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<br /> <br />both segments. The best estimate maximum magnitude for the Leadville faults is considered <br />to be MW 6.5 based on the maximum extent of surface faulting. <br />We have assigned a 50% probability that the northern segment of the Mosquito fault is active <br />based on the lack of definitive data for Quaternary movement. We have assigned a <br />probability of 70% that the southern segment is active. Amulti-segment rupture, however, <br />is only given a probability of 5% because of the lack of geomorphic features associated with <br />a Mw 7 earthquake. The Leadville faults aze considered to be active. <br />Sawatch Fault System <br />' The Sawatch fault system forms the northwestern boundary of the Rio Grande rift for <br />approximately 90 km from Leadville to west of the town of Salida (Noller et al., 1992). We <br />have divided the Sawatch fault into two segments (Plate 1). <br />The northern segment extends for a distance of 21 km southward from neaz Leadville. Based <br />on a study of mountain-front morphology and by comparing it to the better-studied southern <br />segment, Ostenaa et al. (1981) inferred the rate of Quaternary activity along ¢he northern <br />segment is equal to or less than the rate of activity on the southern segment. They noted, <br />however, that the geomorphic expression is subdued north of Twin Lakes, and that <br />Quaternary-aged terraces that project across the area are not deformed. Aerial reconnaissance <br />by Noller et al. (1992) showed no scarps, lineaments or other features related to late <br />Pleistocene deformation north of Twin Lakes. <br />The southern segment of the Sawatch fault extends about 50 km northwazd just west of <br />Salida. This section of the fault shows evidence of movement during the late Pleistocene and <br />Holocene (Ostenaa et al., 1981). Trenching studies suggest that there have been six events <br />in the past 100 to 150 ka, with slip rates during the Bull Lake stage of about 0.07 to 0.08 <br />mm/yr, and during the last 50 ka between 0.05 to 0.17 mm/yr. Ostenaa et al. (1981) <br />estimated a recurrence interval of 10 to 40 ka for events on the southern segment of the fault, <br />with the most recent event occurring less that about 4000 yeazs ago. We estimate a mean <br />slip rate of 0.1 mm/yr for both the southern and northern segments of the Sawatch fault and <br />best estimate maximum magnitudes of MW 7.0 and 6.6, respectively. We consider each <br />segment as a sepazate fault source both of which are considered to be active <br />' H:\CONIRACI\234561DUP~12 12 M0309951609 <br />