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At least 37 records · Page 2Linked to original sources
USGS1963

Structure of the crust and upper mantle in the western United States

Seismic waves generated by underground nuclear and chemical explosions have been recorded in a network of nearly 2,000 stations in the western conterminous United States as a part of the VELA UNIFORM program. The network extends from eastern Colorado to the California coastline and from central Idaho to the border of the United States and Mexico. The speed of compressional waves in the upper-mantle rocks ranges from 7.7 km/sec in the southern part of the Basin and Range province to 8.2 km/sec in the Great Plains province. In general, the speed of compressional waves in the upper-mantle rocks tends to be nearly the same over large areas within individual geologic provinces. Measured crustal thickness ranges from less than 20 km in the Central Valley of California to 50 km in the Great Plains province. Changes in crustal thickness across provincial boundaries are not controlled by regional altitude above sea level unless the properties of the upper mantle are the same across those boundaries. The crust tends to be thick in regions where the speed of compressional waves in the upper-mantle rocks (and presumably the density) is high, and tends to be relatively thin where the speed of compressional waves in the upper-mantle rocks (and density) is lower. With in the Basin and Range province, crustal thickness seems to vary directly with regional altitude above sea level. Evidence that a layer of intermediate compressional-wave speed exists in the lower part of the crust has been accumulated from seismic waves that have traveled least-time paths, as well as secondary arrivals (particularly reflections). On a scale that includes many geologic provinces, isostatic compensation is related largely to variations in the density of the upper- mantle rocks. Within geologic provinces or adjacent provinces, isostatic compensation may be related to variations in the thickness of crustal layers. Regions of thick crust and dense upper mantle have been relatively stable in Cenozoic time. Regions of thinner crust and low-density upper mantle have had a Cenozoic history of intense diastrophism and silicic volcanism.

Crustal Studies Technical Letter↗
USGS1963

Crustal structure in the western United States; study of seismic propagation paths and regional traveltimes in the California-Nevada region

The U.S. Geological Survey, with the assistance of United ElectroDynamics, Inc., completed ten weeks of seismic-refraction field work during the summer of 1962 in the southwestern part of the United States. This work was a continuation of a program initiated in 1961 to study traveltimes and seismic propagation paths in the earth?s crust and upper mantle in the western United States. A total of 761 seismograms were recorded along 10 profiles from 86 explosions at 18 shotpoints. Analysis of the data is continuing, but a few conclusions can be made from a preliminary study: (1) Variations in traveltimes in the Basin and Range province are large but measurable, and perhaps predictable. (2) Traveltimes of seismic waves in adjacent geologic provinces are usually significantly different. (3) The velocity of P g along all of the profiles recorded in 1962 ranges from 5.0 to 6.5 km/sec, and averages 6.0 km/sec. (4) The average velocity of P g in extreme northern Nevada and southern Idaho is 5.6 km/sec, and it is 6.1 km/sec in most of Nevada and California. (5) The average velocity of P n is 7–9 km/ sec and ranges from 7.85 to 7.95 km/sec on reversed profiles where the true P n velocity could be computed. (6) A shallow "intermediate" layer with a velocity of approximately 6.8 km/sec was found in the Snake River Plain. (7) Refraction arrivals from the mantle (P n ) were recorded in the Sierra Nevada. They indicate that the thickness of the crust in the Sierra Nevada is much greater than that in the Basin and Range province. (8) Many refinements in field techniques were made during the 1962 field season.

