SCEC Award Number 11060
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Collaborative Proposal: Paleoseismology of the Borrego and Pescadores faults in northern Baja California: characterizing the past rupture history of a complex transtensional fault zone
Investigator(s)
Name Organization
John Fletcher Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (Mexico) Thomas Rockwell San Diego State University Kate Scharer Appalachian State University Sinan Akciz United States Geological Survey Karl Mueller University of Colorado Lewis Owen University of Cincinnati Warren Sharp Berkeley Geochronology Center
Other Participants Roman Manjarrez (CICESE)
SCEC Priorities A9, A3, A1 SCEC Groups Geology, EFP, SHRA
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 The April 4th, 2010 El Mayor-Cucapah earthquake was the largest earthquake to strike northern Baja California during the historical period and involved rupture of at least five discrete fault zones. Recent earthquakes in the U.S., specifically the 1992 Landers and 1999 Hector Mine earthquakes, also had complex ruptures that involved multiple fault strands. Thus, a major question is whether multi-fault earthquakes are really unusual, or are they common for larger, cascade-type rupture sequences. Towards answering this and other questions, we excavated seven trenches along the Borrego, Laguna Salada and Paso Superior faults to initiate study of their past history of motion.
Four trenches were excavated across the Borrego fault in Borrego Valley. A late Pleistocene deposit buries evidence for the penultimate event, thereby indicating that it is pre-Holocene. Another trench, excavated across the 1892 scarp along the Laguna Salada fault which also sustained re-rupture in the 2010 earthquake, demonstrated evidence for at least two and possibly three older Laguna Salada ruptures, probably all since the middle Holocene based on soil stratigraphy. Altogether, preliminary interpretation of the stratigraphy exposed in these trenches, along with the soils and prior age control on local alluvial units, indicates that the penultimate rupture along the Borrego fault, and possibly also the Paso Superior fault, occurred in the late Pleistocene. In contrast, the Laguna Salada fault has sustained several Holocene ruptures, all of which are inferred to have occurred in the past 4-6 ka, supporting a relatively short recurrence interval for this major basin-bounding fault.
Intellectual Merit Recent large earthquakes southern California, specifically the 1992 Landers and 1999 Hector Mine earthquakes, had complex ruptures that involved multiple fault strands. A major question is whether multi-fault earthquakes are really unusual, or are they common for larger, cascade-type rupture sequences. The 2010 El Mayor-Cucapah earthquake was the largest earthquake to strike northern Baja California during the historical period and involved rupture of at least five discrete fault zones, none of which ruptured in their entirety, and thus joins the 1992 and 1999 earthquakes as complex events. A first order question is: have previous large earthquakes along this system involved the same fault elements, or have some or all previous events been limited to single discrete faults, such as the individual Borrego or Pescadores faults? Or have past earthquakes involved other combinations of faults that chained together to produce a large earthquake? Is this behavior typical of low slip and low slip rate faults that may produce large earthquakes on the order of every 5-50 ka? This project aims to answer some of these questions on the fundamental behavior of faults and fault systems by studying the rupture histories of the individual faults that ruptured in 2010.
Broader Impacts This project is training students at CICESE in Ensenada (two male, one female) in paleoseismic techniques. This is important in that there are currently no paleoseismologists in Baja California and nearly all prior paleoseismic work was conducted from north of the border. This project is leading to a close partnership between SDSU and CICESE, and future collaborative studies are likely to result in a major increase in the understanding of earthquake hazard in northern Baja California.
Exemplary Figure Figure 2. Detailed map on GoogleEarth imagery of the 2010 rupture (red) and the 1892 rupture (blue). Trench sites are shown by the stars.