A computer program to plot wavefronts and rays from a point source in a two-dimensional velocity field is discussed. The program is used to plot ‘fan’ refraction diagrams from London for first-mode Rayleigh waves over a selected area of the North Atlantic, the refraction being assumed to be caused by the variation of ocean depth over an infinite elastic solid. Such diagrams are prepared for frequencies 0.4, 02, 0.133, and 0.1 cps and the theoretical ratio of energy observed on land to the energy actually present, when the sources are situated at different points in the region, is computed for each of the frequencies. It is found that the continental shelf has a ‘low-pass’ filter characteristic over the frequency range. The relevance of this effect in the study of microseismic noise in the ocean bottom is discussed.
Research Article| November 01, 1981 A deep low-velocity body under the Yellowstone caldera, Wyoming: Delineation using teleseismic P-wave residuals and tectonic interpretation: Summary H. M. IYER; H. M. IYER 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar J. R. EVANS; J. R. EVANS 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar G. ZANDT; G. ZANDT 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar R. M. STEWART; R. M. STEWART 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar J. M. COAKLEY; J. M. COAKLEY 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar J. N. ROLOFF J. N. ROLOFF 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information H. M. IYER 1U.S. Geological Survey, Menlo Park, California 94025 J. R. EVANS 1U.S. Geological Survey, Menlo Park, California 94025 G. ZANDT 1U.S. Geological Survey, Menlo Park, California 94025 R. M. STEWART 1U.S. Geological Survey, Menlo Park, California 94025 J. M. COAKLEY 1U.S. Geological Survey, Menlo Park, California 94025 J. N. ROLOFF 1U.S. Geological Survey, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1981) 92 (11): 792–798. https://doi.org/10.1130/0016-7606(1981)92<792:ADLBUT>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation H. M. IYER, J. R. EVANS, G. ZANDT, R. M. STEWART, J. M. COAKLEY, J. N. ROLOFF; A deep low-velocity body under the Yellowstone caldera, Wyoming: Delineation using teleseismic P-wave residuals and tectonic interpretation: Summary. GSA Bulletin 1981;; 92 (11): 792–798. doi: https://doi.org/10.1130/0016-7606(1981)92<792:ADLBUT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract No Abstract Available. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
To assess the nature of the heat source associated with the Roosevelt Hot Springs geothermal area, we have investigated the P‐wave velocity structure of the crust and uppermost mantle in the vicinity of the Mineral Mountains, southwest Utah, a region of late Cenozoic rhyolitic and basaltic volcanic activity. A roughly square (30 × 30 km) array of 15 seismographs, centered on the mountains, was operated for a period of 46 days, during which 72 teleseismic events were recorded with sufficient quality for calculation of P‐wave traveltime residuals. Relative residuals, using the array average for each event as reference, show a clear pattern of azimuthal variation of up to 0.3 sec. This pattern implies the existence of a localized region of relatively low‐velocity material extending up from the upper mantle to depths of about 5 km under the Mineral Mountains. A three‐dimensional (3-D) inversion of the data confirms this conclusion and yields a model featuring a region of low velocity (5 to 7 percent less than the surrounding rock) centered under the geothermal area and extending from about 5-km depth down into the uppermost mantle. The near‐surface velocities obtained in the inversion clearly reveal the structure of the region, part of the Basin and Range province. An azimuthally changing pattern of wave‐form distortion, restricted to the central Mineral Mountains, indicates the presence of a small but intensely anomalous region of low velocity and high attenuation at depths of about 15 km. Although we cannot rule out an explanation for the low velocity purely in terms of compositional changes, in view of the geothermal and volcanic manifestations found in the region we prefer an explanation in terms of abnormally high temperature and a small fraction of partial melt. A partial melt model implies a much greater heat reservoir than does a model involving only circulation along deep fault zones.
