摘要:

On planets where most of the heat is transported to the surface by igneous activity (extrusive volcanism or near‐surface intrusions), the surface heatflow at localities well away from regions of current igneous activity need not be even approximately the conductive heatflow through the entire lithosphere but may instead be dominated by the residual heat leaking out from the last igneous event in that locality. On Io, it is likely that (κτ)1/2« lithosphere thickness ( κ = thermal diffusivity, τ = typical time between “resurfacing” events) and the background heatflow may be very large, comparable or even larger than the current observational heatflow, which is associated with the hotspots alone. This upward revision of Io's heatflow is compatible with observations and with recent indications of a non‐steady tidal and thermal evolution. On Venus, (κτ)1/2is probably comparable to the lithosphere thickness and the resulting upward revision of heatflow may be only marginally significant, unless magmatic activity is enormously greater

ISSN:0094-8276    

DOI:10.1029/GL015i013p01455

年代:1988

数据来源: WILEY

摘要:

An experimental method for directly determining the degree of interconnectivity of melt in a partially molten system is discussed using an olivine‐basalt system as an example. Samarium 151 is allowed time to diffuse through mixtures of olivine and basalt powder which have texturally equilibrated at 1350°C and 13 to 15 kbars. The final distribution of samarium is determined through examination of developed radiographs of the samples. A uniform distribution of samarium throughout an entire sample implies that melt is present as an interconnected network because diffusion of samarium along melt‐free grain boundaries is shown not to occur in the time frame of these experiments. Quantitative determination of the degree of interconnectivity is done by establishing a one to one correspondence between a sample and its radiograph. Results suggest an interconnected melt network is established at melt fractions at least as low as 1 wt % and all melt is completely interconnected at melt fractions at least as low as 2 wt % for the system exam

ISSN:0094-8276    

DOI:10.1029/GL015i013p01459

年代:1988

数据来源: WILEY

摘要:

A laboratory experimental study has been carried out to investigate the mode trapping characteristics of a fluid‐filled fracture between two elastic solids. Using a small circular cylindrical receiver of 2.7 mm diameter, we were able to measure the wave motion directly inside a 2.8 mm thick fracture and to obtain array data for the propagating waves. The data was processed using Prony's method to give velocity of the wave modes as a function of frequency. The experimental results agree with the theoretical predictions quite well. Specifically, in a “hard” (aluminum) fracture where the shear velocity of the solid is greater than the fluid velocity, four normal modes were detected in the frequency range up to 2.4 MHz. Whereas in a “soft” (lucite) fracture where the shear velocity is smaller than the fluid velocity, four leaky‐P modes were detected in the same frequency range. In both cases, a fundamental mode analogous to Stoneley waves in a borehole was detected. In particular, the velocity of this mode approaches zero in the low frequency limit, as indicated by the theory and confirmed by the experiment in a low frequency range dow

ISSN:0094-8276    

DOI:10.1029/GL015i013p01463

年代:1988

数据来源: WILEY

摘要:

A covariance analysis shows that Global Positioning System (GPS) baseline precision in northern South America is substantially improved when fiducial stations (used to improve satellite orbit solutions and establish a reference frame) in North America are supplemented by stations in Hawaii, Australia and New Zealand. The formal errors for a variety of fiducial networks, some including stations in South America, are almost identical when their locations are held fixed. However, the consider error, which is the computed sensitivity to uncertainties in the fiducial station locations and is an estimate of the systematic error, is strongly dependent on the fiducial network geometry. If fiducial station locations are estimated with small a priori uncertainties, the various networks are practically identical, although the total uncertainty is slightly larger for the fiducials‐fixed networks. The sensitivity analysis indicates that the baselines of interest are very sensitive to uncertainties in the locations of the closest fiducial stations. If the uncertainty in the fiducial station locations is reduced to approximately 1 cm from the 4 cm assumed, any of the networks used in this analysis would be acceptable for GPS geodesy in northern South Americ

ISSN:0094-8276    

DOI:10.1029/GL015i013p01467

年代:1988

数据来源: WILEY

摘要:

Mineralogical characterisation of insoluble microparticles entrapped in ice layers successively deposited on the Antarctic continent is a useful tool for paleoclimate reconstruction. Indeed, it is likely to provide information on the distribution of potential source areas and of climatic conditions prevailing over different time periods. A first study, covering the last 30 kyr and including the end of the last glacial age, has shown that, over central East Antarctica, continental input was soil derived and originated probably from South America. In this work, the time period covered is l60 kyrs which includes a complete climate cycle. Clay species and particle size considerations suggest that local sources and Australia have been of minor importance for the studied area over the last 160 kyr. They support the hypothesis of colder and drier conditions prevailing over the source areas during glacial extrema. Lastly the predominance of illite, feldspar and quartz in association with presumably weathered volcanic products is consistent with a continental input originating from South America for the East Antarctic area, but is not in complete agreement with the results of general circulation models.