California;Nevada;Idaho↗
USGS1963

Traveltimes and amplitudes from nuclear explosions; Nevada Test Site to Ordway, Colorado

This paper treats the results of a study of seismic waves generated by eight nuclear explosions and recorded at 31 locations between the Nevada Test Site (NTS) and Ordway, Colorado. The line of recording stations crosses the eastern part of the Basin and Range Province, the Colorado Plateau, the southern Rocky Mountains, and extends into the Great Plains. In the eastern Basin and Range Province and the western margin of the Colorado Plateau (0 ≤ Δ ≤ 385 km ), the time-distance curves for P g and P n can be expressed, respectively, as T 1 = 0.8 + Δ/6.0. T 3 = 5.8 + Δ/7.6. A third phase, tentatively identified as P*, is represented by the equation T2 = 3.8 + Δ/6.5. Using the crustal structure and P n velocity (7.9 km/ sec) found for the NTS region by other authors, these relations indicate that the thickness of the crust increases from about 25 km at NTS to about 42 km in the western part of the Colorado Plateau Province. East of this boundary the velocity of P in the upper mantle increases to 8.0 km/sec; depth to the Mohorovicic discontinuity is approximately constant over the range 435 ≤ Δ ≤ 645 km. Beyond 850 km, first arrivals indicate an apparent velocity of about 8.4 km/sec. Amplitudes of P n attenuate according to the equation A = A o Δ -1/2 (Δ -d) -3/2 e -0.0022Δ over the distance range 150 ≤ Δ ≤ 850 km. This relation yields a value of Q, for P n of about 520. The amplitudes of P g attenuates extremely rapidly, and beyond about 130 km this phase cannot be identified with certainty. An extension of the P g traveltime branch at large distances could be associated with waves reflected beyond the critical angle, from the base of the crust. This phase, called ?P after Mohorovicic, appears to attenuate as A = Ao e -0.076Δ Δ -1/2 . The value of Q indicated by this equation is about 200.

Nevada;Colorado↗
USGS1963

Crustal structure from San Francisco, California, to Eureka, Nevada, from seismic-refraction measurements

Seismic-refraction measurements from chemical explosions near San Francisco, California, and Fallon and Eureka, Nevada, were made along a line extending nearly 700 km inland from San Francisco across the Coast Ranges, Great Valley, Sierra Nevada, and Basin and Range Province. The velocity of P g in the Basin and Range Province was found to be 6.0 km/sec. Between Fallon and Eureka the velocity of P n is 7.8 km/sec, and just east of the Sierra Nevada it is about 7.9 km/sec. Two prominent phases closely following the first arrival between 50 and 250 km from the source in the Basin and Range Province were interpreted as reflections from an intermediate layer and from the Mohorovicic discontinuity. The velocity of P in the possible intermediate layer, deduced from the reflected phases be cause the refracted wave expected from this layer is nowhere a first arrival, seems to be 6.6 km/sec at the top of the layer and probably increases with depth.

California;Nevada↗
USGS1964

Variations in regional traveltimes

Precise epicentral location of a seismic event is made difficult by variations in regional traveltimes. A discussion is presented on delays to be expected in the various segments of a generalized travel path of seismic waves. Traveltime variations caused by changes in crustal structure and velocity introduce a major part of the uncertainty in traveltime at both the seismic source and receiver. Consideration of geologic factors that tend to be related to crustal thickness and mantle velocity may permit an estimate of the amount of delay introduced at the source. Delay at the seismic receiving stations can be determined and corrected for by a study of crustal thickness and a calibration of the velocity structure under the stations.

Crustal Studies Technical Letter↗
USGS1964

Compilation of seismic-refraction crustal data in the Soviet Union

The U.S. Geological Survey is preparing a series of terrain atlases of the Sino-Soviet bloc of nations for use in a possible nuclear-test detection program. Part of this project is concerned with the compilation and evaluation of crustal-structure data. To date, a compilation has been made of data from Russian publications that discuss seismic refraction and gravity studies of crustal structure. Although this compilation deals mainly with explosion seismic-refraction measurements, some results from earthquake studies are also included. None of the data have been evaluated.

