Variability in the location of mapped fault traces based on geomorphic mapping of remote-sensing datasets from 23 mappers

Mapping of neotectonic faults is critical to the scientific study of earthquake processes and surface rupture hazard analysis. Geologists commonly map fault traces from remote sensing datasets by interpreting tectonic landforms formed from past earthquakes. However, the evidence for faulting is not always straightforward to observe and interpret. Even experts map faults differently. We seek to understand the variability in fault trace mapping by mappers with different experience, ranging from undergraduate students to professional geologists with decades of experience. An individual’s understanding of faulting is impacted by their experience, yet people with similar experiences can interpret areas differently. No matter their experience level, mappers all have gaps in knowledge, and faults can rupture in unexpected ways. We anticipate that the results will improve the development of standardized mapping practices.

To evaluate the effect of differing knowledge on mapped fault trace locations, 23 mappers of varying experience levels produced fault maps from pre-rupture topography and imagery acquired before the earthquake of interest. Mappers include four undergraduate students, eleven graduate students, two postdocs, and three mid- and three senior-level professional geologists. The mappers used a systematic approach to map faults based on geomorphology.

To assess map quality, we compared the pre-rupture fault maps to published coseismic rupture maps. We evaluated 1) the percentage of coseismic ruptures that were predicted by the mapped faults, 2) the percentage of the mapped faults that ruptured in the recent earthquake, 3) the distribution of mapped faults around indicative geomorphic landforms, and 4) the impact of data type and the use of a geologic map. We found slight improvement by experience level in the portion of ruptures predicted and mapped faults that ruptured. For ruptures near geomorphic landforms, professional geologists best predicted the rupture location, and undergraduate students mapped with the highest error.  Less experienced mappers tended to misinterpret some geomorphology. Despite these differences in experience level, all participants mapped some faults that did not rupture. Mappers of all experience levels were most successful with the high-resolution topography (~1m/pix). Aerial imagery was less useful in areas with high vegetation and anthropogenic activity. Using a geologic map did not improve the maps.

While experience level has a small effect on fault trace mapping accuracy, with our results we can start defining the epistemic uncertainty for geomorphic fault mapping. Our results also suggest that mapping fault traces remains challenging regardless of expertise and highlight the need for improved and standardized mapping practices.