ISSN:0094-8276    

DOI:10.1029/GL015i013p01471

年代:1988

数据来源: WILEY

摘要:

Three earthquakes of Ms6.3, 6.4 and 6.7 near Tennant Creek, Northern Territory, Australia on January 22, 1988 produced 35 km of surface rupture in the Precambrian shield. The surface deformation consists of low‐angle (20°‐30°) thrust faulting on 2 main scarps separated by a 7 km gap. The Kunayungku (western) fault and the eastern end of the Lake Surprise (eastern) fault dip to the south, while the western end of the Lake Surprise fault dips to the north. Although these earthquakes are among the largest ever recorded on the Australian continent, there is no record of earthquakes larger than magnitude 5 within 500 km prior to 1987. Data from the Warramunga (WRA) seismic array located 30 km east of the rupture show no local activity between 1965 and 1986. Two small events in the fault area were observed in February, 1986 and were followed by a series of 6 mb4 to 5.2 earthquakes in January, 1987 and their aftershocks. The 1986, 1987 and January 22, 1988 mainshocks all nucleated within a zone about 5 km across from north to south, but elongated radially away from WRA by up to 30 km. Although the distribution of these events is in the direction of poorest hypocentral resolution, we can place meaningful constraints on the epicenters by examining the range of solutions that fit the P arrival time data. The 1986 and largest 1987 events were restricted to the western part of the fault zone. The three mainshocks progressed from west to east suggesting that the Kunayungku scarp was produced by the first mainshock, and the west and east ends of the Lake Surprise scarp by the second and third, respect

ISSN:0094-8276    

DOI:10.1029/GL015i013p01475

年代:1988

数据来源: WILEY

摘要:

Strong phases are sometimes observed between P and PP for earthquakes at epicentral distances 80° – 120°. The present study analyzes as an example broadband data from a Chilean earthquake recorded in Europe by the NARS network, and by worldwide distributed GEOSCOPE stations. Clear phases are observed in the P‐coda only for the European stations.Underside reflections at upper mantle discontinuities beneath the PP bounce point at the Mid‐Atlantic Ridge are unable to explain these phases. A discontinuity in the lowermost mantle at the top of D″ is also discarded as a possible explanation.Reflections and conversions at a strong reflector about 200 km deep beneath Chile provide a satisfying explanation of the observations. The P‐coda is dominated by waves leaving the focus as S‐waves. The focal solution and the resulting radiation pattern explain the variations with azimuth and epicentral distance of the phases observed in the P‐coda.The reflector at 200 km beneath Chile may be the upper boundary of a piece of slab detached from the Nazca plate. Alternatively, it may result from the chemical differenciation of the slab during its subduction. Modelling P‐coda generated in other subduction zones may help to specify this differ

ISSN:0094-8276    

DOI:10.1029/GL015i013p01479

年代:1988

数据来源: WILEY

摘要:

Two recent Ms7.6 strike‐slip earthquakes in the northern Gulf of Alaska ruptured a composite 250‐km‐long north‐striking zone in the Pacific plate. These shocks are attributed to a combination of enhanced tensional stress in the Pacific plate seaward of and following the great (Mw9.2) Alaska earthquake of 1964, and compressional stress resulting from collision of the Yakutat terrane with North America. The occurrence of these shocks reflects fragmentation of the northeast corner of the Pacific plate, possibly the initial step in establishing a new plate boundary seaward of the current b

ISSN:0094-8276    

DOI:10.1029/GL015i013p01483

年代:1988

数据来源: WILEY

摘要:

In a previous study, models of the crust and magma chamber at 12°50′N on the East Pacific Rise were derived with a two‐dimensional ray‐tracing analysis of rise‐normal refraction lines. The best model featured a 4 km wide axial magma chamber, but the data contained diffraction effects that were not modeled by geometric ray theory. This raised questions as to whether this model would reproduce the waveform data and whether diffraction might be obscuring a much larger magma chamber. We have tested the model with a finite element calculation of synthetic seismograms. This method incorporates full wave effects, including diffraction. The synthetics accurately reproduce the data and exhibit diffraction phenomena similar to those in the data. The previous travel time analysis was largely successful and was only slightly compromised by diffraction. Diffraction in rise‐normal refraction experiments is, therefore, not obscuring a large, low velocity mag

ISSN:0094-8276    

DOI:10.1029/GL015i013p01487

年代:1988

数据来源: WILEY

摘要:

A twelve day record of temperature collected from the diffuse flow area of a Guaymas Basin hydrothermal site exhibits variations, from a minimum of 3.05°C to a maximum of 4.87°C, whose periodicity is correlated with tides measured at the nearby town of Guaymas. A simple model, based on the hypothesis that temperature variations result as changes in tidal bottom currents induce changes in the height of the thermal boundary layer, is in good quantitative agreement with observed temperatures for most of the record. The success of this model illustrates that the effects of tidal currents can be strong enough to dominate the time variability of a temperature signal at a fixed point in hydrothermal flow. Therefore, tidal currents must be taken into account when using temperature measurements to estimate time varying heat fluxes from hydrothermal diffuse flow region

ISSN:0094-8276    

DOI:10.1029/GL015i013p01491

年代:1988

数据来源: WILEY