摘要:

Temperature‐depth profiles in borehole GC‐1, northwestern Utah, were measured in 1978, 1990, and 1992. Borehole temperatures below 80 m depth are highly reproducible over the 14 year period indicating long term thermal stability. A slowly changing temperature field above 80 m depth has similiar characteristics to synthetic temperature profiles computed from a 100 year record of air temperature changes at Park Valley weather station 50 km northeast of the borehole s

ISSN:0094-8276    

DOI:10.1029/93GL01877

年代:1993

数据来源: WILEY

摘要:

Global geoid data from the Geosat altimeter satellite and topography data have been analysed in the Pacific and Atlantic oceans to study present (along‐axis) and past (off‐axis) regional mid‐ocean ridge segmentation. Along the Mid‐Atlantic Ridge, East Pacific Rise and Pacific‐Antarctic Ridge, axial geoid and depth variations are positively correlated at length scale 800–1000 km. Respective peak to peak amplitudes are in the range of 0.4–0.8 m and 200–400 m. Axial variations are correlated with the position of large offset transform faults which coincide with geoid and topography lows. Triple junctions are also located in geoid and topography lows. The observed admittance between geoid and depth is estimated to 2 ± 0.5 m/km, a value indicative of a dynamic (convective) origin for the along‐axis regional variability.Off‐axis analysis shows a regional pattern of geoid (topography) anomalies elongated in the direction of spreading. This pattern has also a length scale of 800–1000 km and possibly represents past regional segmentation. Large offset fracture zones coincide with off‐axis

ISSN:0094-8276    

DOI:10.1029/93GL00988

年代:1993

数据来源: WILEY

摘要:

In November 1991, a geodetic network was installed across the actively rifting plate boundary zone between Africa and Arabia. Using the microwave signals transmitted by the satellites of the Global Positioning System (GPS), the relative positions of 37 stations are estimated with an average WRMS scatter of 12 or 14 mm for lines shorter or longer than 100 km, respectively. Assuming that the measurements can be repeated with the same precision in 1999, we expect to obtain horizontal uncertainties of 2.1 or 2.4 mm/yr in the rate of change in the short and long lines, respectively. This level of precision will tightly constrain geological models for the spatial distribution of deformation around the rift in Djibouti, and for the far‐field plate motions surrounding the Afar depressio

ISSN:0094-8276    

DOI:10.1029/93GL01882

年代:1993

数据来源: WILEY

摘要:

The South Iceland seismic zone is a 20–60 km‐wide (north‐south) and up to 70 km‐long zone of north and north‐northeast trending Holocene arrays of en echelon tension fractures. These fracture arrays are related to dextral strike‐slip faults buried by Holocene lava flows. In this zone, major destructive earthquake sequences occur at intervals of 45–112 years, the largest events reaching magnitude 7 (Ms). We propose that this seismic zone is located between overlapping rift‐zone segments (spreading centers), where the eastern segment has been propagating to the south during the past 3 Ma. We made a finite element study of this configuration with the segments modeled as mode I cracks loaded in tension. The results suggest that the South Iceland seismic zone in general, and the north and north‐northeast trending dextral faults in particular, develop in response to the shear stresses generated between the r

ISSN:0094-8276    

DOI:10.1029/93GL01888

年代:1993

数据来源: WILEY

摘要:

The June 28, 1992 Big Bear earthquake in southern California was assumed to have ruptured along a northeast‐trending plane, as suggested by long‐term aftershock distribution. No surface rupture was found, however, and mainshock locations determined from both strong motion and TERRAscope data are mutually consistent and do not lie on the assumed fault plane. An integrated study involving waveform modeling, directivity and seismicity analyses suggests a complex rupture pattern, with significant short‐ and long‐period energy propagating northwest along the presumed conjugate faul

ISSN:0094-8276    

DOI:10.1029/93GL02046

年代:1993

数据来源: WILEY

摘要:

