Abstract
Studies of active fault zones have flourished with the availability of high-resolution topographic data, particularly where airborne light detection and ranging (lidar) and structure from motion (SfM) data sets provide a means to remotely analyze submeter- scale fault geomorphology. To determine surface offset at a point along a strike-slip earthquake rupture, geomorphic features (e.g., stream channels) are measured days to centuries after the event. Analysis of these and cumulatively offset features produces offset distributions for successive earthquakes that are used to understand earthquake rupture behavior. As researchers expand studies to more varied terrain types, climates, and vegetation regimes, there is an increasing need to standardize and uniformly validate measurements of tectonically displaced geomorphic features. A recently compiled catalog of nearly 5000 earthquake offsets across a range of measurement and reporting styles provides insight into quality rating and uncertainty trends from which we formulate best-practice and reporting recommendations for remote studies. In addition, a series of public and beginner-level studies validate the remote methodology for a number of tools and emphasize considerations to enhance measurement accuracy and precision for beginners and professionals. Our investigation revealed that (1) standardizing remote measurement methods and reporting quality rating schemes is essential for the utility and repeatability of fault-offset measurements; (2) measurement discrepancies often involve misinterpretation of the offset geomorphic feature and are a function of the investigator's experience; (3) comparison of measurements made by a single investigator in different climatic regions reveals systematic differences in measurement uncertainties attributable to variation in feature preservation; (4) measuring more components of a displaced geomorphic landform produces more consistently repeatable estimates of offset; and (5) inadequate understanding of preevent morphology and post-event modifications represents a greater epistemic limitation than the aleatoric limitations of the measurement process. © 2016 Geological Society of America.
Discussions with many colleagues have helped to focus our thinking on the problems identified in this paper. We thank the many participants in our surveys and Tim Dawson, Suzanne Hecker, and two anonymous reviewers for constructive comments. This work was supported by the U.S. Geological Survey National Earthquake Hazards Reduction Program (G11AP20029 and G11AP20020). The Uniform California Earthquake Rupture Forecast version 3 (UCERF3) was supported by the California Earthquake Authority, U.S. Geological Survey, and the Southern California Earthquake Center. The topographic data presented here were gathered by the National Center for Airborne Laser Mapping and processed and delivered by OpenTopography (http://www.opentopography.org/).