IN KHM.Y RKKKR'IO. United States Department of the Interior GEOLOGICAL SURVEY BOX 25016 'M.S._911_ DENVER FEDERAL CENTER DENVER, COLORADO 80225 May 1A, 1981 Memorandum To: Through: From: Mr. Mr. Earl W. Bean Ronald Foster R. F. Mast, Regional Geologist, Central Region S. M. Colman, F. W. Osterwald, G. R. Scott, (Task 1 and 2) S. T. Algermissen, D. M. Perkins, M. G. Hopper and S. R. Brockman (Task 3 and 4) 0. Subject: Review of Dames and Moore Draft Final Report Enclosed is our review of the Dames and Moore draft final report in accordance with our agreement (MOD E(29-2)-3627, Amendment #1, April 28, 1980) to review the final report and previously scheduled interim reports of "the DOE contractor study of the 'Golden Fault' or any other faults in the vicinity of the Rocky Flats Plant Area." Dames and Moore's report on geologic and seismologic investigations for the Rocky Flats Plant is comprehensive and thorough, and with reservations we agree with the major conclusions and recommendations. However, because of these reservations we do not believe the Dames and Moore report completely resolves the problem of the earthquake hazard at Rocky Flats. Concerning the report as a whole, we have one major question: What criteria were used to judge the seismic significance or potential as an earthquake source of the features investigated? If NRC "capable fault" criteria were used, these criteria need to be explicitly stated. Our comments on specific interpretations are organized according to the "Statement of work for Geologic and Seismologic Investiations for the Rocky Flats Plant." The principal geological investigations included in the report are: Task 1- Golden Fault Investigations Our only differences of opinion on the Interpretation of the Golden Fault investigations concern the origin and significance of the graben on the Coors property. We do not believe that the geometry and mechanics of the Golden Fault and the configuration of the local stress field are sufficiently understood to use these parameters to eliminate a structural relationship between the graben and the main fault. Even if the graben is not related to the Golden Fault, the argument for a nontectonic origin of the graben is weak;
the graben is too poorly understood to absolutely preclude a tectonic origin. The origin and significance of the graben will probably remain as one of the unsolved problems. Additional infonnationtd.il be difficult and (or) expensive to obtain, and is not likely to yield definitive answers. However, the postulated origins are speculative in light of the available data. For conservatism the graben should be assumed to be structually related. TTask 2 Rocky Flats Plant Vicinity Investigations The conclusions regarding all features investigated appear valid. However, a number of faults to the west and north of the plant, within the specified 5- mile radius, are not discussed. If these faults were eliminated as seismic hazards, the criteria by which they were eliminated need to be discussed. All faults in the investigated area should be shown on a map. The principal seismological investigations included in the report are: task 3 monitoring of seismic activity in the vicinity of Rocky Flats for a period of 184 days; and task 4 an investigation of the November 7, 1882 earthquake Judged by Hadsell (1968) to be the largest known earthquake in Colorado* Task 3 Monitoring of Seismic Activity: The seismic monitoring program appears to have been conducted with a station network adequate to detect the level of seismicity in the vicinity of the Rocky Flats Plant. However, the array of stations used is areally limited in extent and could reliably locate earthquakes only within the array, an area o only about 250 km. The problems of mine blasting and cultural noise have been thoroughly discussed in the report. The assessment of a low level of earthquake activity in the' vicinity of the plant during the monitoring period appears to be correct. Task 4 The November 7, 1882 Earthquake Dames and Moore have done by far the most extensive search of historical data so far attempted for this earthquake. The 1882 earthquake is important because: (a) it may have been large; and (b) the poor quality of the intensity data available for the earthquake makes it difficult to fix the location and magnitude of the shock. It was investigated by Dames and Moore because of the possibility that the shock could have occurred near Rocky Flats. Hadsell (1968) lists only one felt earthquake as being known to have occurred at Rocky Flats in historical times, the January 5, 1965 earthquake (maximum Modified Mercalli intensity, I Q - III). Other known felt earthquaki activity near Rocky Flats is restricted to one shock I - III at Boulder and the well known seismicity in the 1960's (and recently) in the Commerce City area.
The following possibilities should be considered with regard to the locatla and maximum Modified Mercalli intensity of the 1882 earthquake: (a) The epicenter of the 1882 earthquake may be at or near the location postulated by Dames and Moore. Two'sets of observations argue against this interpretation: the lack of appreciable felt reports in central Colorado a the relatively high intensities near Salt Lake City and Denver. On the oth hand, the lack of many felt reports' in central Colorado may be due, in part to low ground motion amplitudes in the mountains associated with well indurated, high velocity rocks. The reports from Salt Lake City and Denver may reflect relatively high site amplification at these locations. High ground amplification is known from instrumental studies to occur in the Sal; Lake City area. (b) The earthquake occurred in the Front,Range area north of Denver or at least somewhat east of the location assigned by Dames and Moore. If the epicenter is in or near the Front Range or at least to the east of the Daae and Moore epicenter, the maximum Modified Mercalli intensity, I Q could possibly be as high as VIII or IX. This high I Q would be necessary to aeon for the Salt Lake City intensity level. The hypothesis of a large earthqma] (I = VIII-IX) near the Front Range is' weakened by the total absence of fd; reports from eastern Colorado, Kansas, Nebraska, South Dakota, New Mexico, ; Texas. It should be noted.that Danes and Moore did not look for reports of the earthquake in New Mexico, Texas, or South Dakota. In addition, it vodi have been useful if Dames and Moore had distinguished on their isoseismal between locations where the earthquake was not felt and locations where historical sources were searched and no data obtained. (c) The 1882 earthquake is actually two^ earthquakes. There is the possibility that one earthquake occurred north of Denver and another oconm along the Wasatch front in Utah at about the same time. The probability of two earthquakes occurring in these two areas within about one hour of eadh other is low. Small earthquakes are, however, fairly common along the Tfaatai front and particularly in northern Utah in the Cache Valley-Bear Lake region. The reported times of effects of the 1882 earthquake at different locations vary by approximately an hour. These time variations do not prow or disprove the two earthquake hypothesis. The time variations reported abn consistent with the time keeping methods and accuracy of that era. The EM order of magnitude variation is found in reports of other, better documeata earthquakes that occurred in the west at that time. (d) The earthquake was a large earthquake at or. near the epicenter of the August 17, 1959 Hebgen Lake, Montana earthquake. An earthquake with mfnrf»ni Modified Mercalli intensity X-XI and ML - 7.5 (approximately) slightly largi than the 1959 shock would account for the intensity observations reported fc Dames and Moore. Considerable work on the possibility of observed intensit: in this area would be necessary to prove or disporve this hypothesis.
