摘要:

Oxygen fugacities of olivine‐orthopyroxene‐ilmenite‐bearing, high‐pressure megacrysts, mineral intergrowths, and peridotites from kimberlites and from a minette have been calculated from the equilibrium 2Fe2O3+ 4FeSiO3= 8FeSi0.5O2+ O2. Resultant fO2's, which are interpreted to reflect the subcontinental source regions of alkalic magmas, are clustered between the quartz‐fayalite‐magnetite and magnetite‐wustite buffers. These relatively oxidized conditions are compatible with at least some estimates of fO2of typical basalts. They are also compatible with existence of carbonate minerals and diamond or graphite and, if fluids are present, with water‐rich H2O‐CO2fluids rather th

ISSN:0094-8276    

DOI:10.1029/GL010i005p00365

年代:1983

数据来源: WILEY

摘要:

Quantitative study of the westward drift (w.d.) was initiated by Bullard et al (1950) and then followed by many others, using more or less similar methods (e.g. Yukutake, 1962; Harwood and Malin, 1976). More recently, MAGSAT results have enhanced the interest in the subject (see for example fig. 1 of Peddie, 1982). In the present paper, we compute the w.d. rate in the 1950‐1980 time interval, using different methods, and we establish that the 1969‐1970 secular acceleration impulse (the so‐called "jerk") clearly shows up in the re

ISSN:0094-8276    

DOI:10.1029/GL010i005p00369

年代:1983

数据来源: WILEY

摘要:

Seismograms observed at Longmire, Washington, for four eruptions of Mt. St. Helens (May 18, June 13, August 7, and August 8, 1980), can be interpreted as Lamb pulses excited by a nearly vertical single force that represents the counter force of the eruption. These data provide reliable estimates of the impulse of the force K (time integral of the force) from which the total momentum and the kinetic energy, E, of the ejecta associated with the eruption can be estimated. The values of K are estimated to be 1.4 × 1019, 1.4 × 1016, 3.7 × 1015, and 2.8 × 1015dyne·s for the four eruptions (chronological order), respectively. The corresponding values of E are estimated to range from 0.70 to 2.6 × 1023, 0.70 to 2.6 × 1020, 1.9 to 6.9 × 1019, and 1.4 to 5.3 × 1019ergs using values of ejecta velocity ranging from 100 to 375 m/s. The ratio of K to the amplitude of the air wave excited by the eruption is 20 to 40 times larger for the main event on May 18 than for the other events suggesting a significant difference in the eruptive mechanism. Our results demonstrate that a digital seismograph in the vicinity of volcanoes provides a simple means for quantification of the explosive power of a volcanic

ISSN:0094-8276    

DOI:10.1029/GL010i005p00373

年代:1983

数据来源: WILEY

摘要:

We have conducted 8 precise seismic surveys near Hollister, CA, over a period of 1 y in an attempt to detect the traveltime variation caused by the solid‐earth tidal stress. The surveys were conducted along a 600 m baseline located in quartz monzonite hills 2 km west of the San Andreas fault. A 656 cm³ air gun fired in a mud‐filled pit 2 m deep provided a repeatable seismic source. The signals from two 2.3 Hz vertical‐component geophones 600 m apart were digitized at a nominal rate of 600 samples/s by two cassette recorders modified for precise synchronization of data sampling against a master clock. Each survey consists of ∼ 100 traveltime measurements over the 12 h period between 6 p.m. and 6 a.m. local time; the time frame of each experiment was limited by daytime cultural noises. Analysis of traveltime variation is done either by timing of amplitude extrema or by cross‐correlation of a waveform constructed from the digital data by a cubic‐spline interpolation. Fractional error of the repeatability of traveltime measurement is typically ±3.3 × 10−4for the first high‐frequency, large amplitude arrival following the direct body waves. The first survey, conducted at a spring tide in August, 1981, showed a variation of Δt/t ∼ 2 × 10−3and correlated in time with the extensional tidal strain component along the baseline direction. The next two surveys, conducted at two neap tides, showed variation of Δt/t ∼ 6 × 10−4and also correlated with the same tidal strain component. However, the other 5 surveys conducted after the onset of 1981 rainy season and into the 1982 dry season, 4 at spring tides and 1 between a spring and a neap tide, showed traveltime constant to within 1 standard deviation. These results corroborate only partially the previously reported tidal stress variation of tr

ISSN:0094-8276    

DOI:10.1029/GL010i005p00377

年代:1983

数据来源: WILEY

摘要:

