摘要:

Balloon borne corona probes designed to measure vertical electric fields have yielded data on the vertical electric structure of four thunderstorms studied during the Thunderstorm Research International Program (TRIP), 1981. All four thunderclouds exhibited a bipolar charge structure with an upper positive charge region and a lower negative charge region. The negative region was consistently centered between the 0° C and −10° C isotherm altitudes. The altitudes of the concentrated positive regions varied considerably. The average vertical extent of the negative regions was 1 km with estimated average charge concentrations ranging from −.7 nC/m³ to −1.8 nC/m³. The average thickness of the positive regions was 1.5 km with estimated charge concentrations averaging .5 nC/M³ to

ISSN:0094-8276    

DOI:10.1029/GL010i001p00039

年代:1983

数据来源: WILEY

摘要:

The isotopic composition of rainwater near Nahariya, Israel, was measured in 12 individual storms during the winter months of 1980/1981. Rains most depleted in the heavy stable isotopes and enriched in tritium are associated with air masses which come from North East Europe and have a short, intense interaction with the East Mediterranean Sea. Storms most enriched in18O and D and depleted in tritium are associated with air masses which come from the Atlantic Ocean and enter the Mediterranean Sea on its western part, travelling along the North African Coast, sometimes with a southerly shift. A relatively high correlation coefficient was found between the stable isotopic content of precipitation and the 800 mb temperature.

ISSN:0094-8276    

DOI:10.1029/GL010i001p00043

年代:1983

数据来源: WILEY

摘要:

Fault motion during a dip‐slip earthquake induces elastic stresses in the earth which are well modeled by elastic dislocation theory. Shortly after the earthquake, flow in the asthenosphere begins to relax these stresses and leads to postseismic motions observable at the earth's surface. Recent work has led to apparently contradictory results on the sign of the vertical motion following a dip‐slip earthquake: some workers find downwarping in the region overlying a thrust fault; others find upwarping. This controversy is now resolved by a finite element study of postseismic rebound following a thrust earthquake on a 30° dip fault in an initially planar elastic lithosphere which overlies a Maxwell viscoelastic asthenosphere. If the fault penetrates less than 0.68 of the way through the lithosphere, downwarping follows the earthquake. If the fault penetrates more than 0.68 of the lithosphere, upwarping results. The magnitude as well as the sign of the postseismic rebound is a sensitive indicator of the ratio between the thickness of the elastic lithosphere and the fault depth. Observations of postseismic rebound may thus be used to measure lithosphere thicknesses at time scales on the order of y

ISSN:0094-8276    

DOI:10.1029/GL010i001p00047

年代:1983

数据来源: WILEY

摘要:

Measurements of magnetic fields along the San Andreas fault indicate that inhomogeneous tidally generated current systems flow in and around the fault system. These currents limit measurements of short‐term local fields of tectonomagnetic origin to about 0.3 nT but can be easily removed. Ocean‐tidal induction into a complex fault zone with higher than average electrical conductivity appears to be a more likely explanation than either piezomagnetic effects due to solid‐earth tides or ionospheric‐tidal induction. The amplitudes of the induced diurnal harmonics decrease fairly linearly to the southeast along the fault. This result is consistent with expectations from a hotter and more conductive crust and upper mantle in southern California, as indicated by heat‐flow data for th

ISSN:0094-8276    

DOI:10.1029/GL010i001p00051

年代:1983

数据来源: WILEY

摘要:

A profile of the strain accumulation rate along a line trending N50°E across the subparallel Elsinore, San Jacinto, and San Andreas faults near Palm Springs, California, has been constructed from trilateration surveys in the 1973‐81 interval. The strain accumulation is principally right‐lateral shear across a vertical plane parallel to fault strike (N40°W). The strain rate profile for that component exhibits two clearly resolved maxima, one centered on the San Jacinto fault (γmax= 0.35 ± 0.02 µrad/a) and the other on the San Andreas fault (γmax= 0.40 ± 0.02 µrad/a); no maximum is associated with the Elsinore fault. This result clearly implies that slip at depth on both the San Andreas and San Jacinto faults is loading the shallower sections of these faults, and that eventually rupture can be expec

ISSN:0094-8276    

DOI:10.1029/GL010i001p00055

年代:1983

数据来源: WILEY

摘要:

