8, 3.25, and 4.0 ka, with another event just before 4.5 ka, indicating return intervals for this time period ranging between 450 and 750 years. Using the previously determined slip rate for the Temecula segment of the Elsinore fault of 5 mm/yr, and assuming that 550-600-year average return time is correct, the likelihood for an earthquake on the Temecula segment in the next 50 years is less than 5%.