TIMING OF LATE HOLOCENE EARTHQUAKES ON THE ROSE CANYON FAULT AT OLD TOWN, SAN DIEGO CA

We excavated new paleoseismic trenches across the main trace of the Rose Canyon fault in Old Town, San Diego to determine the timing of late Holocene earthquakes. The trench was excavated across a historical drainage and alluvial fan, which previously emptied onto the flood plain of the San Diego River in the heart of Old Town, California’s first settlement. The stratigraphy at the site consists of historical fluvial and alluvial fan deposits, several buried soil A horizons, massive silt strata, and older San Diego River gravelly secondary channel deposits. There is evidence for four surface-rupturing events, the youngest of which cuts the early historical living surface that contains glass, ceramics, and cow bones. This event is likely related to the 1862 San Diego earthquake, which had an estimated magnitude close the M6 and was described as “The day of terror in San Diego” in the Los Angeles Star. An even younger “cracking event” resulting in fissures through the historical alluvial deposits, and filled with historical-aged sand, suggests either a triggered event or minor creep. Three prehistoric events are recognized that produced substantially more localized deformation than the 1862 event, which suggests that they are larger earthquakes: these events appear as displaced soil horizons, rotated silt beds, offset channel deposits, and fissures filled with overlying sediments. The youngest of these is immediately below the historical horizon and likely correlates with the most recent event recognized at multiple trench sites along the Rose Canyon fault in San Diego, as well as from offshore work in San Diego Bay and from faulted sediment below the San Diego airport, and dates to the past 400 years. The two older events both occurred in the past 4,100 years with the penultimate large event dated to about AD 600. The results of this paleoseismic trench combined with earlier results indicate that the Rose Canyon Fault has sustained activity throughout the Holocene and into the Historical period.