The January 6, Divide Colorado Earthquake

Transcription

1 ^si*'v'm.^^''^i^t'lft^'' 5 1V<--'--"V^v;" ^'^'"' "^.'"-^.v : " ',vn.. ' ' 7 r ' ' ' " : > r - ^ ' ^ The January 6, 1979.!t: Divide Colorado Earthquake..'Vv :."V- AJ MlCROGEOPHYSICS CORPORATION West 44th Avenue / Wheat Ridge, Colorado / (303)

2 .-'" 1 UJ \TQi-/- THE DIVIDE EARTHQUAKE OF JANUARY 6, 1979 by: David Butler John J. Nicholl, Jr -MCROGECPHVSICSCORPORATION \VEST 44th AVENUE 'AHEAT RiDCE. COLOR ADO SCO 3 3

3 I FIGURES 1. Location and Isoseismal Map

4 Introduction At 01:58 UCT, January 6, 1979 a magnitude 2.9 (USGS) earthquake was felt in the area of Divide, Colorado about 30 km WNW of Colorado Springs, Colorado. The depth-constrained solution for the epicenter is N, W (see Figure 1). This paper will discuss data from these sources; a microearthquake array deployed in an aftershock study, an intensity study, and the site-safety implications of this earthquake. A Maximum Credible Earthquake and its Return Time will be estimated based on this preliminary data. Main Shock and Aftershock Data From recordings at 10 diffe determined the epicenter Considering the audible eismic stations, the USGS ove and shown in Figure 1. below), a source somewhat shallower than the 5.0 km assigned depth is indicated. If the depth was allowed to be unconstrained or was assigned a shallower value, an epicenter determined might be more consistent with the other data. I.e., a location 5-7 km south of the given epicenter is certainly feasible and more consistent with the aftershock and intensity data. The closest station used in the determination is GOL at a distance of 85 km. Intensity Data The earthquake produced two loud sounds audible in an area of more o than 600 km, which lead to an extensive air search for an air

5 crash or meteor source for the event. Once the air and ground search by the Colorado CAP was curtailed, the enthusiasm of the CAP volunteers was pressed into service to do an intensity survey. More than 50 reports were generated. The reports varied from intensity III (felt indoors, hanging objects swinging) to V (sleeper awakened, small unstable objects displaced). The intensity map prepared from this data is also shown in Figure 1. a*.^ > ik:> The population density is highesjjjkin the eastern portion of the ^S* area, but the lack of reports fjft J*Cqlorado Springs provide a good ^ vo boundary for the felt area.jj^bqht^gjjfco km 2 event. were disturbed by this ef^&ii^ ~=^<.^f. m *J VW : $ ^ Note that the felt area centers somewhat south of the government epicenter. Aftershock Study Seasonal weather damped the investigation which could usually accompany such an event. However, MicroGeophysics Corporation did field five seismographs for two days within 48 hours after the event. Interference by snow plows and sub zero weather limited the recording time, and only a single aftershock was recorded. It plots significantly south and west of the main shock epicenter. While consistent with a more southerly location of the main shock and with the short aftershock sequence usual for small events, no

6 Isoseismal Map of the January 6, 1979 Divide,Colorado Earthquake r Cheesmon Lake 10 lu (O 5 il. Cb X5 I Q Eleverimile Canyon Reservoir AFTERz SHOCK SEISMOGRAPHS USGS EPICENTER WOODLAND PARK COLORADO 1 x. 03 c: I Q DISTURBED WELLS CRIPPLE CREEK o COLORADO Survey Area f \t Denver KM Figure 1

