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1 li 1 1 1 1 i 1 BEVCH>1ARK ASSESS~IE~T OF THE SEI5,IIC HAZARD FOR THE SAX LUIS SEGME`T OF THE SANGRE DE CRISTO FAULT ZONE, COLORADO INTRODUCTION This preliminary assessment of seismic potential for the Sangre de Cristo fault has relied solely on data published in Kirkham and Rogers (1981). The purpose of this brief report is to provide a benchmark assessment of the earthquake potential of the San Luis segment (fault 113 et a1. in Kirkham z~nd Rogers, 1981) of the Sangre de Cristo fault zone. This assessment should be considered as preliminary in that more information on the tectonics of the Rio Grande Rift and the geomorpholog~• of fault scarps in San Luis Valley together with a more detailed probabil- istic evaluation will improve confidence in the earthouake hazard values. In my opinion, however, the probability values presented herein are well within an order of magnitude of final values which may be obtained through more detailed studies. SEISMICITY A map of earthquake epicenters in Kirkham and Rogers indicates that although the San Luis Valley is a northern extension of the seismically active Rio Grande Rift, only two earthquakes which could reasonably be considered as associated with the Sangre de Cristo fault system occurred in the area between 1870 and 1979. The first was an Intensity IV earthquake Located 65 km northeast of San Luis in the Dead Mule Gulch fault (fault 131) in 1888; the second was an Intensity V earthquake Located 35 km west of San Luis along the southern projection of the Manassa fault (fault 107) in 1952. This extremely Low level of historical seismic activity virtually eliminates the use of frequency-magnitude curves (typically constructed using historical earthquake data) for a determination of earthquake recurrence. I consider the applicat- ion of such a concept by Kirkham and Rogers (page 37) to derive recurrence intervals for magnitude 6.5 earthquakes to be misleading and unjustified.