Crustal Studies Technical Letter↗
USGS1964

Crustal structure in the eastern Colorado Plateaus Provence from seismic-refraction measurements

A reversed seismic-refraction profile was recorded in the Colorado Plateaus Province from Hanksville, Utah, to Chinle, Arizona, The velocity of P g is 6.2 km/sec, and the true velocity of P n is 7.8 km/sec, Waves identified as reflections indicate that an intermediate layer in the crust has a velocity of approximately 6.8 km/sec. Thickness of the crust is 43 km at Chinle and 40 km at Hanksville. The P n velocity in the Colorado Plateaus Province is the same as that in the Basin and Range Province, but is significantly lower than Pn in the High Plains of Colorado.

Arizona;Colorado;Utah↗
USGS1964

Continental crust

The structure of the Earth’s crust (the outer shell of the earth above the M-discontinuity) has been intensively studied in many places by use of geophysical methods. The velocity of seismic compressional waves in the crust and in the upper mantle varies from place to place in the conterminous United States. The average crust is thick in the eastern two-thirds of the United States, in which the crustal and upper-mantle velocities tend to be high. The average crust is thinner in the western one-third of the United States, in which these velocities tend to be low. The concept of eastern and western superprovinces can be used to classify these differences. Crustal and upper-mantle densities probably vary directly with compressional-wave velocity, leading to the conclusion that isostasy is accomplished by the variation in densities of crustal and upper-mantle rocks as well as in crustal thickness, and that there is no single, generally valid isostatic model. The nature of the M-discontinuity is still speculative.

Crustal Studies Technical Letter↗
USGS1964

Seismic-refraction measurements of crustal structure between American Falls Reservoir, Idaho, and Flaming Gorge Reservoir, Utah

Interpretation of a reversed seismic-refraction profile recorded between American Falls reservoir and Flaming Gorge reservoir in May 1963 indicates that the depth to the Mohorovicic discontinuity is about 31 km at American Falls and 37 km at Flaming Gorge. The existence of an intermediate crustal layer at a depth of about 19 to 21 km beneath the profile is well supported by refractions and reflections. The velocity of compressional waves in the mantle just beneath the Mohorovicic discontinuity is about 7.8 km/sec, their velocity in the intermediate layer is about 6.9 km/sec, and their velocity in the upper crust (beneath the near-surface low-velocity material) is about 5.9 km/sec. A prominent phase with an apparent velocity of 8.4 km/sec was recorded at distances of 210 km to 325 km from shots at American Falls. This phase is believed to be a reflection from a boundary within the mantle.

Idaho, Utah↗
USGS1964

Crustal structure between Lake Mead, Nevada, and Mono Lake, California

Interpretation of a reversed seismic-refraction profile between Lake Mead, Nevada, and Mono Lake, California, indicates velocities of 6.15 km/sec for the upper layer of the crust, 7.10 km/sec for an intermediate layer, and 7.80 km/sec for the uppermost mantle. Phases interpreted to be reflections from the top of the intermediate layer and the Mohorovicic discontinuity were used with the refraction data to calculate depths. The depth to the Moho increases from about 30 km near Lake Mead to about 40 km near Mono Lake. Variations in arrival times provide evidence for fairly sharp flexures in the Moho. Offsets in the Moho of 4 km at one point and 2 1/2 km at another correspond to large faults at the surface, and it is suggested that fracture zones in the upper crust may displace the Moho and extend into the upper mantle. The phase P appears to be an extension of the reflection from the top of the intermediate layer beyond the critical angle. Bouguer gravity, computed for the seismic model of the crust, is in good agreement with the measured Bouguer gravity. Thus a model of the crustal structure is presented which is consistent with three semi-independent sources of geophysical data: seismic-refraction, seismic-reflection, and gravity.

