Copyright (c) 2009 All rights reserved. http://works.bepress.com/jack_pelton Recent documents in John (Jack) Pelton en-us Sun, 30 Aug 2009 13:55:12 PDT 3600 http://works.bepress.com/jack_pelton/24 http://works.bepress.com/jack_pelton/24 Wed, 14 Jan 2009 15:13:10 PST High-resolution reflection seismic and detailed gravity data were acquired across an exposure of the Wasatch fault, Utah, near a trench excavated for dating of Quaternary fault displacement. The seismic data across the Quaternary Wasatch fault are interpreted to show three subparallel surfaces dipping from 70°W at the surface to an estimated 45°W at 40 m, displaced unconsolidated sediments, and colluvial material abutting the main fault. Detailed gravity data were of assistance in mapping the sediment-bedrock interface from the rangefront across the Wasatch Fault Zone. A combined high-resolution seismic and gravity interpretation reveals that bedrock is 80 m deeper in the hanging wall than in the footwall, suggesting that displacement has occurred on several en echelon blocks in the near-surface of the Wasatch fault. Movement on the most recent Wasatch fault trace began prior to deposition of Pleistocene lacustrine sediments. The twelvefold, high-resolution reflection seismic data were recorded and processed to achieve vertical resolution of approximately 1 m and horizontal resolution (Fresnel zone radii) of as good as 5 m on the deepest reflecting interfaces at 40 m. Reasonably high frequencies and good bandwidth (about 80-300 Hz) in the stacked seismic data permitted a direct comparison to the trenched cross section. Good correlation between the trench stratigraphy and structure and the seismic data suggests high-resolution seismic data can be a valuable tool for analyzing near-surface faulting in unconsolidated sediment and for locating potential trench sites. John R. Pelton http://works.bepress.com/jack_pelton/23 http://works.bepress.com/jack_pelton/23 Wed, 14 Jan 2009 15:13:05 PST We describe an electrical seismic gun that is capable of firing 8-gauge blank black powder shells in a water-filled borehole under relatively high hydrostatic pressures. The new seismic gun is a modified version of the electrical shotgun source for engineering seismic surveys introduced by Pullan and MacAulay (1987). Our modifications seal the firing circuit and 8-gauge shell against water entry so underwater detonation will occur reliably at depths to at least 80 m (0.9 MPa atmospheric pressure). Source energy is controlled by varying the size of the black powder load in the shell from 50 grains to 500 grains (10 kJ to 100 kJ). Although our seismic gun may be used in any seismic application suitable for modest explosive charges, it was initially developed as a versatile source for use in seismic investigations of the shallow subsurface (primarily engineering and groundwater studies). As of this writing, the gun has been used for optimum offset and CMP high-resolution seismic reflection profiling, engineering refraction surveys, fixed-source and variable-source noise tests, and vertical traveltime measurements in water wells. Other potential uses include VSP and borehole-to-surface or borehole-to-borehole seismic tomography. John R. Pelton http://works.bepress.com/jack_pelton/22 http://works.bepress.com/jack_pelton/22 Wed, 14 Jan 2009 15:13:00 PST Relative vertical displacements and average relative vertical velocities have been determined for bench marks located throughout Yellowstone National Park (YNP) by comparing geodetic leveling measurements made in 1923 (second order), 1936 (third order), 1941 (third order), 1955 (third order), 1960 (first order), and 1975-1977 (first order). The 1923-1975 data indicate uplift greater than 400 mm (7.7 mm/yr) of an area approximately 15-20 km wide and at least 40 km long which is centered along the northeast trending axis of the 600,000-year-old Yellowstone caldera; the maximum 1923-1975 vertical displacement in this area is 726 mm (14.0 mm/yr). The existence of a well-defined and untilted Yellowstone Lake terrace implies that the 1923-1975 average vertical velocities cannot have been maintained for more than a few hundred years. The 1955-1977 data from central YNP indicate that the northeastern caldera half has risen relative to the southwestern half at an average velocity of about 8 mm/yr; a similar bias is present in the 1923-1975 data but it is less pronounced. The 1923-1960 and 1960-1975 data from northwestern YNP, and a 200 mm depression in the 1923-1975 data near Norris Junction, may be interpreted in terms of deformation associated with the 1959 Hebgen Lake earthquake and the 1975 Yellowstone Park