Concerning the Long Valley Meeting Report (Eos, January 19, 1988, p. 43), in the section describing the presentation of results from the teleseismic imaging experiment, it is stated that “…subsequent questions by meeting attendees brought out the idea that the [low‐velocity] anomaly [beneath the caldera] could also be heavily influenced by fractured and hydrothermally altered rocks to depths of 4 – 5 km.” For several reasons, we think that this comment is a significant misrepresentation of facts.
Seismic noise measurements were made in the East Mesa area of Imperial Valley, California, to find out if a noise anomaly was associated with the Mesa thermal anomaly. Thirty-three locations were occupied in the area using slow-speed tape-recording seismic systems. One of the stations (CEN) was operated close to where a geothermal test well was subsequently drilled by the U. S. Bureau of Reclamation. Several sources of cultural noise are present in the area. Large fluctuations in noise level, superposed on a constant high level of noise, occur from traffic on a freeway to the south of the region. There is noise generated by canals to the west and south and agricultural activity to the west of the region. Noise at 2.5 Hz frequency generated by a small waterfall (power drop) on the All American Canal propagates as far as 10 km. Average noise levels were computed at each station using several quiet samples selected from 4-hour sections of data recorded at night and contoured. Spatial distribution of 2 to 3 Hz noise shows noise radiating from the power drop. Noise in 0 to 2, 3 to 5, and 5 to 10 Hz bands show high levels extending along the freeway to the south and East High Line Canal to the west of the area. The Mesa thermal anomaly is centered about 2.5 km from the freeway and canal and does not seem to have any anomalous noise amplitudes associated with it. Additional results using data from two arrays of closely-spaced instruments extending from the freeway to the Mesa thermal anomaly also show no indications of high noise levels over the anomaly. This conclusion differs from the results of two previous surveys in the area (Douze and Sorrells, 1972; Geothermal Staff of Teledyne-Geotech, 1972) which show well defined noise anomalies in the 0 to 2, and 3 to 5 Hz frequency bands. A search was also made for anomalous features in noise spectra and for coherent wave trains indicating the presence of discrete sources of noise.
Microseisms were recorded by two separate arrays within 5 km of Norris Geyser basin, Yellowstone National Park, Wyoming. The data were analyzed using frequency‐wavenumber (f-k) spectral techniques to investigate whether the microseisms are originating at the geyser basin and, if so, whether body waves emanating from a deep source could be distinguished from surface waves on the basis of phase velocity. Array aperture and seismometer spacing were systematically varied to examine a continuous wavenumber range of 0 to 100 cycles/km. Results from high‐resolution f-k analysis show that the microseisms indeed originate at the geyser basin in the frequency range 1.5 to 6.3 Hz with phase velocities of 1.1 to 2.5 and 2.0 to 4.0 km/sec on arrays southwest and east of the geyser basin, respectively. Although we could not distinguish between surface waves and body waves originating near the surface solely on the basis of phase‐velocity information, observed velocities clearly preclude the possibility that a deep hydrothermal system is responsible for body‐wave microseisms in this area.
The Juan de Fuca plate subducts under the western margin of the North American plate in southern British Columbia, Washington, Oregon, and northern California. Benioff zone seismicity delineates shallow parts of the plate in Washington and northern California, whereas in Oregon there is a lack of seismicity. This study images the Juan de Fuca plate in southern Oregon using seismic tomography. We inverted P wave travel time residuals from a 366‐km‐long seismic array operated in southern Oregon in 1982. The southeast striking array extended from the Coast ranges to the Modoc Plateau and crossed the High Cascades at Crater Lake, Oregon. We imaged three features under the array: one high‐velocity zone and two low‐velocity zones. The high‐velocity zone is 3–4% faster than the surrounding upper mantle. It dips steeply at 65° to the east beneath the Cascade Range and extends down to at least 200 km. We propose that this high‐velocity feature is subducted Juan de Fuca plate. We also imaged two low‐velocity zones, both of which are 3–4% slower than the surrounding earth structure. The southeastern low‐velocity zone may be caused by partially molten crust underlying the Crater Lake volcano region.
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