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L. C. Pakiser

Publications and source records attributed to L. C. Pakiser.

15 recordsLinked to original sources
USGS2012

Man-made earthquakes and earthquake prediction

Convincing evidence that man can trigger earthquakes has been developed since the 1963–1967 report. The fact that man can start earthquakes has increased our understanding of earthquake mechanisms and reinforced our judgment that we are approaching the possibility of earthquake prediction. Traditionally, seismologists have avoided the subject of earthquake prediction because of its distasteful association with people who claim to be able to predict earthquakes by mystical nonscientific methods. Research on prediction has been intensified since the reports of the Panel on Earthquake Prediction [ Press , 1965] and the Interagency Working Group for Earthquake Research [ Pecora , 1968],. however, and it is appropriate to assess our progress toward that goal. Pakiser et al . [1969] and Oliver [1970] have also summarized recent U.S. progress toward earthquake prediction.

Eos Science News↗
USGS1989

Geophysical framework of the continental United States: Progress, problems, and opportunities for research

Significant progress has been made over the past five decades in determining the geophysical framework of the continental United States. Highlights include detailed maps of gravity and aeromagnetic anomalies, heat flow, crustal thickness, seismicity, state of stress, and paleomagnetic pole positions. Important tectonic insights have come from earthquake studies, and from knowledge of lithospheric structure derived from seismic reflection, refraction/wide-angle reflection, surface-wave, and teleseismic data. Additional major advances in lithospheric geophysics will depend on four key factors: the reduction of uncertainties in the measurement and interpretation of geophysical data, the widespread application of coincident geophysical methods in concert with geological investigations, the collection of a more uniform continent-scale data base for all geophysical measurements, and the investigation of topical geophysical questions regarding the physical state and properties of the lithosphere. The impracticality of repeating most geophysical field measurements introduces poorly known, but likely large, uncertainties. Since most measurements are not repeated, high priority must be given to the reduction and quantification of uncertainties in measurements and interpretations. The most productive future investigations, in terms of resolution and minimum uncertainties in interpretation, will be those that apply different geophysical methods along identical profiles or areas, and that include geological investigations as a vital ingredient. Important gaps remain in our knowledge of the geophysical framework of the United States on a continent-wide scale, including the deep conductivity structure, the nature of the Moho discontinuity, the structure of the subcrustal lithosphere, and the depth of the lithosphere/asthenosphere boundary. Most transition zones separating geologic or physiographic provinces are poorly studied, yet these zones are likely to be the locations of the most profound changes in the physical properties of the lithosphere. Application of coincident geophysical techniques is needed to study these transition zones. Several topical geophysical questions warrant special emphasis in the future. These questions include the rheology of the crust and subcrustal lithosphere; the distribution, composition, and abundance of fluids in the crust; the genesis and evolution of the Moho; the origin of crustal conductivity zones and deep crustal reflections; the evidence for seismic anisotropy; and the short-term prediction of earthquakes.

GSA Memoirs↗
USGS1970

Use of microearthquakes in the study of the mechanics of earthquake generation along the San Andreas fault in central California

A small, dense network of independently recording portable seismograph stations was used to delineate the slip surface associated with the 1966 Parkfield-Cholame earthquake by precise three dimensional mapping of the hypocenters of its aftershocks. The aftershocks were concentrated in a very narrow vertical zone beneath or immediately adjacent to the zone of surf ace fracturing that accompanied the main shock. Focal depths ranged from less than 1 km to a maximum of 15 km. The same type of portable network was used to study microearthquakes associated with an actively creeping section of the San Andreas fault south of Hollister during the summer of 1967. Microearthquake activity during the 6-week operation of this network was dominated by aftershocks of a magnitude-4 earthquake that occurred within the network near Bear Valley on July 23. Most of the aftershocks were concentrated in an equidimensional region about 2 1 2km across that contained the hypocenter of the main shock. The zone of the concentrated aftershocks was centered near the middle of the rift zone at a depth of about 3 1 2km. Hypocenters of other aftershocks outlined a 25 km long zone of activity beneath the actively creeping strand of the fault and extending from the surface to a depth of about 13 km. A continuing study of microearthquakes along the San Andreas, Hayward, and Calaveras faults between Hollister and San Francisco has been under way for about 2 years. The permanent telemetered network constructed for this purpose has grown from about 30 stations in early 1968 to about 45 stations in late 1969. Microearthquakes between Hollister and San Francisco are heavily concentrated in narrow, nearly vertical zones along sections of the Sargent, San Andreas, and Calaveras faults. Focal depths range from less than 1 km to about 14 km.

