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SECT NFOUR Seismic NEW Inputs <br /> was developed which included earthquakes located within the Central Rocky Mountains,the Rio <br /> Grande Rift and the Western Rocky Mountains (Figure 1). These regions were combined in this <br /> study as the Henderson Mill tailings dam is located close to the boundary between the Rocky <br /> Mountains and the Rio Grande rift and there were too few earthquakes to calculate recurrence for <br /> each seismotectonic province separately. The recurrence parameters for the SRM were <br /> developed using the historical seismicity record for the period 1855 to 2015. <br /> Completeness intervals were estimated for the SRM based on the history of settlement, the <br /> seismographic installation and operation in the region,and using Stepp plot analyses which were <br /> developed by calculating the average annual number of independently occurring events in each <br /> half magnitude increment for the SRM catalog and completeness estimates and number of <br /> earthquakes within each interval are shown in Figure 6. <br /> In the western U.S., the conventional approach has been to assume that the minimum threshold <br /> for surface faulting represents the upper size limit for background earthquakes. In much of the <br /> western U.S.,this threshold ranges from M 6 to 63/4(e.g.,dePolo, 1994). It is believed that larger <br /> earthquakes will be accompanied by surface rupture, and repeated events of this size will <br /> produce recognizable fault-related geomorphic features. As stated in Section 3.2.1, we have <br /> adopted a maximum magnitude of M 63/4± '/4 for the SRM. This magnitude range is at the upper <br /> end of the typical range of western U.S. values because of the thicker seismogenic crust in the <br /> SRM and the occurrence of the 1882 earthquake,which has no known surface expression. <br /> The recurrence relationship was estimated using the maximum likelihood procedure developed <br /> by Weichert (1980) and the estimated completeness intervals for the region. The resulting <br /> recurrence for the SRM is shown on Figure 6 and the b-value is estimated to be 0.73 f 0.11. <br /> Felzer (2008) used California seismicity for the U.S. National Hazard Maps to indicate that <br /> corrections should be made for both magnitude rounding and errors, before calculating <br /> seismicity rates (a-values). Felzer (2008) reports that a-values can also be overestimated as a <br /> result of magnitude errors in historical seismicity catalogs. The Gaussian distribution for <br /> magnitude errors is symmetrical but that of earthquake occurrence is asymmetrical. Thus <br /> magnitudes above a specific magnitude (e.g., M 5.2) have equal probabilities as magnitudes <br /> below a specific magnitude (e.g., M 4.8) because of the symmetrical Gaussian distribution, <br /> which does not reflect the reality that larger earthquakes are less frequent than smaller <br /> earthquakes. This results in an apparent increase in earthquakes for a specific magnitude (e.g., <br /> M 5.0), which is carried into the recurrence calculations, increasing the seismicity rate. Again, if <br /> magnitude error is uniform throughout the catalog, then it is easy to fix, but when it varies <br /> throughout the catalog a correction can be made for each magnitude based on its error and the <br /> Gutenberg-Richter relationship and substituted for the reported magnitude. Then the actual <br /> seismicity rate (a-value) can be estimated. Again Felzer (2008) has tested this methodology <br /> using simulated catalogs and was able to recover the correct a-value with this method. <br /> Both the corrections for magnitude error and magnitude rounding are incorporated into the <br /> recurrence estimates reported here for the SRM catalog. We were able to determine rounding by <br /> observing the magnitudes reported in the catalogs over time and estimated magnitude errors <br /> based on Felzer(2008). For those earthquakes whose errors have been reported in the literature, <br /> these errors are included. Otherwise a standard error of 0.33 was assigned to pre-1962 <br /> earthquakes,0.22 for 1962-1982 and 0.11 for earthquakes occurring after 1982. These errors are <br /> tS '27-M4R48\16 <br />