Late-Holocene Earthquakes on the Rose Canyon Fault at Old Town, San Diego CA

Drake M. Singleton, Thomas K. Rockwell, Monte Murbach, Diane Murbach, Jillian M. Maloney, Yuval Levy, Eui-Jo Marquez, & Luke Weidman

Published August 15, 2017, SCEC Contribution #7719, 2017 SCEC Annual Meeting Poster #151

We present the results of new paleoseismic trenches excavated across the main trace of the Rose Canyon fault (RCF) in Old Town, San Diego, to determine the timing of late Holocene earthquakes. 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 large surface-rupturing events, as well as two smaller events, the youngest of which cuts the early historical living surface that contains glass, ceramics, cow bones, and a historical era foundation. This event is likely related to the 1862 San Diego earthquake, which had an estimated magnitude close to 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. The possibility exists that additional smaller magnitude events have occurred on the RCF, but the stratigraphy at Old Town limits the resolution needed to distinguish evidence for every small surface rupture or cracking event. The four larger events produced substantially more deformation, and over a broader width of the fault zone, than the 1862 event: 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 and dates to the past 400 years. The three older events have all occurred in the past 3,500 years, with the penultimate large event dated to about 1300 AD. The results of this paleoseismic study, combined with earlier results, indicate that the Rose Canyon Fault has sustained activity throughout the Holocene and into the Historical period. Comparison of paleoseismic results from the Newport-Inglewood fault (NIF) indicates that some RCF earthquakes have similar timing with NIF events, most likely indicating the occurrence of a sequence or cluster of events on the coastal system of strike-slip faults. The alternative explanation – large earthquakes rupturing both faults simultaneously – is unlikely when both the slip rate and recurrence intervals for these faults are considered.

Key Words
Paleoseismology, Neotectonics, Newport-Inglewood-Rose Canyon Fault zone

Citation
Singleton, D. M., Rockwell, T. K., Murbach, M., Murbach, D., Maloney, J. M., Levy, Y., Marquez, E., & Weidman, L. (2017, 08). Late-Holocene Earthquakes on the Rose Canyon Fault at Old Town, San Diego CA. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology