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<br />Sawatch Fault, South Graben, Upper Arkansas Valley <br />Studies performed by USBR indicate the Sawatch fault in the south <br />graben of the upper Arkansas Valley is active (seismogenic) based on <br />the presence of fault scarps on late Pleistocene to Holocene deposits. <br />Paleoseismological studies indicate that up to six surface faulting <br />events producing M,,=6.25 to 7.25 earthquakes have occurred in the <br />last 100,000 to 150,000 yeazs (Ostenaa and others, 1981). The latest <br />event occurred less than 4000 years before present. USBR (1980, <br />1990) considers the Sawatch fault in the south graben to be a seismo- <br />genic structure and assigned an MCE of M,,=7.0 at a focal depth of 7 to <br />0 km. <br />We agree that the Sawatch fault is seismogenic and assign a maximum <br />earthquake of 7.25 with an average recurrence of 20,000 years. <br />Sawatch Fault, North Graben, Upper Arkansas Valley <br />Geologic and seismologic studies conducted by USBR (1980, 1990) in <br />the north graben did not disclose conclusive evidence regarding the <br />age of displacement on the Sawatch fault in this area. However, <br />because of similarities to the south graben, USBR (1980, 1990) <br />assigned and maximum credible earthquake (MCE) of M,,=6.8 at a <br />focal depth of 7 to 10 km to the Sawatch fault in the north graben. <br />Based on the absence of evidence for late Quaternary displacement and <br />recurrence or slip rate data in the north graben, we consider the <br />Sawatch fault in the north graben to be inactive (non-seismogenic) for <br />the purposes of this study. Moreover, we believe that the proximity of <br />the seismogenic segment of the Sawatch fault in the south graben to <br />the Trout Creek site diminishes the importance of the north graben in <br />the current PSHA, especially when compared to deterministic method- <br />ology employed by USBR in the early 1980s. <br />Sangre de Cristo Fault, San Luis Valley <br />McCalpin (1981, 1982) indicates the Sangre de Cristo fault in the San <br />Luis Valley produced major earthquakes (M,=6.6 to 7.0) at intervals <br />ranging from 2500 to 60,000 years with the most recent event <br />27 <br />