摘要:

The locations of teleseismically well‐recorded earthquakes suggest that up to 80% of the Shumagin Gap has ruptured seismically during the last 90 years. An earthquake in 1917 (Ms=7.9) nucleated near the gap's eastern edge and appears to have ruptured as much as a 200 km long section of the gap, and an event in 1948 (Ms=7.5) appears to have ruptured the deepest portion of the main thrust‐zone (>30 km) in the central section of the gap. The rupture extents of these earthquakes suggest that the gap can be divided into two or three smaller arc segments. These observations do not preclude the possibility of failure of the entire region in a single great earthquake (Mw=8.5) in the near future. The two historically documented recurrence intervals for the eastern portion of the gap are 59 and 70 years. If the average recurrence interval for large earthquakes near the Shumagin Islands is 65 years, the conditional probability of a large earthquake rupturing this portion of the arc in the next 20 years is between 62 and

ISSN:0094-8276    

DOI:10.1029/GL015i003p00201

年代:1988

数据来源: WILEY

摘要:

Surface‐wave data previously used by the authors to constrain upper mantle anisotropy in the Pacific Ocean are re‐examined after an error was found in the calculations of the Love‐wave phase‐velocities. Sub‐lithospheric anisotropy is still required by the corrected data but it extends to shallower depth than we originally proposed. Using different parameterizations of the upper‐mantle, we present strong evidence that the bottom of the anisotropic layer has been reached around 250 km in the Pacific. Beneath North‐America, no revision of our data is required and anisotropy could extend to around 400 km‐depth. Our results are in agreement with an upper mantle anisotropy due to preferred orientation of olivine crystals in an overall horizontal shear strain parallel to the present‐day plate motion. We suggest that this preferred orientation of the crystals mainly occurs within the upper mantle l

ISSN:0094-8276    

DOI:10.1029/GL015i003p00205

年代:1988

数据来源: WILEY

摘要:

We have constructed a parametric many‐body potential that gives accurate predictions of high pressure properties of alkali‐earth oxides. The model incorporates ionic deformational energy and a repulsive potential that are sensitive to the crystal environment. This feature is accomplished by an explicit dependence of the size of oxygen on the crystal potential. The parameters of the model are constrained by equation of state and zero pressure elastic constants of the B1 (NaCl) phases. The model predicts accurate B1‐B2 transition pressures and densities of the B2 (CsCl) phases. Elastic moduli are reproduced exactly, and accurate pressure derivatives are pred

ISSN:0094-8276    

DOI:10.1029/GL015i003p00209

年代:1988

数据来源: WILEY

摘要:

The depth to the brittle‐ductile transition in the crust is often defined by the intersection of a shear resistance relation in the brittle upper crust that increases linearly with depth and a power law relation for ductile flow in the lower crust that depends strongly on temperature. Transient variation of this depth caused by a magmatic intrusion at a depth near the regional transition can be modeled by a heat conduction model for a rectangular parallelepiped superimposed on a linear geothermal gradient. When parameters appropriate for the southeastern United States are used, a moderate‐sized intrusion is found to decrease the transition depth by as much as 7 km; significant variations last about 10 m.y. Since the base of the seismogenic zone is identified with the brittle‐ductile transition and since significant depth variations provide loci for strain energy concentration, these results imply that intrusions of late Tertiary age or younger could be important sources of clustered seism

ISSN:0094-8276    

DOI:10.1029/GL015i003p00213

年代:1988

数据来源: WILEY

摘要:

Paleomagnetic poles from two Late Jurassic to Late Cretaceous stratigraphic sections on the northern boundary of the Tarim Craton in western China are concordant with poles of similar age from eastern China. When combined with previously published data, these new results suggest that the major tectonic blocks which comprise China (the Tarim, Yangtze and Sino‐Korean cratons) have been arranged in approximately their present relative configuration since Cretaceous times. The averaged Cretaceous paleomagnetic pole for China is significantly different from the averaged Cretaceous pole for Siberia. The difference may reflect significant relative post‐Cretaceous motion between the composite China terrane and the Siberian craton. Alternatively, the Cretaceous pole for Siberia may not be relia

ISSN:0094-8276    

DOI:10.1029/GL015i003p00217

年代:1988

数据来源: WILEY

摘要:

