Using Landslides to Locate the Epicenter for the Historic 1882 Earthquake in Colorado

ABSTRACT On November 7, 1882, the largest earthquake in Colorado’s recorded history impacted an area estimated to be 470,000 km2 (181,000 mi2). While differing accounts of intensity were recorded by various communities at the time, the distribution of reported damages indicated a likely magnitude between 6 and 7. This study aimed to refine this estimate using landslides within the region that were likely to be co-seismic. Previously proposed methodology was used to calculate surface roughness values for landslides mapped at various scales digitized and compiled by the Colorado Geological Survey. A search radius of 54 km surrounding the epicentral estimate for the earthquake was chosen to evaluate the relative ages of landslides within the defined region. While the proposed methodology requires a calibration using age-dated landslides within the data set, the use of surface roughness as a relative age scale proved to be sufficiently useful to constrain an area within the study that showed anomalously high surface roughness values. Based on this earthquake being one of the only large earthquakes to have occurred in recorded history in Colorado, as well as a lack of wide distribution in the anomalous region expected from a storm event, this anomalous area containing high surface roughness landslides across varying base lithologies was concluded to have a high probability of being co-seismically induced. Utilizing this information, a new initiation ellipse was generated from these data.