Earthquakes at Rangely, Colorado
Rick Moran - April 2007
ES 767, Tectonics, Emporia State University
Since 1963 nineteen small earthquakes with magnitudes > 2.5 have occurred within 25 km of Rangely, Rio Blanco County, northwestern Colorado. The strongest was 4.9 on the Richter scale in 1995. Extensive research was done by the United States Geological Survey (USGS) from 1969 to 1973 to examine why so many earthquakes were occurring. The USGS study became known as The Rangely Experiment which established a correlation between deep water well injection in the Rangely oil field and seismicity. A fault of Pennsylvanian age oriented N50E was identified as the source of the seismicity. Most all the area earthquakes appear to be originating from Pennsylvanian faults with the same orientation.
Several small earthquakes have been recorded near the towns of Rangely and Dinosaur since a seismograph was installed to record the area seismicity. In the 1960's it was noted that many were associated with the Rangely oil field. The giant oil field began production in 1945 from the Weber formation of Permian age at a depth of 1700 to 2100 meters. In 1958 secondary recovery began with the injection of water which increased the fluid pressure in the Weber above the original 190 bars. Subsequent studies by the USGS in the early 1970's determined the critical fluid pressure above which earthquakes could be trigger is 257 bars (3730 psi). Haimson's research concluded the critical fluid pressure was 237 bars (3450 psi). Fluid pressures in some areas had reached 275 bars by the early 1970's. More than 1500 minor seismic events were identified by researchers in the Rangely area from the 1963 to 1973. Based on the findings of the research, water injection was adjusted to maintain pressures below the critical pressure.
To the west and north of Rangely are 2 fault systems. One is a major NW trending thrust fault system of Eocene age including the Willow Creek Fault. The other fault system consists of much smaller Pennsylvanian age normal faults trending N50E. The normal faults were reactivated during the early Cretaceous. In the Rangely oil field they cut the Lower Permian Weber formation by up to 20 meters. The largest identified is the Main Field fault located in the eastern half of the oil field. No apparent surface expression of the Main Field fault exists within the oil field. The Main Field fault may express itself at the surface as the White River 15 km to the SW over a 30 km length. On the western side of the oil field is another normal fault trending N50E not as well defined.
The first seismograph station close enough to Rangely to record the small earthquakes was installed November 1962. It is located 70 km WNW of Rangely and continues to operate from there. In 1967 four seismographs were installed around Rangely oil field by the USGS and increased to 16 in 1969. They were later removed in the mid-1970's. One seismograph remained in the Rangely area until removed in 1982.
In reviewing data extractions from earthquake databases done over the last 15 years the difference in interpretation of seismograph records is apparent. Earthquake epicenters for the same event have moved up to 10 km and magnitudes adjusted by up to 0.8. Press reports provided within a few days of an earthquake don't always match the earthquake's location and intensity currently in the Advanced National Seismic System (ANSS) catalog used here. Five earthquakes were logged from the mid-1960's by people living in Rangely that are not listed in the ANSS catalog.
When examining the last 40 years of seismic activity within 100 km of Rangely it is apparent a cluster of events exists west of the Rangely and towards Redwash, Utah. Online USGS maps of seismicity of Colorado and Utah from 1990 - 2001 provide a quick reference. Earthquakes from the ANSS catalog in the Rangely area can be grouped into 6 episodes covering the last 40 years: three in 1966, two in 1967, two in 1970, one in 1973, two in 1979, and eight in 1995-6. Missing from the catalog is a 3.4 earthquake that occurred 8/5/1964 that was felt in Rangely.
Map of Rangely area earthquakes covering November 1962 to April 2007 from the ANSS catalog and the NCEDC. Only faults discussed are included on the map. The colored circles correspond to estimated depths with red 2 to 4 km, yellow 4 to 6 km, and green > 6 km. There are no earthquakes cataloged covering the 40 km south of this map. A satellite photo of approximately the same area is available from Google Maps.
