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1.
Seth Stein Joseph F. Engeln Douglas A. Wiens Robert C. Speed Kazuya Fujita 《Tectonophysics》1983,99(2-4)
The Lesser Antilles subduction zone is an extreme case of the subduction of old (~ 90 m.y.) lithosphere at a slow (~ 2 cm/y) convergence rate. Focal mechanisms of the largest earthquakes in the area have been obtained using body and surface wave data. During the time period (1950–1978) studied the subduction seismicity appears to represent primarily intraplate rather than interplate deformation. All three large (magnitude seven) earthquakes were from intraplate normal faults; no large thrust faulting earthquakes and few small ones occurred. These observations suggest that the plate boundary is largely decoupled, that subduction is at least partially aseismic, and that the downgoing slab is in a state of extension. 相似文献
2.
D.T Young F.J Crary J.E Nordholt D Boice A Eviatar J.J Hanley D.J McComas D Reisenfeld R.C Wiens 《Icarus》2004,167(1):80-88
The Plasma Experiment for Planetary Exploration (PEPE) made detailed observations of the plasma environment of Comet 19P/Borrelly during the Deep Space 1 (DS1) flyby on September 22, 2001. Several distinct regions and boundaries have been identified on both inbound and outbound trajectories, including an upstream region of decelerated solar wind plasma and cometary ion pickup, the cometary bow shock, a sheath of heated and mixed solar wind and cometary ions, and a collisional inner coma dominated by cometary ions. All of these features were significantly offset to the north of the nucleus-Sun line, suggesting that the coma itself produces this offset, possibly because of well-collimated large dayside jets directed 8°-10° northward from the nucleus as observed by the DS1 MICAS camera. The maximum observed ion density was 1640 ion/cm3 at a distance of 2650 km from the nucleus while the flow speed dropped from 360 km/s in the solar wind to 8 km/s at closest approach. Preliminary analysis of PEPE mass spectra suggest that the ratio of CO+/H2O+ is lower than that observed with Giotto at 1P/Halley. 相似文献
3.
R. Pillet D. Rouland G. Roult D. A. Wiens 《Physics of the Earth and Planetary Interiors》1999,110(3-4):211-234
Direct earthquake-to-station Rayleigh and Love wave data observed on high gain broadband records are analyzed in order to improve the lateral resolution of the uppermost mantle in the southwest Pacific region. We used data of nine permanent Geoscope and Iris stations located in the southern hemisphere and nine other stations as part of two temporary networks, the first one installed in New Caledonia and Vanuatu (hereafter named Cavascope network) by ORSTOM and the EOST from Louis Pasteur University in Strasbourg (France) and the second one installed in the Fiji, Tonga and Niue islands (hereafter named Spase network) by Washington University in St. Louis (USA). In order to collect more significant details on the surficial structures, we included the analysis of short period waves down to 8 s. A multiple frequency filtering technique has been used to recover phase velocities of Rayleigh and Love waves for selected earthquakes with magnitude greater than 5.5 and with known centroid moment tensor (CMT). About 1100 well-distributed seismograms have been processed in the period range 8–100 s and corrections for topography and water depth have been applied to the observed phase velocities. The geographical distribution of phase velocity anomalies have then been computed using the tomographic method developed by Montagner [Montagner, J.P., 1986a. Regional three-dimensional structures using long-period surface waves. Ann. Geophys. 4 (B3), 283–294]. Due to a poor knowledge of dense, well-distributed, crustal thickness values and corresponding velocity models, we did not perform or speculate on the construction of an S-wave 3D velocity model; therefore, we limited this study to the interpretation of the phase velocity distribution. The location of phase velocity anomalies are well determined and the deviations are discussed within the framework of the geological context and compared with other tomographic models. At long periods, from 40 s to 100 s, our results agree well with most of previous studies: the tomographic imaging shows a large contrast between low and high phase velocities along the Solomon, New Hebrides and Fiji–Tonga trenches. The lowest phase velocity anomalies are distributed beneath northern and southern Fiji basins and the Lau basin (corresponding to the volume situated just above the dipping slabs), whereas the highest values are displayed beneath the Pacific plate and the eastern part of Indian plate downgoing under the North Fiji basin. At shorter periods, our results show that the phase velocity distributions are well correlated with the large structural crustal domains. The use of local temporary broadband stations in the central part of the studied area gives us the opportunity to observe surface waves showing well-dispersed trains, allowing extended velocity measurements down to 8 s although aliasing due to multipaths become important. The continental regions (Eastern Australia, New Guinea, Fiji islands and New Zealand) show low velocities which are likely due to thick continental crust, whereas the Tasmanian, D'Entrecasteaux, and the Northern and Southern Fiji basins are characterized by higher velocities suggesting thinner oceanic crust. Additional analysis including the anisotropic case and S-wave velocity inversion with depth is in progress. 相似文献
4.
Yanhua Ma R. N. Peterson S. P. Zhang I. C. McDade R. H. Wiens G. G. Shepherd 《Annales Geophysicae》1995,13(2):189-195
A spectral imager specifically designed to measure the O+(2P-2D) emission in the thermosphere during twilight has been constructed and tested in Toronto (43.8°N, 79.3°W), and found to show promise for long-term and campaign-mode operations. A modification of the mesopause oxygen rotational temperature imager (MORTI), it consists basically of a narrow-band interference filter (0.14 nm bandwidth) to separate wavelengths as a function of off-axis angle, a lens to focus the spectrum into a series of concentric rings, and a focal plane array (CCD) to record the spectral images in digital form. The instrument was built with two fields of view, one for the zenith and one for 20° above the horizon, movable to track the azimuth of the Sun, in order to provide appropriate data for inversion. Data gathered during June 1991 provided measurements of the column-integrated emission rate with a precision of about 3%. An atomic oxygen profile was deduced that showed good agreement with that predicted by the MSIS-90 model atmosphere. Geomagnetically induced variations of the O+ lines, calcium spectra resulting from meteor showers, and OH nightglow were also observed. 相似文献
5.
