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In August and September 1986, an earthquake swarm of possibly several hundred earthquakes occurred near Crested Butte, Colorado. The epicentral area is located within the Ruby Range in a region of extensive middle Tertiary volcanic and intrusive activity. The recording of this sequence has provided the best data to date to evaluate the source chara...
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... location program HYPOELLIPSE (Lahr, 1984) was used to locate all 78 earth- quakes. A simple plane-layered crustal velocity model for west-central Colorado (Wong, 1991) was used in the loca- tion calculations (Table 2). Station elevation corrections were calculated assuming a velocity equal to that of the ve- locity of the uppermost layer in the crustal model. ...
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... Post-glacial stress release has caused local tectonic features such as sackungen (Radbruch-Hall et al., 1977;Soldati, 2013) surrounding Mount Crested Butte (Gaskill, 1991). In a more regional context, most of modern-day southwestern Colorado is experiencing an extensional stress environment (Bott & Wong, 1995;Levandowski et al., 2018) in response to complete subduction of the Farallon Plate underneath the west coast of North America (Liu et al., 2008). ...
... Since the end of the Laramide Orogeny southwest Colorado has been experiencing extensional stresses. These extensional stresses have activated normal faults within the vicinity of the East River Watershed (Bott & Wong, 1995). Normal faults, based on Anderson's theory of faulting, generally dip steeper than reverse faults, around 50°-70°. ...
As climate changes and populations grow, groundwater sustainability is becoming increasingly important. Hydrogeologic models, which are based on a conceptual understanding of the subsurface, are crucial tools for informing decisions. Conceptual models of the subsurface incorporate knowledge of geological processes, and, frequently, observations from geophysical data into a common subsurface parameterization where the parameters may still be uncertain. Many methods exist to test how different conceptual subsurface parameterizations compare to geophysical data, but a frequent problem in hydrogeologic model development occurs when multiple geological phenomena could explain a single subsurface parameterization. In this work, we present a framework for testing geological hypotheses in conditions where a geological feature is observed in geophysical data, but its physical characteristics are uncertain. The framework uses Popper‐Bayes methods developed in previous studies, and is applied to study a fractured bedrock zone in a mountainous watershed in southwest Colorado. First, we propose six hypotheses based on the geological history of the watershed. Then, using the proposed Popper‐Bayes approach, we demonstrate that two of the hypotheses are inconsistent with the electrical resistivity tomography data. Finally, we discuss the importance of the prior model, and in what other scenarios the framework can be applied to.
... Fourteen faults around the state have been assigned Maximum Credible Earthquakes from M 6.25 -7.5 (Widmann et al., 1998). Contemporary crustal stress in western Colorado includes a roughly northeast-oriented extensional axis, based on inversion of earthquake focal mechanisms (Bott and Wong, 1995). The nature and rates of the tectonic processes responsible for creating seismic hazard in Colorado are not well understood. ...
We use 3 years of measurements from 25 continuous GPS stations across the Rio Grande Rift in New Mexico and Colorado to estimate surface velocities, time series and strain rates. The stations are part of the EarthScope Rio Grande Rift experiment, a collaboration between researchers at the University of Colorado at Boulder, the University of New Mexico, and Utah State University. The network includes 5 east-west station profiles transecting the rift, with the southernmost line in southern New Mexico and the northernmost line in northern Colorado. Most of the stations have shallow-drilled braced monuments installed in 2006-2007 and will remain occupied until 2010-2011 or longer. Data from regional Plate Boundary Observatory (PBO) GPS stations are included in the processing to increase station density and extend profiles further to the east and west of the Rio Grande Rift. Time series from the first three years of the experiment show excellent monument stability. Our results suggest that steady-state extension across the southern Rio Grande rift has a rate of 0.48 0.38 mm/yr. We did not detect significant steady-state extension across the northern reaches of the Rio Grande rift, but results give an upper bound of 0.9 mm/yr. We find that east-west extension rates from the 4 northern profiles increase from north to south, which is consistent with a clockwise rotation of the Colorado Plateau around a pole of rotation in northern Colorado.
... Perhaps the bestknown earthquakes in Colorado have been those induced by the disposal of waste fluids at the Rocky Mountain Arsenal near Denver (Evans 1966;Healy et al. 1968;Herrmann et al. 1981) and secondary oil recovery in western Colorado at the Rangely oil field (Gibbs et al. 1973). Earthquake swarms in Colorado are not uncommon: a swarm of earthquakes occurred in August and September 1986 near Crested Butte, with at least 200 events and a maximum magnitude (M L ) of 3.5 (Bott and Wong 1995); another swarm of earthquakes occurred in the fall of 2001 near Trinidad, with a maximum magnitude of 4.6 (Meremonte et al. 2002). ...