Apatite fission‐track ages of basement from the Superior craton in eastern Manitoba, western Ontario and eastern North Dakota record two periods of Late Proterozoic epeirogeny and erosion. The latter period, which involves basement beneath and adjacent to the eastern flank of the Williston Basin, precedes basin subsidence and roughly coincides in time with breakup of the Proterozoic supercontinent. We speculate that these events are genetically relate

ISSN:0094-8276    

DOI:10.1029/GL015i003p00221

年代:1988

数据来源: WILEY

摘要:

Voyager observations of Ganymede show broad, high‐albedo, topographic domes situated within the central pits of some impact craters, referred to in this study as “dome craters”. Of 56 dome craters identified on Ganymede, all but two can be placed into one of two classes, based on the ratio of dome diameter to crater rim diameter. Two new hypotheses for the origin of the domes involving diapirism as an agent of dome formation are offered. Implicit in both hypotheses are possible regional heat flux variations. Under these scenarios, “relaxation” of crater relief may not be homogeneous in space and/or time, and crater morphology may not be a consistent indicator of crater age. Plutonic intrusions within the upper lithospheres of Ganymede (and Callisto) may have played a far more important role in heat transport on these satellites than previou

ISSN:0094-8276    

DOI:10.1029/GL015i003p00225

年代:1988

数据来源: WILEY

摘要:

The origin of the fundamental crustal dichotomy on Mars remains the outstanding unsolved problem in martian geologic evolution. The hypothesis that this dichotomy is due to a single giant (mega) impact early in martian history is tested by comparing the observed number of large impact basins on Mars with the number expected from a D−2distribution. This test is based on the assumption that the mega‐impact Borealis Basin is the largest member of such a distribution. The difference between observed and “expected” basins is the number of “missing” basins that might have been obscured by subsequent geologic events. If the Borealis Basin were the largest member of a D−2impact population, there would be a very large number of “missing” basins on Mars: 1 larger than 5000 km, 10 larger than 2000 km and 40‐50 larger than about 1000 km diameter. The area occupied by these “missing” basins (allowing for overlap) is too large to be hidden by the younger surface units. A large population of presently unrecognized large impact basins would have to be located in the well preserved cratered highlands. By contrast, if Chryse is the largest member of a D−2impact population, the more modest number of “missing” basins could in principle be confined to areas of Mars that have been resurfaced or reworked by subsequent geologic processes. A possible alternative to the mega‐impact hypothesis is that the crustal dichotomy is due to the cumulative effects of multiple overlappin

ISSN:0094-8276    

DOI:10.1029/GL015i003p00229

年代:1988

数据来源: WILEY

摘要:

We have surveyed eight years of interplanetary magnetic field (IMF) and energetic particle observations obtained by the IMP‐8 spacecraft upstream from the bow shock and accumulated 63 cases when the upstream IMF remained radial for extended periods of time (>1 hour). Of these, two cases have been selected during which measurable fluxes of ambient solar or corotating energetic particle events were absent. These conditions provide an excellent test to the theories of the origin of upstream energetic ions. It is shown that there are extended periods with radial IMF when no upstream energetic ions (E ≥ 50 keV) were detected. It is further shown that energetic ions in the range 50 keV ≤ E ≤ 1 MeV are not continuously present but appear in bursts of intensities varying by more than an order of magnitude under persistently radial IMF. These measurements contradict a fundamental prediction of the Fermi mechanism for the origin of the upstream energetic ions, namely that such ions should always be present on radial IMF lines. The observations are consistent with the hypothesis that energetic (≳ 50 keV) ions leak out from, and appear in the upstream medium sporadically, following the onset of magnetic activity within the magn

ISSN:0094-8276    

DOI:10.1029/GL015i003p00233

年代:1988

数据来源: WILEY

摘要:

This paper uses annual mean counting rate data from detectors on two long‐lived spacecraft, Pioneer 11 and IMP 8, to study the temporal and heliocentric radial distance variations of the intensity of interplanetary protons (Ep≈ 1 MeV) over solar activity cycle 21. The Pioneer 11 data cover the time period April 1973 through 1986 and the heliocentric radial distance range 1.0

ISSN:0094-8276    

DOI:10.1029/GL015i003p00237

年代:1988

数据来源: WILEY