One of the most powerful applications of fault-zone LiDAR scans is to serve as the before image for comparison with a survey acquired after a future surface-rupturing earthquake. Then, every displaced feature acts as a geodetic marker from which an ultra-high resolution map of the surface displacement field may be constructed. Such a detailed displacement field shows how faults and their containing rock volume act together to accommodate deformation and grow geologic structures over successive earthquakes. This provides new understanding of how earthquake ruptures connect faults to generate larger, more destructive events, and illuminates cryptic, distributed components of deformation needed for improving estimates of long-term deformation rates and seismic hazard. This grant through the NSF EarthScope Program and the Americas Program of the NSF Office of International Science and Engineering supports the development of fully 3-dimensional approaches to unraveling deformation from successive airborne LiDAR scans of a fault zone. The focus of the project is the before and after airborne lidar scans of the April 4, 2010 El Mayor-Cucapah (EMC) earthquake rupture in northern Baja California, Mexco. The project objectives address three challenges in working with this data set: (1) reprocessing of the pre-earthquake data to reduce scanning artifacts and improve accuracy; (2) development of methods for rigorous, high-resolution displacement measures from point-cloud data of vastly different resolutions (9 to 18 pts/m2 post-earthquake compared with 0.013 pts/m2 pre-earthquake); (3) preliminary 3-D mechanical modeling of fault-zone deformation from this event. Meeting these challenges will advances knowledge of fault-zone deformation gained from this earthquake, as well as advance techniques for analysis of the next earthquake captured by differential LiDAR -- quite possibly along one of the numerous active faults imaged as part of the Earthscope facility.
Coseismic surface rupture is an important, accessible record of earthquake slip, and the primary record of prehistoric seismicity. Near-field deformation measurements from differential LiDAR can transform our understanding of how coseismic surface ruptures are produced and distributed within fault zones. New knowledge to be gained includes understanding the mechanical coupling of fault slip to near-field distributed deformation, quantifying distributed components of fault slip otherwise difficult to measure, and predicting the style, extent, and magnitude of high strains around fault zones that could damage buildings and critical infrastructure. The techniques developed for this project will also prove valuable for other applications, such as in geomorphology, civil engineering, and robotics. This research brings together the expertise from five U.S.-based research groups that are leaders in the study of differential airborne LiDAR: UC Davis, Arizona State, UC San Diego, University of Houston, and Caltech / USGS. This project also broadens international collaborations formed following the EMC earthquake by involving researchers and students from CICESE, Baja California, in the development and deployment of new differential LiDAR algorithms and the open-source LiDAR-visualization software.
PI: Jean-Bernard Minster (jbminster@ucsd.edu) Project Period: 6/9/2012 - 5/31/2014 Institution: University of California, San Diego Project Overview. Airborne lidar is a technology that provides decimeter-precision maps of surface topography that can be compared to look for surface changes over period of days to decades. This project uses before/after airborne lidar imaging of the 2010 El Mayor-Cucapah (EMC) earthquake to (1) develop high resolution 3-D measurements of coseismic surface displacement and (2) analyze this surface displacement field to examine relationships between fault slip and distributed deformation and to elucidate the mechanical properties of the fault that ruptured in this event. Project Outcomes. The UC San Diego team undertook several tasks as part of this project and obtained the results described below. Manuscripts describing these results are being prepared for publication. 1.) Pre-event data validation. We analyzed the pre-event lidar dataset to provide all investigators on this project a visual representation of the internal consistency of the pre-event data (Figure 1). A perfect data set would report the same elevations wherever the survey swaths overlapped, however the interswath comparison shows evidence of systematic meter-scale geolocation errors in the preevent data. These results were part of the justification for the pre-event dataset reprocessing described in Glennie et al. (2014). 2.) Vertical displacement due to the EMC earthquake. We performed 3D differencing for all areas of overlap between the pre-event and post-event lidar data using the technique described in Borsa and Minster [2012a] (Figure 2). Because of the low resolution and systematic errors in the pre-event dataset, we could achieve highquality results for vertical displacements only. Our 3D differential vertical displacements improve on the 2D displacements from Oskin et al. [2012], especially in areas of steeper topography. 3.) Comparison of observed vertical displacements with other estimates. We compared our observed vertical displacements with predicted displacements from a simple finite-fault slip model embedded in an elastic halfspace (Yuri Fialko, personal communication) and found close correlation between the overall shape of surface displacement, with better agreement further away from the fault trace [Borsa and Minster, 2012b] (Figure 3a and 3b). The broad shape of the vertical surface displacement curve is dominated by slip at depth, so this agreement between observed and predicted displacements indicates that simple models adequately describe deep slip. At the same time, we obtained evidence for complex rupture in the near surface. Our observed vertical displacements diverged from modeled displacements 1.) in the deformable sediments of the Laguna Salada, and 2.) where high strain resulted in the fracturing of surface rocks, producing small scarps in areas of high curvature (Figure 3a and 3b). These two areas are where surface effects introduce significant modification of the vertical deformation because of deviation from simple elastic rheology. Relevant Publications Borsa, A, and Minster, J.B. (2012a) Rapid determination of near-fault earthquake deformation using LIDAR, Bulletin of the Seismological Society of America, 102 (4), pp. 1335-1347. Borsa, A., and Minster, J.B. (2012b) Coseismic deformation for the 2010 El Mayor-Cucapah Earthquake estimated from cross-correlation of pre- and post-event airborne lidar surveys. SSA Annual Meeting, Seismological Research Letters, 83 (2). Glennie, C., Hinojosa, A., Nissen, E., Kusari, A., Oskin M., Arrowsmith, R., Borsa, A. (accepted 2014) Optimization of Legacy LiDAR Datasets for Measuring Near-Field Earthquake Displacements. Geophysical Review Letters. Additional References Toda, S., Stein, R.S., Sevilgen, B., Lin, J. (2011) Graphic-rich deformation and stress-change software for earthquake, tectonic, and volcano research and teaching—user guide: U.S. Geological Survey Open-File Report 2011–1060, 63 pp., available at http://pubs.usgs.gov/of/2011/1060/
