Paleoseismology of the Elsinore fault at Agua Tibia Mountain, southern California

We investigated two trench sites to determine Holocene paleoseismicity on t he flanks of Agua Tibia Mountain along the Elsinore fault, southern Califor nia. Our investigation revealed that a minimum of four earthquakes produced surface rupture on this southern reach of the Temecula segment of the faul t in the past 4.5 ka. We also recognized evidence for an earthquake that pr edated 4.5 ka, possibly by only a short period of time. Radiocarbon and his torical 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 abo ut. A.D. 1810. Four of the recognized events occurred between 2.7 ka and just before 4.5 k a, whereas the fifth occurred less than 340 years ago. These observations s uggest 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, the n 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 even t just before 4.5 ka, indicating return intervals for this time period rang ing between 450 and 750 years. Using the previously determined slip rate fo r 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 earthqua ke on the Temecula segment in the next 50 years is less than 5%.