THE INCOMPLETE INVENTORY OF QUATERNARY FAULTS IN THE NORTHERN ROCKY MOUNTAINS OF COLORADO AND WYOMING AND THE IMPLICATIONS FOR THE NEXT BIG EARTHQUAKE

The historical rates and patterns of ~M 4 seismicity are relatively uniform over a broad region of the Northern Rocky Mountains of Colorado and Wyoming. The two largest historical earthquakes in this region, the 1882 ~ M 6.4 event in northern Colorado and the 1984 M 5.4Laramie Mountains earthquake, have not been associated with known surface ruptures or Quaternary faults and may typify many characteristics of the largest earthquakes in the region. Extrapolation of recurrence curves based on this historical seismicity record implies recurrence on the order of ~500 years for events of M 6.8 or larger, an earthquake magnitude often associated with surface rupture and existing Quaternary faults in the western United States. As one hypothetical example to match this recurrence rate, using typical fault rupture length and displacement parameters, about 20 independent fault sources, 500-800 km in total length, with slip rates in the range of 0.13-0.2 mm/yr would be required. The existing fault inventory in the Quaternary Fault and Fold Database within this region for Class 1-3 Quaternary faults, with ages <130 ka, has sufficient length, but existing slip rate estimates are mostly an order of magnitude too low. A series of similar examples, with varied magnitudes, source lengths, and slip rates all indicate a large deficiency in the number of recognized sources for larger events. These examples imply that as many as 100 - 200 additional low slip rate fault sources may remain to be identified throughout this region.

There are two conflicting implications from these comparisons: 1) the rates of the largest earthquakes in this region are indeed very low, and recurrence models for the regional source should be strongly tapered or truncated at magnitudes above ~ 6.5, or 2) the existing inventory of Quaternary faults in the Colorado and Wyoming is significantly under estimating the number of low-slip rate, 0.005 – 0.05 mm/yr, fault sources in the region. These two extremes have less impact on hazard estimates for typical building codes, but are potentially more significant for siting of critical facilities in locations where these sources may be not evaluated in detail. These comparisons also imply that the next large earthquake in this region will most likely be associated with an earthquake source not presently recognized.