7 additional significance should be implied from the aftershock location. Geologic Considerations Kirkham and Rogers (1981) present two potentially active faults in i this region which might be the source of this earthquake. These two structures are the Oil Creek fault which passes north/south through the town of Divide and the Woodland Park Graben. The high population and low reported intensity at and north of Woodland Park make it improbable that the event was located east of the given location. In the course of the intensil^f investigations and afterword (in the spring) signs or evideng^pf-^round breakage was sought. The cursory investigation Discussion The following geologic analysis has been offered for the Divide event: "Kirkham and Rogers (1981) assign this event to the Oil Creek fault which passes north/south through the town of Divide. Kirkham and Rogers show the mapped length of the fault as about 65 kilometers. From Taylor (1975) and the map of Scott, Taylor, Epis and Wobus (1978), it is clear that the Oil Creek fault is contained within Precambrian rocks and the Pikes Peak Granite. Near Divide the Oil Creek fault is covered by Pliocene and Miocene deposits. Taylor (1975) shows an offset of these beds in a sketch cross section of the area. Even if the Pliocene and Miocene cover is broken, this evidence merely states that the fault breakage is post-miocene, i.e., post-20 million years before the present. If

8 the Oil Creek fault is an active structure, its length of 65 km might imply a magnitude 7.0 Maximum Credible Earthquake (Slemmons, 1977; Butler, et al, 1983)." The potentially active faults of Kirkham and Rogers (1981) are structures which have exhibited post-neogene (28 million years before the present) movement. Without proof of Quarternary fault breakage, the probability of renewed activity on these structures is slight. One should note that geologic investigation to date has not proven the continuity of this Miocene structure nor the presence of Quarternary surface rupture anywhere along its length. The decision on the site safety, the Return Time for events eter given above centers on the Divide event or larger, Population in this area h since the Gold Rush of 1859 due to the presence of the nearby Cripple Creek gold fields. However, in the early days, another boom in the night would have been assigned to mining activity rather than a natural phenomena. If we conservatively estimate that no event as large as the Divide event has occurred in 50 years, a Return Time in excess of 500,000 years would be estimated for a magnitude 7.0 magnitude Maximum Credible Earthquake (using a b-slope of 1.0). In summary, the very conservative selection of the entire Oil Creek structure as active and the very conservative selection of 50 years as a return period for events like this one leads to a

9 negligible probability of a damaging earthquake (usually defined as an event with magnitude greater than 5.0) in the lifetime of critical structures now being considered for this area. Design lifetimes are typically less than 100 years. The most probable values of the input parameters are a shorter surface-breakage fault length (maybe km) and a longer return time for events similar to this magnitude 2.9 event (maybe greater than 100 years). Conclusion Based on seismological extrapolations from the Divide earthquake, a preliminary Maximum Credible Earthquake of magnitude 7.0 for the Oil Creek fault is estimated to hav^n^ preliminary return period «x\kv of greater than 500,000 years. Acknowledgments The Civil Air Patrol of Colorado should be congratulated for their enthusiastic public service in this investigation. Roger Bowman and Andy Staatz did much of the field work and planning under the sponsorship of MicroGeophysics Corporation. The Golden office of NEIS was also very cooperative during this study.

10 REFERENCES Butler, David, Hornback, V. Q., Parsons, J. P., Wahlstrom, E. E., Wengler, Wengler, Geology and seismicity of the Strontia Springs area and adjacent areas, Colorado, Report for the Denver Water Department, 108 pages. Kirkham, R. M., and Rogers, W. P., 1981, Earthquake potential in Colorado a preliminary evaluation: Bull. 43, Colorado Geological Survey, pp Scott, G. R., Taylor, R. B., Epis, R. C. and Wobus, R. A. 1978, Geologic map of the Pueblo 1 x 2 quadrangle south-central Colorado: Miscellaneous Investigations Series, U. S. Geological Survey Map Slemmons, David B., 1977, State-of-the-art for assessing earthquake hazards in the United States: Faults and eathquake magnitude: U. S. Army Corps of Engineers Waterways Exp. Sta. Misc. Paper , rept. 6, pp Staatz, A., Bowman, J. R. the Divide, Earthquake Taylor, R. B., 1975, in Curtis, B. F Mountains: Geol. t j. ^Butler, David, 1978, Location \, v. 49, no. 4 pp. 21. Ne onism in south-central Colorado, history of the southern Rocky :em. 144, pp of