earthquake. A composite profile of the 1936-1977 and 1941-1977 average vertical velocity data which skirts the edge of the Yellowstone caldera in northeastern YNP is most easily explained as the result of random leveling error and/or bench mark instabilities. The apparent youth of the 1923-1975 Yellowstone uplift and its close association with the Yellowstone caldera suggest that the most likely cause of the uplift is a recent (not more than 500 years ago) influx of molten material into the upper crust beneath the caldera. John R. Pelton http://works.bepress.com/jack_pelton/21 http://works.bepress.com/jack_pelton/21 Wed, 14 Jan 2009 15:12:55 PST "We briefly describe a computer-based system for interactive processing of earthquake seismograms recorded on up to four reels of 16 mm microfilm which eliminates the major weaknesses in the multi-step processing procedure described previously. This is an important development because for the forseeable future, 16mm microfilm will remain the principal recording media for the USGS southern Alaska seismic network. In addition, the system provides a convenient tool for special seismological research requiring repetitive waveform measurements made from microfilm. The basic system design is outlined below, and the interested reader may consult a report by Astrue et al. (1983) for details." John R. Pelton http://works.bepress.com/jack_pelton/20 http://works.bepress.com/jack_pelton/20 Wed, 14 Jan 2009 15:12:50 PST An electrically detonated downhole seismic gun (EDG) that will fire blank 8-guage shells underwater has been constructed and tested to 80m depth (hydrostatic pressures of 130 psi or 8.9 x 10⁵ Pa). Although other engineering seismic guns which fire blank or projectile sources are available, they are for near-surface shots and are not meant to be used for downhold seismic surveys in water-filled boreholes. The EDG was designed primarily for checkshot surveys (well travel-time surveys) and high-quality reflection/refraction tests, but potential applications include shallow vertical seismic profiling and borehole to borehole or borehole to surface tomography, as well as optimum offset and common midpoint seismic reflection surveys. The EDG consists off four steel subassemblies: (1) chamber; (2) breech; (3) pipe; and (4) hanger. A blank 8-gauge electrical shell is held by the chamber and is detonated by an electrode located inside the breech. O-ring seals prevent water from entering the breech and causing short circuits. The breech is screwed into a pipe which is also fitted with o-ring seals to keep the internal wiring dry. A hanger subassembly provides a convenient attaching point for the hoist cable. Arming of the EDG with an explosives blaster occurs only after lowering to operating depth. The EDG has been tested with various size black powder loads up to 750 grains. Frequency bandwidth and repeatability tests were carried out under saturated conditions in a fluvial and lacustrine sedimentary section. These preliminary tests show significant frequency content in the 25-200 Hz band (peak near 100 Hz) for reflections from depths of 150-300 m, and acceptable waveform repeatability for different shot records obtained with identical geometry and acquisition parameters. James C. Parker http://works.bepress.com/jack_pelton/19 http://works.bepress.com/jack_pelton/19 Tue, 13 Jan 2009 15:54:26 PST The Pamplona zone is a region of complex deformation and moderate seismicity located within the Yakutat block, a region that has been relatively aseismic since a series of large (M>7.8) earthquakes in 1899. In 1970 a sequence of moderate to large sized earthquakes occurred within the Pamplona zone (largest event of Mw =6.7). Together with a Mw =6.1 event in 1958, these events are the only M≥5.5 events known to have occurred in the Pamplona region since 1900. Thus these events give important information on internal deformational processes within the Yakutat block. Waveform modeling of three earthquakes in April 1970, showed rupture complexity along low angle, thrust faults. Focal depths indicate that two of the events occurred above the Wrangell-Aleutian megathrust, while the largest event may have occurred on the megathrust. Events in 1958 and February 1970 indicate that deformation within the western Pamplona zone is occurring along high angle (>60°) faults with reverse-oblique motion. We believe the Pamplona spur, the easternmost part of the Pamplona zone, may have behaved as an asperity during the 1899 sequence. The location of the spur may be influenced by a north-south trending fault zone in the subducting Pacific plate that appears to be responsible for