Options (b) and (c) above have implications with regard to significant ground motion at Rocky Flats. Option (b), as previously stated, would require epicentral intensities of the order of VIII-IX in order to produce the effects observed in Utah. We do not favor this hypothesis because of the lack of intensity observations east of Denver. Option (c), would result in epicentral intensities in the range VII-VIII for an earthquake.epicenter somewhere nortk of Denver. A comment should be made concerning the assignment of intensities by Dames Moore. While we agreed in general with the intensities assigned, one serious discrepancy should be identified. Dames and Moore assigned high intensities to landslides and associated geological effects. Sometimes the intensities assigned these effects were as high as X. All of the intensity X assignments In the Dames and Moore report are based on geologic effects (landslides, etc.). While this association of high intensity with geologic effects is technically In agreement with the Modified Mercalli scale, it has been widely recognized for a number of years by workers In this field that landsliding, liquif action, etc. can occur with levels of shaking that will result in mlnor. or no damage to structures (V-VI). Thus, landslides occur over a wide range of the intensity scale. They should only be used as a corroboratlon of high intensity when other intensity data are available and landsliding is widespread under a broad range of geologic and slope conditions. The use of geologic effects as evidence of high levels of ground shaking In the Dames and Moore reports is in error since the resulting isoseismal map has been used in the report to locate the epicenter of the 1882 earthquake. f In an effort to assess the maximum intensity of the 1882 shock we plotted the. shaken area vs. intensity for a number. of earthquakes in Colorado and Utah (Figure 1). Also included In the figure are the shaken area of the 1882 earthquake as presented by Dames and Moore (d in Figure 1) and the areas shaken if the intensity data of Dames and Moore are somewhat revised (m In Figure 1). Figure 1 indicates that the 1882 earthquake at the Dames and Moore. location would probably have a maximum intensity of VIII or IX. In Figure 2 the areas shaken by the earthquake used in Figure 1 have been grouped and averaged by maximum intensity. Once again, the areas shaken as indicated by Dames and Moore (d) and a revised set of Dames and Moore data are presented. These results are in agreement with the assessment of a maximum Intensity of VIII-IX for the 1882 earthquake using basically the Dames and Moore data. Figure 3 compares the Dames and Moore Isoseismal map with isoseismal maps of two recent, well studied shocks. One occurred in the Bear Lake region of northern Utah, the other in Commerce City, Colorado.
CONCLUSIONS AND RECOMMENDATIONS /. CONCERNING THE SEISMOLOGICAL INVESTIGATIONS 1. Using basically the Dames and Moore intensity data, the maximum intensity of the 1882 earthquake is in the range of VIII-IX, assuming that the earthquake epicenter is located in the Piceance Basin. 2. The location of the 1882 earthquake is uncertain and could range over a wide area. The assignment of the earthquake epicenter by Dames and Moore to the Piceance Basin has no special significance since there are many geological features in Colorado that would be connected with the occurrence of a minimum intensity VIII-IX earthquake.. 3. The occurrence of two earthquakes with nearly the same origin time cannot be ruled out with the existing data and is reasonably suggested by the observed intensity pattern. 4. The location of the 1882 earthquake near Denver, or east of the Dames and Moore locations, is possible, considering the existing data. 5. We believe that the Dames and Moore report does not completely resolve the problem of the evaluation of the earthquake hazard at Rocky Flats. This is not to imply that their investigation of the /- 1882 earthquake has not been valuable. As previously stated, the V. Dames and Moore report is the most concentrated and extensive effort undertaken to resolve the 1882 earthquake question. However, if their interpretation of the 1882 earthquake is correct, ground motion from the 1882 source does not significantly effect hazard estimation at Rocky Flats. Below are listed additional seismological investigatons that may be of benefit. 6. Further work that might be useful In resolving the problem of earthquake hazard in the vicinity of Rocky Flats might include: a. a search for additional felt data to the east of Denver as noted previously; b. further search for intensity data in Utah that might prove or disprove the two earthquake hypothesis; c. a careful historical investigation of intensity data for other earthquakes along the Front Range and their relation to the Rocky Flats area; d. a longer term monitoring of the Rocky Flats area with a somewhat larger areal array of instruments conducted over'a longer time period.
References cited Hadsell, F. A., 1968, History of earthquake activity in Colorado, Colo. School of Mines Quarterly, v. 63, no. 1, pp. 57-72.