Spectral characteristics of P phases from 4 shallow focus earthquakes and 8 underground explosions, and of 52 samples of ocean bottom background noise, are examined by using tape recordings of ocean bottom hydrophones near Wake Island from July 1979 through March 1981. Significant differences are found between spectra of large shallow focus earthquakes and explosions (5.7 ≤ mb ≤ 6.3) observed at 61° to 77° epicentral distance. For similar magnitudes, explosions were found to have less energy at frequencies below 1.5 Hz and more energy at frequencies above 2.0 Hz. Earthquakes were found to have a spectral slope of −28 dB/octave (relative to pressure) over the band 1 to 6 Hz. Explosions were found to have the same spectral slope over the band 2.2 to 6 Hz, but a different slope of −12 dB/octave over the band 1.1 to 2.2 Hz. High frequencies (>6 Hz) observed in the teleseismic P phases indicate high Q values for the deep mantle. Ambient noise levels on the ocean bottom near Wake are comparable to levels at the quietest continental sites for frequencies between 3 and 15 Hz. Also high levels of coherence (at least as high as 0.85) have been observed for P phases recorded on sensors with 40‐km

ISSN:0094-8276    

DOI:10.1029/GL010i001p00059

年代:1983

数据来源: WILEY

摘要:

A lower mantle S‐wave triplication is detected using short‐ and long‐period SH seismograms in the distance range 70° to 95°. Modeling of the observations with synthetic seismograms indicates that the triplication is produced by a 2.75 ± 0.25% shear velocity increase about 280 km above the core‐mantle boundary. The SH data from intermediate and deep focus events for three distinct source region‐receiver array combinations show generally consistent travel time and relative amplitude behavior of the triplication. There are small, systematic travel time shifts between the source regions which suggest regional variations in the depth of the discontinuity of

ISSN:0094-8276    

DOI:10.1029/GL010i001p00063

年代:1983

数据来源: WILEY

摘要:

About 50% of the Doppler observations made on a Navy Navigation Satellite over a 9 year period were analyzed to determine the motion of 8 sites on the North American plate and 12 sites on 7 other plates. The computed plate motions were not statistically significant compared with the standard errors of measurement of 1 to 5 cm/yr except for the Australian plate, European plate and Pacific plate. The measured motions of these plates are about twice those inferred from geologic records, but are in the proper direction.Processing of the balance of the data on one satellite over the 9 year time interval would improve the accuracy of the determination by about 60%, improving the possibility of detecting additional statistically significant motions.Unreasonable altitude changes at most sites are probably due to neglected higher order ionospheric refraction effects on the observations.

ISSN:0094-8276    

DOI:10.1029/GL010i001p00067

年代:1983

数据来源: WILEY

摘要:

Brittle failure in rocks is generally preceded by a localization of deformation in the region of the final macrofracture. This process is associated with a non‐uniform distribution of subcritical microcracks which should be reflected in the distribution of acoustic velocities and surface deformation. The temporal and spatial development of such deformation zones has been studied by monitoring these quantities during the deformation of Westerly granite samples.Holographic interferometry revealed localization of surface deformation above about 70% of the tangential strain at failure. As loading proceeded, acoustic velocities became highly anisotropic, finally varying by a factor of 2 in different directions. They were smoothly dependent on path orientation, but not on path position within the sample. Thus, surprisingly, they did not reflect the localized deformation observed with holograph

ISSN:0094-8276    

DOI:10.1029/GL010i001p00071

年代:1983

数据来源: WILEY

摘要:

Holographic interferometry has been used to observe the surface deformation of Westerly granite during creep experiments at 50 MPa (500 bars) confining pressure. Interference fringes on the holograms produced by this technique constitute contour maps of the sample surface shape change between exposures. Several distinctive characteristics of the surface shape change have been observed. In general, surface deformation during primary and secondary creep, while highly inhomogeneous, is distributed fairly uniformly. No large scale pattern is seen in the surface shape changes. At approximately the onset of tertiary creep, surface displacements become more locallized. Eventually (∼85‐90% of time‐to‐failure) a distinct ridge is formed which exactly coincides with the location of the final f

ISSN:0094-8276    

DOI:10.1029/GL010i001p00075

年代:1983

数据来源: WILEY