the 1987-1992 Gulf of Alaska sequence, occurring 50 to 200 km south of the Pamplona zone. John R. Pelton http://works.bepress.com/jack_pelton/18 http://works.bepress.com/jack_pelton/18 Tue, 13 Jan 2009 15:54:22 PST Normal surface faulting associated with the Borah Peak, Idaho earthquake (preliminary Ms 7.3) of 28 October 1983 was witnessed in the vicinity of Arentson Gulch by two elk hunters, D. Hendriksen and J. Turner of Boise, Idaho. On the morning of the earthquake, the hunters were traveling in a four-wheel drive vehicle on a dirt road (Figure 1) with Hendriksen driving. Another pair of hunters, Mr. and Mrs. William Knox of Challis, Idaho, were on foot in the same area. Mr. Knox was to the northwest and on a hill above Hendriksen and Turner, and was driving elk toward his wife who was waiting near some power lines in a narrow valley (Figure 1). At 8:06 a.m. local time (14:06 UTC) the earthquake occurred, while Hendriksen and Turner drove their vehicle northeastward at a slow rate of speed (about 5 mph). A prominent fault scarp formed about 20 m directly in front of the vehicle, with the vehicle on the downthrown side (Figure 2). We interviewed Hendriksen and Turner at this site on the evening of 28 October and again at this site on the morning of 29 October. Hendriksen alone was interviewed at Boise State University on 8 November and 30 November. The 29 October interview and portions of the 8 November interview were tape-recorded and transcribed. Excerpts from these transcriptions and an interpretation of the events are given below. Observations made during the surface faulting by Mr. and Mrs. Knox are documented by Wallace (1984). John R. Pelton http://works.bepress.com/jack_pelton/17 http://works.bepress.com/jack_pelton/17 Tue, 13 Jan 2009 15:49:44 PST Numerous compilations of instrumental and non-instrumental seismicity demonstrate that the eastern Snake River Plain (ESRP) is seismically quiet relative to surrounding block-faulted mountainous areas. Although the frequency of occurrence of ESRP microearthquakes is very low and cannot be quantified with available data, the documented existence of these events is important because they can provide valuable data for the evaluation of regional tectonic models and seismic hazards. In this paper we present evidence for microearthquake activity on the northeast-trending longitudinal axis of the ESRP, an area where detailed seismic monitoring has not been previously attempted. John R. Pelton http://works.bepress.com/jack_pelton/14 http://works.bepress.com/jack_pelton/14 Mon, 12 Jan 2009 23:37:44 PST "This chapter serves as an introduction to seismic wave propagation for those waves commonly observed in near-surface seismology. A reasonably detailed description is provided for linearly elastic and linearly viscoelastic constitutive models. Dissipation of energy is systematically developed with careful definition of the relationships between energy loss, complex moduli, and the quality factor. The discussion of wave propagation is mostly about plane body waves (viscoelastic case) and plane surface waves (elastic case) in homogeneous (or vertically heterogeneous) isotropic media. The discussion also includes a simple model for head waves, the reflection of plane body waves from a plane interface between homogeneous and isotropic elastic media, and a description of geometric spreading, the radiation pattern, and the near field associated with a point source in an unbounded elastic medium. The appendices contains a significant amount of related information including an introduction to linear system theory, Fourier integral transforms and the convolution theorem, a discussion of the principles that underlie linear viscoelasticity in the general heterogeneous and anistropic case, and details for many of the computations." John R. Pelton http://works.bepress.com/jack_pelton/11 http://works.bepress.com/jack_pelton/11 Mon, 12 Jan 2009 23:37:36 PST "This chapter provides a survey of those near-surface seismic methods that are based on the deployment of sources and receivers at the surface on land. It builds on the review and tutorial articles by Steeples and Miller (1990) and by Lankston (1990), which appeared in the original SEG publication on geotechnical and environmental geophysics (Ward, 1990). The tradition established by those authors is continued here by pulling together essential concepts and presenting them at a uniform level with consistent notation and terminology. Although many details must remain unstated because the subject area is extensive, a lengthy, list of cited references is provided to alleviate this difficulty." John R. Pelton