SCEC Award Number 16190 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Paleoseismology and mechanics of multi-fault ruptures
Investigator(s)
Name Organization
John Fletcher Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (Mexico) Thomas Rockwell San Diego State University Michael Oskin University of California, Davis Lewis Owen University of Cincinnati
Other Participants Ana Paula Hernandez-Flores (CICESE)
Hugo Villaverde (CICESE)
Laura Vallin (CICESE)
Abel Gutierrez Lopez (CICESE)
Froylan Cambron (CICESE)
Keene Karlsson (SDSU)
Mary Barr (UCD)
SCEC Priorities 1a, 2b, 3d SCEC Groups EFP, FARM, Geology
Report Due Date 03/15/2017 Date Report Submitted 03/15/2017
Project Abstract
 In 2016 we continued research to determine whether differences exist in observable seismogenic parameters for three macroscopic fault systems with contrasting hypothetical failure conditions. Our work focused heavily on the Cañada David detachment (CDD), which controls the southern half of the Laguna Salada rift basin and is one of the world’s best examples of an active low-angle normal fault, which are severely misoriented and controversial. We performed drone surveys to generate high resolution (11 cm) orthophotos and DEMs of the entire the CDD mountain-front scarp array, an area of ~125 km2. Systematic mapping of the alluvial fan surfaces and scarps in this area formed the basis of five masters theses and was critical for locating the sites of 27 trenches that were excavated by a group of ~20 scientists throughout the month of November. Our work resulted in the discovery the December 30, 1934 surface rupture, which extends ~20 km and exhibits ~1 m of oblique displacement. This is only the tenth example in the world of an historic earthquake associated with a low-angle normal fault. In addition to documenting intimate structural relationships of coseismic scarps and the detachment in map view, one of our deepest trenches exposed Holocene scarps with meters of coseismic slip emanating from the detachment, which dips 20 degrees and is exposed at the base of the trench along its entire length (34 m). This structural relation has long been hypothesized but this is the first time it has ever been seen in a trench wall or outcrop.
Intellectual Merit Understanding the conditions required to activate fault slip and the factors that affect rupture propagation are key to being able predict the earthquake behavior of faults. Our approach is to let the rocks and the faults speak for themselves, but the problem is that they are generally not very talkative. Thus we organize large-scale scientific campaigns and dig deep to expose key structural relationships. Though this hard work the fault systems in Laguna Salada are beginning to speak loud and clear. They are showing us that faults cannot be treated as isolated entities and that significant differences in behavior of fault systems are associated with easily characterized parameters such as the orientation and configuration of slip surfaces. The rich record of well-preserved prehistoric earthquakes in Laguna Salada is unique, but the types of fault systems that it contains are not. Low-angle normal faults may be limited to extensional tectonic regimes but severely misoriented faults are not only ubiquitious, they are commonly the most important fault strands at any plate margin (e.g. San Andreas fault). We feel that our observation based-approach is perhaps one of the best ways to address important questions concerning earthquake forecasting.
Broader Impacts This proposal formed the basis of five masters theses from CICESE and SDSU. Two of the students are coadvised by Fletcher and Rockwell and many trips were made through the year with a widely varying cast of students related to the four principal investigators. Participants in the fall trenching campaign represented a diverse range of nationalities including Mexico, United States, France, Portugal and India. This is a strong international collaboration that enriches both the scientific and cultural experiences of each and every participant.


Broader impacts of our work are discussed together with its intellectual merit. Please see the previous section.
Exemplary Figure Figure 2. Parts a and b are oblique views of the CDD master fault (green) and Holocene coseismic scarps. Part c is a mosic of trench site 7.1d located in the bottom of a modern arroyo similar to those depicted in parts a and b. Trench wall exposes coseismic scarps with meters of Holocene slip emanating from the 20 degree dipping detachment, which juxtaposes basin fill with cataclastic crystalline basement. This drives the last nail into the coffins of the remaining few who do not acknowledge the occurrence of seismogenic slip on low-angle normal faults. Part d, Log of trench T1B, south wall, showing evidence for two major Holocene surface ruptures.
Fletcher, Rockwell, Cambron, Villaverde and many other scientists (~20) who participated in the 2016 fall trenching campaign (we can provide a group photo if interested)