... These faults indicate potential for much larger earthquakes than those recorded to date (Matthews 2003). The occurrence of earthquakes has been documented in a variety of areas in Colorado, but in most cases the seismograph station coverage has been spatially limited to specific regions (see, e.g., Goter and Presgrave 1986;Keller and Adams 1976;Bott and Wong 1995;Sheehan 2000;Sheehan et al. 2003). ...
... Swarms are common in volcanic regions such as Japan, central Italy, Afar, or oceanic ridges, where they occur before and during eruptions [e.g., Kisslinger, 1975;Noir et al., 1997;Aoki et al., 1999]. They are also observed in zones of Quaternary volcanism such as California, Colorado, French Massif Central, or Vogtland/NW Bohemia (border region between Germany and the Czech republic) [e.g., Bott and Wong, 1995;Hainzl and Fischer, 2002;Fischer and Horálek, 2003;Klinge et al., 2003], where fluid migration in a magmatic environment can be invoked. The dynamic evolution of earthquake swarms in intraplate regions [Špičák, 2000], for instance, those observed in Arkansas, Vosges (France), England, or Scotland [e.g., Chiu et al., 1984;Audin et al., 2002;Assumpção, 1981], remains more mysterious, even if fluid migration is a likely regulating factor. ...
The Ubaye valley, one of the most active seismic zones in the French Alps, was visited in 2003-2004 by a prolific and protracted earthquake swarm with a maximum magnitude M L = 2.7. The seismic activity clustered along a 9-km-long, 3- to 8-km-deep rupture zone which trends NW-SE across the valley and dips 80°SW. Focal mechanisms for the larger shocks show either normal faulting with a SW-NE trending extension direction or NW-SE strike slip with right lateral displacement. The activity initiated in the central part of the rupture zone, diffused to its periphery, and eventually concentrated in its southeastern deeper part. A permanent station situated above the swarm allowed us to monitor the entire phenomenon from its inception to its conclusion. The complete time series includes more than 16,000 events, with shocks down to magnitude M L = -1.3. It shows bursts of activity, separated by quiescent periods, with no well-defined subswarms as observed in other similar studies. The Gutenberg-Richter b value significantly varied between 1.0 and 1.5 in the course of the phenomenon.
... This tectonic stress field is consistent with the stress field inferred from other focal mechanisms in central Colorado (e.g., Wong, 1986;Bott and Wong, 1995). East of the dashed line, most of the focal mechanisms display predominantly reverse faulting in response to NW-SE to E-Wdirected compression (maximum principal stress) as observed elsewhere in the Great Plains and Midcontinent (Zoback and Zoback, 1989). ...
... Perhaps the best known earthquakes in Colorado have been those induced by the disposal of waste fluids at the Rocky Mountain Arsenal near Denver (Evans, 1966;Healy et al. 1968;Herrmann, 1981) and secondary oil recovery in western Colorado at the Rangely oil field (Gibbs et al. 1973). Earthquake swarms in Colorado are not uncommon (Bott & Wong 1995). A swarm of earthquakes, including one of magnitude 4.6, occurred near Trinidad, Colorado in the fall of 2001 (Meremonte et al. 2002). ...
... Occasional microearthquake surveys of limited extent have been conducted (e.g. Goter & Presgrave 1986;Keller & Adams 1975;Bott & Wong 1995;Sheehan, 2000;Godchaux, 2000) and the U.S. Bureau of Reclamation has operated two small seismograph networks in southwest Colorado, mainly to monitor induced seismicity related to a dam near Ridgeway, Colorado, and deep brine injection near Paradox, Colorado. In addition, Microgeophysics Corporation operated a seismograph network in the Front Range under contract to the Denver Water Board from [1983][1984][1985][1986][1987][1988][1989][1990][1991][1992][1993]. ...
... The contemporary crustal stress regime in Colorado includes extension along a roughly northeast-oriented axis (Bott & Wong 1995). A possible mechanism for seismogenesis in the area is reactivation of existing faults, which are oriented favorably to the contemporary stress field, i.e. oriented west-northwest to northwest. ...