California, Nevada↗
USGS1964

A preliminary summary of a seismic-refraction survey in the vicinity of the Tonto Forest Observatory, Arizona

The U.S. Geological Survey complete d a seismic-refraction survey in the vicinity of the Tonto Forest Seismological Observatory (T.F.S.O.) in April and May 1964. More than 1200 km of reversed profiles were surveyed to determine the crustal structure and crustal and upper mantle velocities in this area. The purpose of this work was to provide information on wave-propagation paths of seismic events recorded at T.F.S.O. and to improve the performance of the Observatory in locating and identifying these events. First arrivals indicate that the Mohorovicic discontinuity dips to the northeast by as much as 6 degrees under T.F.S.O., and may even be displaced vertically by as much as 5 km immediately north of the Observatory near the boundary of the Basin and Range and the Colorado Plateau Provinces. A preliminary examination of the first arrivals indicates that the crust at T.F.S.O. is at least 30 km thick and is made up of at least two seismic layers. A thin veneer at the surface with a velocity of approximately 4 km/sec is underlain by a layer with a velocity of approximately 5.9 km/sec to 6.1 km/sec. An intermediate layer with velocity of 6.6 to 7.0 km/sec is probably present in the lower crust, but is not revealed by first arrivals. The velocity of seismic waves in the upper mantle is about 7.9 km/sec.

Arizona↗
USGS1964

Seismic-refraction measurements of crustal structure between Nevada Test Site and Ludlow, California

Seismic-refraction measurements from nuclear and chemical explosions were made along a line from the Nevada Test Site (NTS) to Ludlow, California, and additional recordings from nuclear explosions were made southward toward Calexico, California. The time of first arrivals from the Ludlow shotpoint is expressed as T 0 = 0.00 + Δ/2.50 (assumed), T 1 = 1.00 + Δ6.10, T 2 = 2.81 + Δ/6.80, and T 3 = 5.48 + ~7.76, where T is in seconds and distance Δ is in km. First arrival times from NTS fit the lines T 1 = 0.74 + Δ/6.10, T 2 = 2.81 + Δ/6.80 (assumed), T 3 = 6.70 + Δ/8.04 to a distance of 265 km, beyond 265 km T 3 = 5.83 + Δ/7.75. The difference in the apparent velocities of the P n (T 3 ) arrival is caused by variations in the dip of the Mohorovicic discontinuity. The thickness of the successive layers at NTS are H 0 s 1.0 km (V 0 ~ 2.5 km/sec), H 1 = 13 km (v 1 = 6.1 km/sec), and H 2 = 20 km (v 2 = 6.8 km/ sec); the total crustal thickness is 34 km. The successive crustal layers at Ludlow have a thickness of H 0 = 1.4 km, H 1 = 13 km, and H 2 = 13 km; the total crustal thickness is 27 km.

Arizona, California, Nevada↗
USGS1965

Calculations of upper-mantle velocity from published Soviet earthquake data

The lack of information on mantle velocities and crustal structure of the U.S.S.R. has led to a preliminary examination of published Soviet earthquake bulletins in the hope of deriving useful velocity and structure information from the data they contain. Mantle velocities deduced from earthquake data on several Russian earthquakes are in excellent agreement with results of Soviet deep seismic sounding.

Soviet Union↗
USGS2013

Interactions between a group of Golden Eagles and a herd of North American elk

Raptors are generally considered solitary predators (Schoener 1969), but occasionally they interact socially (Brown and Amadon 1968). Certain raptor species (e.g., Swallow-tailed Kites [ Elanoides forficatus ] and Swainson's Hawks [ Buteo swainsoni ]) concentrate in aggregations in response to localized, abundant food sources (Ellis et al. 1993). Many raptor species engage in group hunting (Ellis et al. 1993), and social foraging is a routine strategy for some species (e.g., Harris's Hawks [ Parabuteo unicinctus ]; Bednarz 1988, Ellis et al. 1993]. Raptors generally engage in group hunting to pursue elusive or large prey (Ellis et al. 1993). Occasionally individuals of conspecific raptors engage in play as a group sometimes involving chases of prey species (Palmer 1988). In this letter, we report interactions between a large group of Golden Eagles and a herd of adult and juvenile Rocky Mountain elk ( Cervus canadensis nelsoni ) in late autumn.

Idaho↗