W. Spence et al., RARE, LARGE EARTHQUAKES AT THE LARAMIDE DEFORMATION FRONT - COLORADO (1882) AND WYOMING (1984), Bulletin of the Seismological Society of America, 86(6), 1996, pp. 1804-1819
The largest historical earthquake known in Colorado occurred on 7 Nove
mber 1882. Knowledge of its size, location, and specific tectonic envi
ronment is important for the design of critical structures in the rapi
dly growing region of the Southern Rocky Mountains. More than one cent
ury later, on 18 October 1984, an m(b) 5.3 earthquake occurred in the
Laramie Mountains, Wyoming. By studying the 1984 earthquake, we are ab
le to provide constraints on the location and size of the 1882 earthqu
ake. Analysis of broadband seismic data shows the 1984 mainshock to ha
ve nucleated at a depth of 27.5 +/- 1.0 km and to have ruptured simila
r to 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 r
egion. A microearthquake survey shows aftershocks to be just above the
mainshock's rupture, mostly in a volume measuring 3 to 4 km across. F
ocal mechanisms for the mainshock and aftershocks have NE-SE trending
T axes, a feature shared by most earthquakes in western Colorado and b
y the induced Denver earthquakes of 1967. The only data for the 1882 e
arthquake were intensity reports from a heterogeneously distributed po
pulation. Interpretation of these reports might be affected by ground-
motion amplification from fluvial deposits and possible significant fo
cal depth for the mainshock. The primary aftershock of the 1882 earthq
uake was felt most strongly in the northern Front Range, leading Kirkh
am and Rogers (1985) to locate the epicenters of the aftershock and th
e mainshock there. The Front Range is a geomorphic extension of the La
ramie Mountains. Both features are part of the eastern deformation fro
nt of the Laramide orogeny. Based on knowledge of regional tectonics a
nd using intensity maps for the 1984 and the 1967 Denver earthquakes,
we reinterpret prior intensity maps for the 1882 earthquake as reflect
ing low population to the north adn 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 850,00
0 km(2), with a corresponding moment magnitude (M(w)) of 6.6 +/- 0.6.
Our study, in the context of regional tectonics, implies that rare ear
thquakes of magnitude up to M(w) 6.6 +/- 0.6, at depths from shallow t
o mid-crustal, are possible throughout the Laramie Mountains and the F
ront Range, approximately from Casper, Wyoming, to Pueblo, Colorado.