Surface ruptures provide a physically important accessible record of the distribution of slip in earthquakes and are the primary record of prehistoric seismic activity. Traditional field mapping and measurements may incompletely characterize surface ruptures due to their often complex, distributed nature. Prehistoric earthquake ruptures are also subject to surface processes that, over time, smooth out displaced features and mask critical components of the deformation field, such as warping of the land surface.
This grant through the NSF EarthScope Program and the Americas Program of the NSF Office of International Science and Engineering supports the acquisition of very high-resolution airborne LiDAR topography over the surface rupture from the 4 April 2010 El Mayor - Cucapah earthquake in northernmost Baja California. The El Mayor - Cucapah earthquake ruptured the Pescadoros-Borrego fault system, which lies adjacent to the Laguna Salada fault that produced a similar-sized earthquake in 1892. Why the earthquake occurred where it did presents an important challenge to our understanding of the physics of earthquake slip and recurrence. Detailed comparison of the geometry of the 2010 and 1892 surface ruptures, engendered by the airborne LiDAR scan, will be especially important for assessing the relationship between these earthquakes.
The El Mayor - Cucapah Earthquake occurred in a hyper-arid desert setting where little vegetation cover exists to obscure the rupture. Thus high-resolution digital elevation models (~25 cm resolution) may be generated with current scanning technology (>10 points/sq. m), providing very high-resolution measurements of the surface rupture from this earthquake in a near-pristine state. The data span the rupture belt as identified by initial aerial and satellite reconnaissance by at least 500-1000 meters. The data acquisition and analysis involve numerous students and strengthen US-Mexico earthquake science collaborations. Terrestrial laser scanning data covering select portions of the rupture at ultra high resolution (1000s of points/sq. m) will be embedded in the airborne data and their seamless visualization and analysis will be enabled using tools developed at the KeckCaves facility at UC Davis (http://keckcaves.ucdavis.edu/). These data, including the embedded ground-based LiDAR scans, will be made immediately available to the research community using the existing infrastructure of the OpenTopography Facility (www.opentopography.org/). They will significantly advance the state of the art in earthquake geology and address important questions on the nature and preservation of earthquake ground ruptures.
The response to this earthquake enhances international collaboration and education opportunities. Close collaboration with scientists in Mexico has been central to the response to the El Mayor - Cucapah Earthquake. All data-gathering and dissemination activities will emphasize students from both the United States and Mexico. Openly available LiDAR data collected from the ground rupture will provide new opportunities for Mexican scientists and students to work with this imagery data, strengthening new collaborative relationships with U.S. scientists that have been established as part of the earthquake response.
The major effort supported by this grant was the acquisition of a high-resolution topographic survey of the ground rupture produced by the 4 April 2010 El Mayor-Cucapah earthquake in northernmost Baja California, Mexico. This was the first opportunity to use airborne laser scanning, also known as LiDAR (light distance and ranging) to precisely measure a complete earthquake rupture. Soon after acquisition in August, 2010, the data were freely released to the research community and to the public on opentopography.org for further research and educational use. The El-Mayor Cucapah earthquake was a magnitude 7.2 event that occurred in a sparsely populated desert region west of Mexicali. The earthquake ruptured at least seven distinct fault segments, and is the latest example of a large earthquake generated by linking together members of the complex secondary fault network that surrounds the San Andreas fault in California (previous examples include the 1992 Landers and 1999 Hector Mine earthquakes). It is important to understand how such large earthquakes are generated along a set of faults in order to better predict seismic hazards closer to the urban region. High resolution topography data collected for this project illuminate the earthquake rupture in unprecedented detail, revealing new features that help to identify past such events in the geologic record. One of the exciting surprises in this data set was the discovery of a previously unrecognized fault within the soft sediments of the Colorado River delta that slipped in this earthquake. The data also lend new understanding to the mechanical interactions between faults, helping to constrain the mechanisms of how ruptures propagate and grow into large earthquakes. One of the important outcomes of this work was to greatly strengthen ties between U.S. and Mexico earthquake researchers. Colleagues from CICESE (Centro de Investigación Científica y de Educación Superior de Ensenada) were instrumental in executing the survey, and have collaborated closely in analyzing the data. One of the unprecedented outcomes of this research, made possible through this collaboration, was the first-ever comparison of pre- and post-event LiDAR topographic surveys (see attached figure A). Pre-event data were collected by the Mexican cartographic agency (INEGI) as part of a regional survey of the Colorado River delta. Though of much lower resolution than the post-event data, the precise vertical resolution of LiDAR allows us to analyze details of distributed warping of the ground surface (see attached figure B). This warping indicates that large elastically generated strains (~10^-3) can occur in the shallow crust between fault ruptures. The deformation absorbed this way constitutes a significant fraction of the ground displacement generated by the earthquake.
