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both segments. The best estimate maximum magnitude for the Leadville faults is considered <br /> to be Ma, 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. A multi-segment rupture, however, <br /> is only given a probability of 5% because of the lack of geomorphic features associated with <br /> a Ma, 7 earthquake. The Leadville faults are 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 near Leadville. <br /> Based on a study of mountain-front morphology and by comparing it to the better-studied <br /> southern segment, Ostenaa et al. (1981) inferred the rate of Quaternary activity along the <br /> northern segment is equal to or less than the rate of activity on the southern segment. They <br /> noted, 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 <br /> reconnaissance by Noller et al. (1992) showed no scarps, lineaments or other features related <br /> to late Pleistocene deformation north of Twin Lakes. <br /> The southern segment of the Sawatch fault extends about 50 km northward 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 rates 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 <br /> fault, with the most recent event occurring less that about 4000 years ago. We estimate a <br /> mean down-dip slip rate of 0.1 mm/yr for both the southern and northern segments of the <br /> Sawatch fault and best estimate maximum magnitudes of Mw 7.0 and 6.6, respectively. We <br /> consider each segment as a separate fault source both of which are considered to be active. <br /> H:\CONTRACT\TENMILE\12 12 M0412951500 <br />