Late-Holocene rupture history of the Rose Canyon Fault in Old Town, San Diego; implications for cascading earthquakes on the Newport-Inglewood-Rose Canyon fault system

We present new results from paleoseismic trenches excavated across the main trace of the Rose Canyon fault zone (RCFZ) in Old Town-San Diego, California, to determine the timing of late-Holocene earthquakes. There is evidence for four large surface-rupturing events, as well as two smaller events, the youngest of which cuts the early historical living surface that contains glass, ceramics, and a historical era foundation. This youngest event is likely related to the 1862 San Diego earthquake, which had an estimated magnitude close to M 6. The age of older ruptures is constrained by 36 radiocarbon dates that exhibit good stratigraphic order. The four larger events produced substantially more ground deformation, and over a broader width of the fault zone, than the 1862 event. The youngest of the four larger events is found immediately below the historical horizon and likely correlates with the most recent event recognized at multiple trench sites along the RCFZ in San Diego and dates to the mid-1700s. The three older events have all occurred in the past 3300 yr, with the penultimate large event dated to about A.D. 1300. The results of this paleoseismic study indicate that the RCFZ has sustained activity throughout the late-Holocene and into the historical period. These results also suggest that the RCFZ has a late-Holocene recurrence interval of approximately 700 yr, which is several hundred years shorter than previous estimates. Comparison of RCFZ paleoseismic results with paleoseismic data from the Newport-Inglewood fault zone (NIFZ) shows that some RCFZ earthquakes have similar timing with NIFZ events, most likely indicating the occurrence of a sequence or cluster of events on the coastal system of strike-slip faults. The alternative explanation-very large earthquakes rupturing both faults simultaneously-is unlikely when both the slip rate and recurrence intervals for these faults are considered.