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The 1986 Crested Butte earthquake swarm and its implications for seismogenesis in Colorado
Abstract and Figures
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 characteristics of an earthquake sequence in Colorado and the associated tectonic stresses. At least 200 events were recorded at regional distances; 30 events were Richter magnitude (M L) 1.6 and greater, and 16 were reported felt. The largest event, M L 3.5, occurred on 3 September. In addition to the regional recordings, a portable seismographic network was deployed from 19 to 26 August. Based on these data, 78 events were relocated using a master event technique. The earthquakes define a 6-km-long, northwest-striking planar zone dipping steeply to the northeast, between the depths of 2 and 11 km. Focal mechanisms indicate pre-dominantly normal faulting with a minor left-lateral component on an approximately northwest-striking, northeast-dipping plane. These observations are all consistent with possible slip on the Treasure Mountain fault, a late Tertiary structure within the Ruby Range. The northeast-oriented T axes exhibited by the Crested Butte focal mechanisms are consistent with the regional extensional stress direction characteristic of the southern Rocky Mountains as indicated by other earthquake focal mechanisms. Within this extensional stress regime, earthquakes in western Colorado appear to be the result of normal slip on reactivated preexisting faults that are favorably oriented to the contemporary stress field.
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... 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). ...
... 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.
... 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). ...
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.
We developed a catalog of small magnitude (ML -0.1 to 4.7) seismicity across Colorado and New Mexico from the EarthScope USArray Transportable Array and CREST (Colorado Rocky Mountains Experiment and Seismic Transects) seismic networks from 2008-2010 to characterize active deformation in the Rio Grande Rift. We recorded over 900 earthquakes in the Rio Grande Rift region, not including induced earthquakes and mine blasts, and find that the rift is actively deforming both broadly and in distinct regions. Seismic events that are likely induced, mostly in the Raton Basin, make up 66% of the catalog (1,837 earthquakes). Neogene faults in the northern rift in north-central Colorado are seismically active in the North Park basin and northwestern Colorado. The central rift from the San Luis Basin (southern Colorado) to south of the Socorro Magma Body is the most seismically active Rift region, and seismicity delineates the deformation in the Colorado Plateau transition zone, which is spatially correlated with volcanic vents, dikes, and faults within the western Jemez Lineament. The eastern Jemez Lineament is nearly aseismic and surrounded by a halo of seismicity culminating in boundaries defined by recent moderate (MW 3.9 and MW 3.3) earthquakes. The southern rift is characterized by diffuse seismicity in Texas and Mexico. This study provides an updated seismic catalog built with uniformity in seismometer coverage and low epicentral uncertainties (~2 km) that allows for regional evaluation of seismicity. During this time period, clusters of seismicity and moderate magnitude earthquakes characterize deformation in a low-strain rate extensional environment.
The Jiashi swarm earthquakes have been relocated using the "master event" method. On the basis of the high-precision relative locations and focal mechanisms of the swarm, we infer that a pair of echelon fault segments with NNW strike, right step and right lateral may exist in the swarm area. The average focal mechanism of the swarm area has been inversed from the spatial distribution of the polarities of 2177 P-wave initial motions in the Jaishi swarm. Based on the Silver's seismic source model, the main fracture characteristics of the swarm have been inferred from source spectra. According to the faulting model inferred mainly from the high-precision locations of the swarm earthquakes, we calculated the perturbing stress field generated by the echelon fault segments and explained some swarm source characteristics.
New helium isotope data measured for geothermal springs in the State of Colorado range from crustal helium isotope values (0.02 R/Ra) to ratios that overlap with subcontinental mantle lithosphere (3-7 R/Ra). The geothermal resource potential for Colorado has generally been overlooked by many industry developers however new geochemical data warrant further investigation of several key resources that have helium isotope signatures that exceed many successfully producing blind Basin and Range resources, such as Dixie Valley. Helium isotope ratios up to 5.88 Rc/Ra measured on gases in carbonic geothermal springs near Rico are the highest yet to be measured in the state, and isotope ratios for gases represent a mantle 3He isotope component of 73%. Quartz geothermometer temperatures of 155°C for Rico indicate minimum equilibration temperatures when considering dilution with shallow groundwater, and geothermal gradients ranging from 60 to 80°C/km predict potential resources between 1.8 and 2.2 km depth within permeable Precambrian basement rocks. Fluid-mineral equilibration temperatures, geothermal gradients, and basin depth for many other resources in Colorado largely imply that equilibrated geothermal fluids originate at depths well within Precambrian basement rocks, and suggest a similar deep origin that is not obvious from the geochemically diverse hot springs that emanate at the surface. The helium isotope distribution for Colorado geothermal springs appears to be strongly associated with faults, extended crust, and Pliocene magmatic intrusions and more discretely associated with diffuse lower to middle crustal permeability and low-velocity mantle anomalies. This research investigates the potential of using helium isotope ratios as an exploration tool in low-temperature systems to vector toward high crustal permeability zones in regions of anomalous crustal heat flow.