Construction of seismic hazard and risk maps depends upon carefully constrained input parameters including background seismicity, seismic attenuation, and slip rates of Quaternary faulting. Incomplete knowledge of any of these parameters reduces the accuracy and usefulness of the resulting hazard maps. The state of Colorado has a rapidly growing population, active tectonics, and seismicity that has never been surveyed statewide. Monitoring of seismicity in Colorado has been sparse at best, and we have relied heavily on a patchy historical record and microearthquake surveys of limited spatial extent for our current state of knowledge regarding the levels of seismicity in Colorado. This paper summarizes two microearthquake experiments conducted in Boulder County and the Northern Front Range, Colorado, in 1996-7 and 1999, and describes the 1992 Rocky Mountain Front broadband seismic experiment. These brief experiments provide data that improves our understanding of Colorado seismicity and seismic hazard, though more comprehensive monitoring is needed. A brief discussion of mine blast practices is included, as mine blasts dominate the seismic record in Colorado at magnitudes less than 2. The potential for additional useful information on Colorado earthquakes and seismicity to come from additional analysis of existing seismic data sets is recognized.
... 4. Focal mechanism of the mainshock (a) and three of the largest aftershocks (b, c, d) (relevant parameters are reported inTable 1). Filled and open symbols correspond to compressions and dilatations, respectively.Table 1) as a master event (Bott and Wong, 1995; Meremonte et al., 1995). The 17 best located events (erh <1 km, rms <0.2) are plotted inFig. ...
We analyse the source process and the aftershock distribution of the April 21, 1995, Ventimiglia, ML 4.7 earthquake using the records of permanent high dynamic broad-band seismic stations and a temporary network deployed on land and at sea few hours after the earthquake. This event occurred on the western Mediterranean coast, near the border between Italy and France, at a depth of 9 km, at a point where Alpine tectonic units and Late Oligocene extensional structure overlap and are currently undergoing compressional stress. The focal solutions of the mainshock and three aftershocks depict a dominant reverse faulting with an important strike-slip component, which underlines two nodal planes: a NW–SE-dipping north fault and a NE–SW-dipping south fault. We operate a careful re-location of the aftershocks using a master-event technique and data from the temporal network and obtain a predominant NW–SE alignment. Then, we analyse the rupture process using an empirical Green function approach. We find that the mainshock broke a 0.5 to 1 km fault length and that the rupture propagated during 0.1–0.2 s probably in a SE direction. Those two arguments, together with the recent fault trace that exists close to the epicentre, leads us to propose that this event expresses the reactivation of an old transverse NW–SE structure with a dextral movement. This study thus emphasizes the role of inherited, deep-rooted, transcurrent features in the tectonic reactivation of this passive margin. It also underlines the importance of combining short-period and broad-band seismology to better resolve and understand regional tectonic processes in areas of moderate seismic activity and complex geology.
... Despite this evidence for past and present earthquake activity, the nature and rates of the tectonic processes responsible for creating this seismic hazard are not well understood. The contemporary crustal stress regime in Colorado includes extension along a roughly northeast-oriented axis [Bott and Wong, 1995]. A possible mechanism for seismogenesis in the area is reactivation of existing faults, which are oriented favorably to the contemporary stress field, i.e. oriented westnorthwest to northwest. ...
Using repeated high precision GPS measurements on existing monuments, we hope to accurately determine the present day crustal strain rates in the State of Colorado on a regional and/or local scale. We will provide the first modern, space-based crustal strain and surface velocity estimates for the southern Rocky Mountain region, including the Front Range and Rio Grande Rift. In addition, this work will provide a critical set of measurements to which to refer future project measurements. Thus, if a future moderate or large earthquake were to occur in Colorado, the earthquake size and faulting geometry might be accurately determined by a second set of measurements.
... Past microseismic (M < 4) monitoring done in the Front Range area (just to the west of Denver) from 1983-1993 by Microgeophysics Corporation showed a moderate amount of microseismicity which tended to occur in swarms . Two such swarms in western Colorado were studied in detail by Goter et al. (1988; Carbondale swarm) and Bott and Wong (1995;the 1986 Crested Butte swarm). ...
... Locations of foci (hypocenters) of these swarm related earthquakes appear to trace out planar fault geometries which most likely represent the reactivation of preexisting faults (Bott and Wong, 1995). Although small in magnitude, the number of earthquakes involved in these swarms sums to a significant amount of released seismic energy . ...
... However, these injected fluids affected seismicity approximately 10 days after their injection (Healy et al., 1968). Bott and Wong, 1995). ...
Colorado has over a hundred years of history in seismic research and monitoring. It has experienced both unintentional and intentional induced seismicity, being one of the first places in which the phenomenon was observed. The state has had numerous tectonic earthquakes, the largest being a historical, estimated magnitude 6.6, that occurred in 1882. Being far away from tectonic plate boundaries, Colorado earthquakes are a unique window into the study of intraplate tectonics and continental rifting. Beginning with the earliest seismometer installed in the state in 1909, this article presents a history of Colorado seismic stations, earthquake activity, and active fault mapping.