California↗
USGS1968

Seismic evidence for the thickness of Cenozoic deposits in Mono Basin, California

From gravity and limited seismic data obtained in 1957, Pakiser and others (1960) reported a thickness of Cenozoic deposits in the deepest part of Mono Basin, California, of 5.5 ± 1.5 km. Later, in 1962, from a series of chemical explosions in the westernmost part of Mono Basin and outside the limits of the main depressed structure, the thickness of Cenozoic deposits was estimated to be 1.6 km. In 1966, a series of ten 1-ton chemical explosions was detonated in Mono Lake near the deepest part of the Mono Basin structure for the purpose of studying the relative effectiveness of different types of explosives in generating seismic energy. Seismic waves recorded at distances 25.0 to 92.3 km from the explosions were delayed by 1.43 seconds (referred to a shot on bedrock) as they descended through the low-velocity Cenozoic deposits of Mono Basin. By using the velocities of Cenozoic deposits as determined during the 1957 field season, the thickness of Cenozoic deposits required to account for the 1.43-second delay determined in 1966 has been estimated to be about 5 ± 1 km. The delay of seismic waves emerging in Long Valley was less than expected, indicating that they were propagated into Long Valley mainly through high-velocity rocks. From the rate of deposition of Cenozoic rocks in Mono Basin based on the age and depth of burial of the Bishop Tuff, it was estimated that Mono Basin began to subside in early or middle Pliocene time.

California↗
USGS1966

Composition and evolution of the continental crust as suggested by seismic observations

The average composition of the continental crust is more mafic than hitherto supposed. The conterminous United States can be divided, on the basis of seismic structure, into ten regions. The seven western and the three eastern regions can be termed western and eastern superprovinces. Seismic studies show that the crust is thinner and more silicic in tectonically active regions (western superprovince — average crustal thickness 34 km), than in stable regions (eastern superprovince — average crustal thickness 44 km). Mafic rocks are estimated to average 55% of the continental crust: 45% in the western and 59% in the eastern superprovince. These results express quantitatively the ideas expressed qualitatively by Pakiser and Zietz (1965). The computations of percentages of major oxides in the crust associate seismic velocities with rock compositions.

Tectonophysics↗
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↗
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↗
USGS1962

Geophysical study of Cenozoic geologic structures of northern Owens Valley, California

A narrow gravity minimum anomaly of amplitude 30 mgals indicates that northern Owens Valley , California , a narrow fault-bounded trough or graben filled with Cenozoic clastic deposits to a depth of as much as 8,000 ft. Seismic-refraction measurements support this conclusion. Aeromagnetic and gravity measurements define a small, dense, and magnetic body buried by the valley fill of northern Owens Valley .

California↗
USGS1962

Crustal structure in Nevada and southern Idaho from nuclear explosions

The time of first arrival of seismic waves generated by 4 underground nuclear explosions at the Nevada Test Site (NTS) and recorded along a line extending north into southern Idaho is expressed as T 0 = 0. 00 + Δ/3.0 (assumed), T 1 = 0 .40 + Δ/6.03, and T 2 = 6.15 + Δ/7.84, where time is in seconds and the shot-detector distance (Δ) is in km. Assuming constant velocities and horizontal layers, crustal thickness in the vicinity of NTS was determined to be 28 km. Delays in the traveltime segment T 2 , which represents P n , indicate that the crust may thicken to 32 km in northern Nevada. A third phase, expressed as T 3 = 14.48 + Δ/7.84, was also recognized and has arrival times appropriate for SPS. Amplitudes of P n were determined at 7 places from recordings of seismic waves from one underground nuclear explosion (ANTLER).

Nevada;Idaho↗
USGS1960

Transcurrent faulting and volcanism in Owens Valley, California

In the Owens Valley region of California , volcanic activity of Cenozoic age was confined mainly to three areas near the ends of important faults. The volcanic eruptions seemingly took place in regions of relative tension, if the horizontal movement along these faults was left lateral. The deep depression of Owens Valley may have resulted from compression associated with left-lateral horizontal fault movement. The transfer of molten rock from beneath this deep depression laterally into the regions of tension and thence to the surface seems to account for the relief of abnormal stresses and the volume of the volcanic rocks.

California↗