This work investigates the spatial distribution of three seismic swarms that occurred in Western Liguria (NW Italy) in 1990 and 1993. The largest events, recorded on July and December 1993, exceeded magnitude 4.0. The routine locations of hypocenters were not able to depict the geometry of the active zone, due to the limitations of the regional seismic network. Also the adoption of different location techniques (“forward” probability estimation, relative locations) did not furnish any particular information on the real spatial distribution of these very closely spaced events.
The analysis of the recorded signals demonstrated the strong coherency among seismograms and suggested the use of doublet-multiplet techniques in order to evaluate the relative position of events. For every swarm, the relative locations obtained (with estimated errors of the order of a few meters) depict clearly the geometry of the very limited area involved in this seismic activity (of the order of 1 km2): all the analyzed events lie (within a few meters) on a plane that is in agreement with one of the planes of the focal mechanism of the mainshocks obtained by first-motion polarities. A strike-slip rupture, with a compressive component, on planes with strike around 80° and dip ranging from 56° to 74°, is in agreement with all the available information. Moreover, using some similar events belonging to the different periods of activity (bridge events), the relative position of the three groups of earthquakes was also determined, showing that the more energetic activity of 1993 reactivated a shallower area contiguous to the sector mobilized in 1990 and characterized by a slightly different dip.
The 1997 Jiashi swarm earthquakes are relocated by using the “master event” method. From the precise relative locations and focal mechanisms of MS ≥ 5.0 earthquakes we infer that a pair of right-step echelon fault segments, striking NNW-SSE and slipping right laterally, may exist in the source region. 2177 polarity readings of P-wave first motions are inverted for the average focal mechanism of the swarm earthquakes. Based on the Silver's source model[1], the main fracture characteristics of the earthquake faults are inferred from the observed source spectra. We have calculated the perturbed stress field generated by the motion of echelon fault segments and explained some source characteristics of the swarm earthquakes. An echelon fault model is introduced to the diversity of focal mechanisms and low stress drop of the Jiashi earthquakes. The paper suggests that the echelon fault structure in the source region is responsible for the multiplicity of strong earthquake occurrence.
We use continuous measurements of GPS sites from across the Rio Grande Rift, Great Plains, and Colorado Plateau to estimate present-day surface velocities and strain rates. Velocity gradients from five east-west profiles suggest an average of ∼1.2 nanostrains/yr east-west extensional strain rate across these three physiographic provinces. The extensional deformation is not concentrated in a narrow zone centered on the Rio Grande Rift but rather is distributed broadly from the western edge of the Colorado Plateau well into the western Great Plains. This unexpected pattern of broadly distributed deformation at the surface has important implications for our understanding of how low strain-rate deformation within continental interiors is accommodated.
From heat-flow data obtained in New Mexico and southern Colorado, one finds: (1) a major geothermal anomaly with heat-flow values greater than 2.5 HFU (heat-flow unit, microcal/(sq cm) sec) coincident with the western part of the Rio Grande rift; (2) a complex heat-flow pattern in the eastern Colorado Plateau with values of 1.5 HFU and less, apparently associated with major structural basins, and values of 2.0 HFU and greater, apparently associated with some intrusions and perhaps major uplifts; and (3) a regional increase in heat-flow values from 1.5 to 2.0 HFU to values greater than 2.5 HFU in southwestern New Mexico, which may be coincident with the north-trending geothermal transition zone between the Colorado Plateau and the Basin and Range provinces.