The three 1966 earthquakes located 15 to 20 km west of Rangely occurred over a 7 hour period. Two more occurred an hour apart in 1967. In 1970 an earthquake was centered on the White River, followed 6 hours later by an aftershock to the SW. The epicenter of these earthquakes identifies a reoccurring pattern in the area. Within a few days most earthquakes are followed by an aftershock > 2.5 centered 5 to 15 km away. Most all aftershocks are located on a NE-SW trend or NW of the original epicenter. This implies movement is occurring along a NE trending fault system which matches the Pennsylvanian fault system identified in the Rangely oil field and on the Douglas Creek Arch to the south. Aftershocks located to the NW may be the result of stress transferred to the next parallel fault. A more detailed study would be necessary to confirm the hypothesis.
In 1979 a 4.1 magnitude earthquake was centered 6 km southwest of Rangely. The Meeker Herald reported an individual saying: "One person in the office here (Rangely) thought somebody had run a truck into the building; but for the most part, it just rattled everybody's windows and shook the ground a little." A sonic boom like sound was also reported with the initial tremor. Later that day a small aftershock was felt but not catalogued in the ANSS database. Ten days later a 2.6 aftershock occurred located 20 km north of Rangely. The aftershock was near the Willow Creek thrust fault, the only earthquake that closely matches that fault's location.
In 1995 at 5:46 MST the 1st of 4 earthquakes occurred between Rangely and the town of Dinosaur. It was reported in Dinosaur lasting 10 to 15 seconds and not felt in Vernal, Utah to the west. The 1st earthquake at 4.9 was centered on the Main Field fault. Original reports had placed the epicenter 3 miles south of Dinosaur at magnitude 4.1. This points to some of the problems of the uncertainty in identifying the epicenter.
In 1973 several surface cracks appeared at the surface west and northwest of Rangely. In 1976 seven miles west of Rangely cutting across highway 64 an ~6 cm vertical crack offset the road without any lateral movement. From the surface cracks and sheared surface casing the movement was identified as localized hydraulic fracturing at a depth of 100 meters along Mancos formation bedding planes. At the time the USGS had seismograph stations in the Rangely area and did not record and seismicity with the slide events. Later media reports associated the cracks with past earthquakes, but evidence indicates they developed over a few minutes to days and not in a few seconds.
There have been several reports since the 1990's of suspected shaking and loud booms by people in the Rangely area. The cause is unknown with possible explanations of military air traffic, shallow Mancos shale shifting, dynamite, and a large meteor entry. No seismicity was catalogued by the ANSS for these events other than April 1995.
Recent research at Livermore Laboratory, UCSC, and the
Colorado School of Mines is investigating fault detection in the Rangely
oil field by soil and vegetation measurements as part of a CO2 sequestration
study. Geobotanical baselines were created from aerial remote sensing
and ground based measurements. The concept is if injected CO2 leaks from
the oil field it would so along faults. A release
would more likely occur from an earthquake which could have a measurable impact
on surface vegetation and soils.
Small earthquakes have been recorded west of Rangely since the 1960's. No nearby seismographs existed prior to that time to know the level of earlier earthquake activity. It was clearly demonstrated in the 1960/70's many if not all were induced by high fluid pressures from water injection. Earthquakes did drop off noticeably after 1972 once fluid pressures were reduced. In the subsequent years earth quake episodes occurred in 1979 and 1995.
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James F. Gibbs, Jon H. Haely, C. Barry Raleigh, John Coakley, 1973. Seismicity in the Rangely, Colorado Area: 1962-1970, Bulletin of the Seismological Society of America, v. 61, no. 5, pp. 1557 - 1570.
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Pickles, Bill, 2003. Bird's-Eye View Clarifies Research on the Ground, Science & Technology.
C.B. Raleigh, J.H. Haely, J.D. Bredehoeft, 1976. An Experiment in Earthquake Control at Rangely, Colorado, Science, v. 191, no. 4233, pp. 1230 - 1237.
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Chris Thompson, Rick Moran, Steve Horne, John Walsh, John Fairborn, 2002. Seismic fracture characterization of a sandstone reservoir - Rangely Field, Colorado. 72nd Annual International Meeting: Society of Exploration Geophysics, 1049-1052
J. Walsh, J. Urdea, J. Jyde, H. Simon, S. Horne, C. Thompson, G. De, R. Moran, 2002. Determining Fracture or Stress Through Casing: A Case Study. Society of Professional Well Log Analysis, 43rd Annual Symposium.
Earthquake measuring 3.4 rattles Rangely area. Meeker Herald. March 22, 1979.
Quake rolls Through. Denver Post. March 21, 1995.