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<br /> <br /> Ideally, it is preferable to calculate earthquake recurrence in the immediate vicinity of a site. <br /> However, the apparent low level of seismicity in Colorado within the study region (Figure <br />`, 2) (partly due to poor seismographic coverage) and hence sparse data preclude this approach. <br /> Earthquake recurrence for the Colorado Rocky Mountain region, which stretches westwazd <br /> from the Front Range to 108°W and from 37°N to 41°N (excluding the Rio Grande rift <br /> province), was estimated based on a historical catalogue containing just over 250 earthquakes <br />' (Unruh et al., 1995a). <br />' The recurrence relationships were estimated following the maximum-likelihood procedure <br />developed by Weichert (1980) and the estimated completeness intervals for the region. <br />Dependent events, either foreshocks, aftershocks or smaller events within azl earthquake <br />' swarm (the lazgest event is assumed to be a mainshock) were identified using empirical <br />criteria for the size in time and space of foreshock-mainshock-aftershock sequena~s developed <br />by Arabasz and Robinson (1976), Gardner and Knopoff (1974), and Uhrhamme:r (1986). If <br />an event were identified as dependent by two of the three criteria, it was delered from the <br />catalogue. <br />' After adjusting the historical records for dependent events and incompleteness, the recurrence <br />for the Rocky Mountain Region was calculated based on 22 earthquakes in the range M~ 3.0 <br />to 6.5. Although there is no specific requirement on the number of events necessary to <br />constitute an adequate database which would justify the assumption of a Poi!;son process <br />(hence, provide an accurate assessment of earthquake recurrence), the small number of events <br />'~ suggests that large uncertainties must be associated with any resulting estimates. The number <br />of earthquakes was normalized on an annual basis and per km2. The area used for the <br />' Colorado Rocky Mountains region is approximately 110,000 km2. Regression was performed <br />on the resulting data points as described by Weichert (1980). The recurrence relationship <br />is of the form of the Gutenberg-Richter relationship: log N = a - bM, where N is the annual <br />number of earthquakes of magnitude z M, the a-value is a constant and represents the log of <br />the annual number of earthquakes for M >_ 0 and the b-value is the slope of the regression. <br />A b-value of 0.90 t 0.14 (standard deviation) was determined for the Colorado Rocky <br />Mountain region (Unruh et al., 1995a). In comparison, Presgrave (1977) computed a b-value <br />of 0.86 for the Steamboat Springs and Elkhead Mountains areas based on the recordings of <br />' earthquakes at GOL from 1966 to 1973 and more recently McGuire (1993) computed a b- <br />H:\CON7RACT23456I.DUPIS 15 M0309951609 <br />