All available data on the Colorado earthquake of November 7, 1882 indicate an epicentral location in northwestern Colorado and a local magnitude (ML) of about 6½. There are few descriptive reports from the meisoseismal region because the area was sparsely populated at the time. An epicenter of 40.5°N, I 08°W has been estimated based on the distribution of Modified Mercalli intensities in Colorado, Wyoming, and Utah. This epicenter is uncertain by at least I 00 kilometers (km). The estimated felt area of 500,000 square km suggests ML is approximately 6½ based on a comparison with more recent, better documented earthquakes in the Rocky Mountain region. This magnitude estimate is uncertain by at least one-half magnitude unit clue to uncertainty in the felt area estimate and in the felt area-magnitude relationship. Given the dearth of earthquake reports in the epicentral region, the exact location will remain uncertain until a causative fault, with documented movement about I 00 years ago, is identified. One possible causative structure is located in the Piceance Basin in northwest Colorado. The fault was identified by investigation of satellite images and field reconnaissance after the preliminary epicenter had been identified. The 1882 shock is the largest earthquake documented in Colorado; a recurrence today would affect many engineering facilities in the Rocky Mountain region.
The orientation and relative magnitudes of in situ tectonic stress in the continental United States have been inferred from a variety of indicators, including earthquake focal mechanisms, stress-induced elliptical borehole enlargement ("breakouts"), hydraulic fracturing stress measurements, and young fault slip and volcanic alignments. The data come from a wide range of depths (0.1 to 20 km) and have been assigned a quality ranking according to their reliability as tectonic stress indicators. The data show that regionally consistent orientations persist throughout the seismic "brittle" upper crust. Stress provinces are defined on the basis of uniform stress orientations and relative stress magnitudes (style of faulting). The sources of stress for all the major stress provinces are believed to be linked either directly or indirectly to plate-tectonic processes. Most of the central and eastern United States is part of a broad "midplate" stress province (which includes most of Canada and possibly the western Atlantic basin) characterized by NE- to ENE-oriented maximum horizontal compression. This orientation range coincides with both absolute plate motion and ridge push directions for North America. With the exception of the San Andreas region and most of the Pacific Northwest area, the remainder of the western United States (generally, the thermally elevated region from the high Great Plains to the west and including the Basin and Range province) is characterized by extensional tectonism. Within areas of "classic" basin-range structure, both those currently active and those largely quiescent, the least horizontal stress is oriented approximately E-W (between WNW and ENE). In the Colorado Plateau interior and the southern Great Plains, the least horizontal stress is NNE, roughly perpendicular to that in surrounding areas. The state of stress in these two regions may be related to pronounced lateral variations in lithosphere thickness beneath them. New focal-mechanism data in western Washington and northern Oregon define a region of NE compression apparently associated with subduction of the Juan de Fuca plate. In the coastal region of central California there appears to be a zone approximately 100 to 125 km wide on both sides of the San Andreas fault in which the maximum horizontal stress is oriented NE, nearly perpendicular to the fault itself. While compression orthogonal to the San Andreas fault explains late Tertiary-Quaternary folding and reverse faulting subparallel to the San Andreas, this NE compression seems incompatible with right-lateral slip on the N40°W-trending San Andreas fault. The observed stress field, however, is consistent with current estimates of the direction of relative plate motions, provided that the shear strength of the San Andreas is appreciably lower than the level of far-field shear stress in the crust.
The contemporary seismicity of the Colorado Plateau based on seismic monitoring in the past 30 yr can be characterized as being of small to moderate magnitude, and contrary to earlier views, of a low to moderate rate of occurrence with earthquakes widely distributed. Concentrations of earthquakes have been observed in a few areas of the plateau. The most seismically active area of the Colorado Plateau is the eastern Wasatch Plateau-Book Cliff. Although very few earthquakes can be associated with known geologic structures or tectonic features in the Colorado Plateau, seismicity appears to be the result of the reactivation of pre-existing faults lacking surficial expression but favorably oriented to the tectonic stress field. The contemporary state of stress within the plateau is characterized by approximate northeast-trending extension in contrast to the previous belief that the plateau was being subjected to east-west tectonic compression. One area of the plateau, the eastern Wasatch Plateau and Book Cliffs, may still be characterized by compressive stresses; however, the nature of these stresses is not well understood. -from Authors
In August and September of 1986, the towns of Crested Butte and Aspen were shaken by a series of earthquakes with Richter magnitudes up to 3.5. Many of the tremors were accompanied by unusual thunder-like sounds. These earthquakes were part of an intense swarm of several hundred events that may have its source along a set of conjugate faults with a system of dikes along the Ruby Range. The fault(s) were probably reactivated in a normal sense by contemporary extensional stresses that characterize much of western and central Colorado.