6.
K. M. Fischer M. J. Fouch D. A. Wiens M. S. Boettcher 《Pure and Applied Geophysics》1998,151(2-4):463-475
—We have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the Tonga, southern Kuril, Japan, and Izu-Bonin subduction zones using shear-wave splitting in S phases from local earthquakes and in teleseismic core phases such as SKS. The observed splitting in all four subduction zones is consistent with a model in which the lower transition zone (520–660 km) and lower mantle are isotropic, and in which significant anisotropy occurs in the back-arc upper mantle. The upper transition zone (410–520 km) beneath the southern Kurils appears to contain weak anisotropy. The observed fast directions indicate that the geometry of back-arc strain in the upper mantle varies systematically across the western Pacific rim. Beneath Izu-Bonin and Tonga, fast directions are aligned with the azimuth of subducting Pacific plate motion and are parallel or sub-parallel to overriding plate extension. However, fast directions beneath the Japan Sea, western Honshu, and Sakhalin Island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing. Models of back-arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in Tonga and Izu-Bonin, but further three-dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern Kurils and Japan. The fast directions in the southern Kurils and Japan may require strain in the back-arc mantle that is driven by regional or global patterns of mantle flow. 相似文献
7.
Seth Stein Sierd Cloetingh Douglas A. Wiens Rinus Wortel 《Earth and Planetary Science Letters》1987,82(1-2)
The possibility that thermoelastic stresses due to plate cooling contribute significantly to the stress field and seismicity in young oceanic lithosphere has been a subject of considerable recent interest. This effect is suggested by three key observations: a decrease in seismicity with lithospheric age, the fact that focal mechanisms show extension perpendicular to the spreading direction, and a depth stratification of mechanism types. A difficulty with this idea is that although thermoelastic stresses should be comparable in different regions, the intraplate seismicity seems to occur in local concentrations. In particular, the ridge-parallel extensional seismicity occurs preferentially in the Central Indian Ocean region.We explore the possibility that much of the data favoring thermoelastic stresses can be interpreted in terms of stresses resulting from individual plate geometry and local boundary effects. In particular, the dramatic concentration of extensional seismicity in the Central Indian Ocean region is consistent with finite element results for the intraplate stress incorporating the effects of the Himalayan collision and the various subduction zones. The ridge parallel extensional stresses show a decrease with age similar to that of the seismicity. As earthquakes in this area provide a major portion of the data for both ridge-parallel extension and depth stratification, these effects may be due more to the regional stress. We thus propose that thermoelastic stresses provide a low level “background” in all plates, but that the dominant effect is that of individual plate geometry and local boundary effects. 相似文献
8.
R.H. Wiens 《Planetary and Space Science》1974,22(7):1059-1065
Filter photometer data on OH intensities reveal a midnight maximum of the average ratio I(8?3)/I(5?0) in midwinter at Adi Ugri. This midnight maximum is a regular feature on those nights when the intensity also goes through a midnight maximum. An interpretation in terms of a dynamic model is presented. On nights of steadily decreasing OH intensity, the pre-midnight ratio variation is uncertain. 相似文献
9.
Donald Lewis Bowden John C. Bridges Agnes Cousin William Rapin Julia Semprich Olivier Gasnault Olivier Forni Patrick Gasda Debarati Das Valerie Payré Violaine Sautter Candice C. Bedford Roger C. Wiens Patrick Pinet Jens Frydenvang 《Meteoritics & planetary science》2023,58(1):41-62
Askival is a light-toned, coarsely crystalline float rock, which was identified near the base of Vera Rubin Ridge in Gale crater. We have studied Askival, principally with the ChemCam instrument but also using APXS compositional data and MAHLI images. Askival and an earlier identified sample, Bindi, represent two rare examples of feldspathic cumulate float rocks in Gale crater with >65% relict plagioclase. Bindi appears unaltered whereas Askival shows textural and compositional signatures of silicification, along with alkali remobilization and hydration. Askival likely experienced multiple stages of alteration, occurring first through acidic hydrolysis of metal cations, followed by deposition of silica and possible phyllosilicates at low T and neutral-alkaline pH. Through laser-induced breakdown spectroscopy compositional analyses and normative calculations, we suggest that an assemblage of Fe-Mg silicates including amphibole and pyroxene, Fe phases, and possibly Mg-rich phyllosilicate are present. Thermodynamic modeling of the more pristine Bindi composition predicts that amphibole and feldspar are stable within an upper crustal setting. This is consistent with the presence of amphibole in the parent igneous rocks of Askival and suggests that the paucity of amphiboles in other known Martian samples reflects the lack of representative samples of the Martian crust rather than their absence on Mars. 相似文献
10.
Average diurnal variation patterns of the nightglow hydroxyl and sodium intensities are studied using data accumulated over several years by filter photometers at stations in the tropics, and north and south temperate latitudes. These average patterns are shown to depend upon season and latitude. They are compared with the frequencies with which various patterns actually occur, and it is shown that with increasing latitude the type of variation seen on a given night becomes less predictable. The sodium emission co-varies with the OH diurnally. The annual variations of OH and Na are presented and it is shown that the sodium in the southern hemisphere behaves like the sodium in the northern hemisphere displaced by six months. The annual OH variation does not exhibit this property. The OH intensity follows the solar activity cycle, but the sodium does not. 相似文献