A segment of the Gulf of Tadjoura (Djibouti, East‐Africa) accreting plate boundary, shows a period of quiescence in the seismic activity since 1974. This segment corresponds to the extension area of the aftershock activity that has occured after a cluster of magnitude 5.5 earthquakes in April 1973. From this example we propose that the seismic gap concept can be extended to moderate earthquakes occuring at extensional plate boundaries. The magnitude of the largest earthquakes at the spreading axis is limited by the size of the rupture length and by the strength of the brittle lithosphere. In the case of the Djibouti ridge recurrence time of 10‐20 years are found for earthquakes of about M

ISSN:0094-8276    

DOI:10.1029/GL010i005p00381

年代:1983

数据来源: WILEY

摘要:

Following a large (Ms=7.0), interplate earthquake on 15 July 1981 in the Vanuatu (New Hebrides) Islands, the area defined by the aftershocks dramatically increased over a two week period of time. The aftershocks were located using the Cornell/ORSTOM seismograph network that has been monitoring earthquakes with magnitudes larger than mb=2.9 in the region since mid‐1978. In the first few hours following the mainshock, the area defined by the aftershocks was consistent with the rupture zone of a typical interplate earthquake with the observed seismic moment of 5.8 × 1026dyne‐cm. However, during the 16 days following the mainshock, the area of the aftershock zone increased to approximately 10 times the area of expected coseismic rupture. The final aftershock zone included earthquakes that were located within the overriding and descending plates as well as along the interplate fault zone. The migration of aftershocks away from the rupture zone of the 1981 mainshock indicates that, at least in some cases, the aftershock zone can overestimate the extent of coseismic rup

ISSN:0094-8276    

DOI:10.1029/GL010i005p00385

年代:1983

数据来源: WILEY

摘要:

Insolation measurements made on clear days at Corvallis, Oregon, since the last week of November, 1982, have indicated a decrease of about 14% in the atmospheric clearness index, an increase by a factor of about 2.3 in the ratio of the diffuse sky irradiation to the global irradiation and a decrease of about 28% in the transmittance of the atmosphere for the direct normal solar irradiation compared to the normal, clear‐day values. It is felt these anomalies are caused by the increased atmospheric turbidity due to the El Chichon volcanic dust clou

ISSN:0094-8276    

DOI:10.1029/GL010i005p00389

年代:1983

数据来源: WILEY

摘要:

Carbonyl sulfide (COS) was found to be supersaturated by a factor averaging 2.6 with respect to the atmosphere in surface waters of the continental shelf of Peru. The degree of supersaturation was correlated with dissolved dimethyl sulfide (DMS) concentrations indicating a possible biogenic source of atmospheric COS. The oceanic source of COS could account for a substantial fraction of the COS flux to the atmosphere.

ISSN:0094-8276    

DOI:10.1029/GL010i005p00393

年代:1983

数据来源: WILEY

摘要:

Recent discoveries of natural gas hydrates in permafrost and sub‐oceanic environments have led us to measure the thermal conductivity of methane hydrate. A guarded hot‐plate cell was constructed and tested with various ice specimens. We find the conductivity of methane hydrate near −60°C is 0.45 W/mK, a value close to that obtained for other clathrate hydrates by us and others. It is much lower than for ice (by a factor of about 5), so that a logging tool based on this property should prove to be of use in identification of gas hydrates in permafrost

ISSN:0094-8276    

DOI:10.1029/GL010i005p00397

年代:1983

数据来源: WILEY

摘要:

The radar interferometer technique first used at the magnetic equator in Peru is also a very powerful means for studying auroral plasma instabilities. We present here the first results, obtained with a 49.92 MHz, 20‐25 KW peak power pulsed radar located in Ithaca, NY (42.5° N, 76.4° W). Strong auroral echoes were obtained during several highly active periods. Phase differences between the signals received on the two antennas accurately determine the E‐W position, within the scattering volume, of localized scattering centers, and changes in this phase determine the corresponding velocity. The signal Doppler shift describes radial (essentially N‐S) motion. The data provide detailed information on the turbulent structure of the echoing region and show clearly that different features in the Doppler power spectrum often represent signals coming from different locations. If we assume that the radial and transverse phase velocities represent real drift velocities, we can determine full horizontal velocity vectors from the data and hence the horizontal electric field, usually with a time resolution of the order of

ISSN:0094-8276    

DOI:10.1029/GL010i005p00401

年代:1983

数据来源: WILEY