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<br />In the absence of geologic evidence, the RMA source zone and its <br />seismogenic potential are defined solely by observed seismicity. The <br />source zone is capable of generating M=St events, but the upper limit <br />of the magnitude range is uncertain. Bollinger and others (1983) <br />concluded, on the basis of source zone dimensions, appazent downdip <br />extent, and continuing seismicity after termination of fluid injection, <br />that ongoing seismicity represents tectonic stress release. <br />Furthermore, because no known geologic structure is coincident with <br />earthquake occurrence, criteria for evaluation of maximum magnitude <br />based on fault rupture pazameters is inapplicable. <br />Maximum earthquake magnitude for the purposes of this study aze <br />estimated for the RMA source zone based on rupture area defined by <br />the pattern of seismicity and the absence of surface fault expression. <br />The maximum depth of the seismogenic crust defined by historic <br />seismicity is about 15 km. Accordingly, this depth and the surface <br />pattem of seismicity indicates the most probable maximum magnitude <br />is 6.5. The recurrence relation for earthquakes in the RMA is illus- <br />trated on Figure 2. <br />Despite the absence of evidence for a specific geologic structure (fault) <br />associated with the RMA source zone, the relatively small nature of <br />the source zone and the generally linear pattem of seismicity aze <br />amenable to treatment as a line source (fault) in SEISRISK III. <br />Moreover, the relatively lazge distance between the RMA source zone <br />and the Trout Creek dam site indicates this simplifying assumption is <br />reasonable. Accordingly, we modeled the RMA source zone as a line <br />source for the purposes of this study. <br />7.1.2 Areal Seismic Source Zones <br />Kirkham and Rogers (1981) divided Colorado into seven seismotec- <br />tonic provinces based on similarities in style and age of faulting and <br />common seismicity characteristics. Maximum credible earthquakes <br />(MCEs) were assigned by Kirkham and Rogers (1981) to each seismo- <br />tectonic province based on fault lengthldisplacement relationships, <br />recency of movement, historic seismicity, stress/strain data and <br />comparisons with similar seismogenic regions. Seismotectonic <br />provinces and associated maximum credible earthquakes aze summa- <br />rized below in Table II. <br />29 <br />