The Easter Day Baja California Earthquake Rupture:
Could This Happen in San Diego?
Dr. Thomas Rockwell
Department of Geological Sciences
San Diego State University
The rupture of the Easter day El Mayor-Borrego M7.3 earthquake extends ~120 km from the northern tip of the Gulf of California to the international border and comprises two distinct geomorphologic and structural domains. The rupture is complex, with breaks along multiple fault strands, including minor re-rupture of the scarps associated with the 1892 Laguna Salada earthquake. The southern part of the rupture consists of a zone of distributed fracturing and liquefaction that cuts across the Colorado River delta. This part of the rupture is incoherent, and it is not clear that actual surface rupture along the entire length of this section of the fault. InSAR and SPOT differencing clearly indicate significant displacement in the delta region, but not necessarily at the surface. The northern half of the rupture propagated 55 km through an imbricate stack of east-dipping detachment faults in the Sierra Cucapah. In the southern Sierra Cucapah, rupture extends 20 km along the Laguna Salada and Pescadores faults and reached a maximum displacement of ~4m of right-lateral strike-slip, with some down to-the-east dip slip. The amount of dip slip is variable and changes polarity along strike along the Pescadores Fault before becoming predominantly east-down with maximum vertical offsets of 150 cm along the northern Pescadores Fault. This ruptured terminates in the high elevations of the sierra and jumps nearly 10 km north in a left step-over to the Borrego fault. Maximum measured displacement along the Borrego fault in Borrego Valley was about 3.1 m of strike slip and another 2 m of down-to-the-east dip slip on an east-dipping fault, yielding oblique slip of nearly 4 m. To the north, the Paso Superior fault ruptured across Mexican Highway 2 with more than a meter of oblique slip. Part of the complexity of the rupture can be attributed to interaction with detachment faults that allow the rupture to expand in the near surface. Damage associated with this earthquake is most dramatic in the delta region, where extensive liquefaction caused tilting and submergence of fields and communities, and destruction of roads, canals, and other infrastructure.
Could this happen in San Diego? San Diego is bisected by the Rose Canyon fault, a predominantly right-lateral system that accumulates strain much faster than the faults responsible for the Easter Day earthquake. Preliminary observations suggest that the previous such earthquake in Baja California may have been more than 10,000 years ago, whereas the Rose Canyon fault has produced at least six or more large (M7+) earthquakes in the same timeframe, with the most recent large earthquake only a few centuries ago. The displacement in this most recent earthquake appears to have been similar to that observed for the Baja event, about 3 m. Based on the pattern of past earthquakes, the Rose Canyon fault may have recently entered a more active period, although the short-term likelihood of another large quake is still relatively low.
Dr. Thomas Rockwell is a nationally and internationally renowned paleoseismologist and geomorphologist. He is an expert on the soils and tectonics of southern California and Baja California. He has conducted extensive trenching programs to date earthquakes on faults in the western U.S., Mexico, Central and South America, the Middle East and Asia, and his research is routinely used to date and reconstruct landscapes, assess rates and determine the age of faulting, as well as assess flood hazards. He often acts as a consultant for local geotechnical and archaeological consulting firms, providing input on the age of the soils or geomorphic surfaces of interest and assisting with fault evaluations. In addition to his duties as a professor of neotectonics and Quaternary geology at San Diego State University, Dr. Rockwell has been the principal investigator for numerous research studies funded by the National Science Foundation, the U.S. Geological Survey, and the Southern California Earthquake Center. He serves on several national-level research panels, including for NEHRP, NSF, NASA, and SCEC, and has served on the Board of Directors for the Seismological Society of America and the Southern California Earthquake Center.