Rare, large earthquakes at the Laramide deformation front; Colorado (1882) and Wyoming (1984)

The largest historical earthquake known in Colorado occurred on 7 November 1882. Knowledge of its size, location, and specific tectonic environment is important for the design of critical structures in the rapidly growing region of the Southern Rocky Mountains. More than one century later, on 18 October 1984, an m (sub b) 5.3 earthquake occurred in the Laramie Mountains, Wyoming. By studying the 1984 earthquake, we are able to provide constraints on the location and size of the 1882 earthquake. Analysis of broadband seismic data shows the 1984 mainshock to have nucleated at a depth of 27.5+ or -1.0 km and to have ruptured approximately 2.7 km updip, with a corresponding average displacement of about 48 cm and average stress drop of about 180 bars. This high stress drop may explain why the earthquake was felt over an area about 3.5 times that expected for a shallow earthquake of the same magnitude in this region. A microearthquake survey shows aftershocks to be just above the mainshock's rupture, mostly in a volume measuring 3 to 4 km across. Focal mechanisms for the mainshock and aftershocks have NE-SW-trending T axes, a feature shared by most earthquakes in western Colorado and by the induced Denver earthquakes of 1967. The only data for the i882 earthquake were intensity reports from a heterogeneously distributed population. Interpretation of these reports also might be affected by ground-motion amplification from fluvial deposits and possible significant focal depth for the mainshock. The primary aftershock of the 1882 earthquake was felt most strongly in the northern Front Range, leading Kirkham and Rogers (1985) to locate the epicenters of the aftershock and mainshock there. The Front Range is a geomorphic extension of the Laramie Mountains. Both features are part of the eastern deformation front of the Laramide orogeny. Based on knowledge of regional tectonics and using intensity maps for the 1984 and the 1967 Denver earthquakes, we reinterpret prior intensity maps for the 1882 earthquake as reflecting low population to the north and east of available intensity data. We estimate that, had there been a more evenly distributed population, the 1882 earthquake would have been felt over an area of about 850,000 km (super 2) , with a corresponding moment magnitude (M (sub W) ) of 6.6+ or -0.6. Our study, in the context of regional tectonics, implies that rare earthquakes of magnitude up to M (sub W) 6.6+ or -0.6, at depths from shallow to mid-crustal, are possible throughout the Laramie Mountains and the Front Range, approximately from Casper, Wyoming, to Pueblo, Colorado.