Three are the essential parameters needed to describe seismicity: theb‐value of the Gutenberg‐Richter relation and the spatial and temporal fractal dimensions (DsandDt). Several cases have been reported when a significant decrease in theb‐value or in the spatial fractal dimension preceded major earthquake sequences or aftershocks. Here we use the fractal method based on the correlation integral to study the temporal changes in the spatial (2‐D) distribution of earthquakes in three important seismic zones of Italy. In all three zonesDsshows significant variability, which correlates well with the major events and clearly marks the beginning and the end of an earthquake cycle. Finally, the nucleation of most major events is associated with a fractal dimension value which corresponds to the topological dimension of

ISSN:0094-8276    

DOI:10.1029/93GL01889

年代:1993

数据来源: WILEY

摘要:

A 200 km anomalous topographic zone was discovered using wavelet scale analysis in a 1600 km linear bathymetric profile taken northeast of Hawaii. A spectral analysis of the zone shows that the power in wavelengths shorter than 25 km averages 5–10 times lower than the surrounding topography. Conversely, wavelengths longer than 25 km have 2–3 times greater power than nearby topography. Further research reveals that this low‐frequency zone correlates with the eastern flank of a regional bathymetric high which has been described by J. Mammerickx [1981]. Details of the features suggest that this zone is a small, abandoned, slow‐spreading rift overprinted by a regional thermal swell. The magnitude of the feature is smaller than other, known, abandoned spreading centers, making it easy to overlook without the application of the wavelet tr

ISSN:0094-8276    

DOI:10.1029/93GL01880

年代:1993

数据来源: WILEY

摘要:

The 10 largest tamped nuclear explosions detonated by the Former Soviet Union in and near two salt domes near Azgir were relocated using seismic data and the locations of shot points on a SPOT satellite image taken in 1988. Many of the shot points are clearly recognized on the satellite image and can be located with an accuracy of 60 m even though testing was carried out at those points many years earlier, i. e. between 1966 and 1979. Onsite inspections and a local seismic monitoring network combined with our accurate locations of previous explosions would insure that any cavities that remain standing from those events could not be used for undetected decoupled nuclear testing down to a very small yield. Since the Azgir area, like much of the Pre‐Caspian depression, is arid, it would not be a suitable place for constructing large cavities in salt by solution mining and then using them for clandestine nuclear testin

ISSN:0094-8276    

DOI:10.1029/93GL01218

年代:1993

数据来源: WILEY

摘要:

Empirical models have long been available for estimating the Newtonian viscosity of single‐phase silicate melts from composition in terms of oxide components [Bottinga and Weill, 1972;Shaw, 1972]. These models are used regularly to estimate or parametrize viscosity in field studies and fluid dynamical calculations of magmatic transport. However, few independent viscosity measurements on multi‐component melts of natural composition have been made to test the accuracy of the empirical estimation models. A series of experimental measurements of viscosity has been performed on a dacitic melt, using the technique of concentric cylinder rheometry. In the ranges of temperature and shear rate examined (1000 to 1150° C and 0.013 to 7.43 s−1, respectively), the melt viscosity is found to be independent of shear rate. When the viscomentric results from the present study and those on a rhyolitic composition [Hochella and Brown, 1984] are compared with the results from the empirical models, the model ofShaw[1972] is found to represent both sets of experimental data better than that ofBottinga and Weill[1972]. This is presumably due to differences in the ranges of temperature and composition represented in the data used to construct the respective

ISSN:0094-8276    

DOI:10.1029/93GL01878

年代:1993

数据来源: WILEY

摘要:

We have studied the effects of rheological boundaries in the upper mantle on the dynamics of inducing pulsating diapiric plumes. We have compared the effects due to shallower rheological interfaces in the upper mantle on the timescales of the pulsations. Comparing with a basic model with a rheological interface at 670 km depth, we find that timescales between pulsations can be reduced considerably down to a few million years by adding another rheological boundary at 400 km depth. These results suggest that short timescale diapiric flows may be generated from relatively stationary upwellings from the lower mantle because of the presence of rheological boundaries in the shallow upper mantle.

ISSN:0094-8276    

DOI:10.1029/93GL01768

年代:1993

数据来源: WILEY