New Observations of Coseismic Fault Zone Deformation from Differencing Pre- and Post-Earthquake Lidar Data
Monday, 15 December 2014
Sub-meter resolution airborne LiDAR topography comprises a critical part of the research infastructure along several major active faults in western North America, where it provides a topographic baseline against which future, post-earthquake LiDAR surveys can be differenced. Here, we describe two differencing methods - one based on the Iterative Closest Point algorithm, the other on cross-correlation techniques - that enable the 3-D surface deformation field to be determined. This contrasts with conventional InSAR and pixel-tracking techniques which measure line-of-sight and horizontal displacement components only. An additional advantage is that LiDAR differencing retains coherence in the presence of steep displacement gradients, such as close to surface ruptures, and it is therefore well-suited for probing the slip distribution and mechanical properties of the shallow part of the fault zone. We illustrate using two recent Mw ~7 earthquakes in Mexico and Japan which are the first near-complete ruptures to be captured in this way. For the 4 April 2010 El Mayor-Cucapah (Mexico) earthquake, the 3-D displacements are used to explore whether significant slip occurred on low-angle detachment faults, as has been postulated by some field geologists. For the 11 April 2011 Fukushima-Hamadori (Japan) earthquake, coherent LiDAR displacements from the interior part of the fault zone are used to help bridge a critical observational gap between surface faulting offsets (measured in the field) and slip occurring at depths of a few kilometers (inferred from InSAR). Challenges include the treatment of vegetation and the dependence on older, "legacy" LiDAR datasets - third party surveys which were not optimized for earthquake studies and for which important data acquisition and processing metrics are unavailable.