摘要:

We determined the variations in seismic structure beneath southern California by using a tomographic method of inversion on teleseismic P delays recorded with the Southern California Array. The algorithm employed was a modified form of an Algebraic Reconstruction Technique (ART) used in medical X‐ray imaging. Deconvolution with an empirically estimated point spread function was also used to help in focusing the image.The inversion reveals two prominent features beneath the region. The first is a thin, vertical wedge directly beneath the Transverse Ranges that is 2‐3% faster than the surrounding region. This feature deepens to the east, attaining a maximum depth of about 250 km beneath the San Bernardino Mountains. The second feature is a major zone of low velocity material that is 2‐4% slow under the Salton Trough rift valley, extending to a depth of about 125 km. Two possible explanations for the spatial association of the Transverse Ranges with the velocity anomaly below are lithospheric subduction or small‐scale sublithospheric convection in the region of the Big Bend of the San Andreas Fault. The low velocity anomaly beneath the Salton Trough is consistent with convective upwellin

ISSN:0094-8276    

DOI:10.1029/GL011i007p00625

年代:1984

数据来源: WILEY

摘要:

To advance our understanding of the earthquake rupture process, we must develop methods that will map observations into the space‐time variation of seismic moment release on the fault plane. While the temporal history of overall moment release is relatively easy to obtain from P‐waves, there has been no objective quantitative method available to directly determine the spatial variation in moment release. The theoretical connection between spatially varying moment release and the resultant seismogram is a linear integral relationship (specifically, a Radon transform). This paper outlines a linear inverse method that provides a high‐resolution tomographic image of the space‐time varying moment density function. Application of the method to P‐waves from the large 1976 Mindanao earthquake demonstrates the ability of this method to reliably extract coherent pulses of moment release in space

ISSN:0094-8276    

DOI:10.1029/GL011i007p00629

年代:1984

数据来源: WILEY

摘要:

Multimode Love and Rayleigh wave data related to the Pacific Ocean and to North America are inverted using two sets of parameters: 1) the isotropic P and S velocities plus the density and 2), five elastic parameters describing a transversely isotropic medium, plus the density. It is found that the assumption of isotropy leads to very unrealistic upper‐mantle models while much more realistic models are obtained under the assumption of transverse isotropy. The lateral variations in density between the Pacific Ocean and North America are then less than 1.5% and, in both regions, a large density gradient is well resolved at a depth near 360 k

ISSN:0094-8276    

DOI:10.1029/GL011i007p00633

年代:1984

数据来源: WILEY

摘要:

Seismic velocities are calculated for two petrological models of the upper mantle, an olivine‐rich assemblage, pyrolite, and a garnet‐clinopyroxene rich, olivine bearing, assemblage, piclogite. These are compared with recent seismic profiles for various tectonic provinces. The shield data is most consistent with a cold olivine and orthopyroxene‐rich LID (the seismic lithosphere) extending to 150 km followed by a high temperature gradient and/or a change in mineralogy that serves to decrease the velocity. From 200 to 400 km the velocities follow a 1400°C adiabat. The rise‐tectonic mantle is much slower, presumably hotter and is likely to be above the solidus to depths of at least 300 km. The high Vp/Vsratio of the lower oceanic lithosphere in the western Pacific is most consistent with

ISSN:0094-8276    

DOI:10.1029/GL011i007p00637

年代:1984

数据来源: WILEY

摘要:

The thickness growth characteristics of flowing lava are investigated using a heat balance model and a two‐dimensional model for flow of a Bingham plastic fluid down an inclined plane. It is found that yield strength plays a crucial role in the thickening of a lava flow of given flow rate. To illustrate this point, downstream thickness profiles and yield strength distributions were calculated for flows with mass flow rates of 104and 105kg/m‐sec. Higher flow rates led to slow cooling rates which resulted in slow rate of increase of yield strength and thus greater flow leng

ISSN:0094-8276    

DOI:10.1029/GL011i007p00641

年代:1984

数据来源: WILEY

摘要:

Measurement of the anisotropy of magnetic susceptibility (AMS) in rock specimens is complicated when magnitude differences between two of the three principal susceptibilities which are conventionally used in defining the susceptibility ellipsoid approach the precision limits of the instrument used. To evaluate this problem, the precision is reported for 5 replicate measurements of low field AMS in 34 samples (including diamagnetic samples) with susceptibilities ranging between 5.73 × 10−3and 2.94 × 10−5in mass SI units using a torsion fiber magnetometer. (These samples are available upon request for interlaboratory calibration.) Precision appears to be controlled by the within‐sample axial susceptibility ratios and not by the susceptibility magnitudes. It is observed for the instruments used in this study that when the axial ratios (χa/χcor χb/χcare equal to or less than 1.002, then the replicate precision for those axes is poor. When the axial ratios are greater than 1.005, then the precision is excellent. Such replicate AMS measurements for any given instrument provide an empirical means to determine a practical method for rejecting AMS data that are not sufficiently anisotropic for reliable

ISSN:0094-8276    

DOI:10.1029/GL011i007p00645

年代:1984

数据来源: WILEY

摘要:

The Mississippian Leadville Formation of Central Colorado is dolomitized near the Sawatch Uplift and hosts numerous Pb‐Zn‐Ag mineral deposits. Paleomagnetic and rock magnetic data from several dolomite types away from centers of Tertiary intrusive activity imply a late Paleozoic (Late Mississippian, ?) age for the dolomitization (e.g., D = 148°, I = −11°, k = 86, α95= 8.6°, N = 3 localities, 31 samples in dolomitized units). The magnetization is of diagenetic origin and was probably acquired during or soon after dolomitization. Tertiary thermochemical activity has overprinted dolomites close to intrusive centers. Data indicate that limestones removed from igneous activity were also remagnetized at the

ISSN:0094-8276    

DOI:10.1029/GL011i007p00649

年代:1984

数据来源: WILEY

摘要:

A 1 mm layer of magnetite powder has been sprinkled into a 3 cm deep, 1 meter long trench in high bioturbation rate tidal flat sediments, Sapelo Island, Georgia (USA). Periodic resampling and replicate remanent magnetization (RM) measurement indicates that within site RM directions and intensities stabilized between 20‐50 days after initial sampling. The RM mean after 50 days exhibits an inclination which is ˜20° less than the geomagnetic field inclination at the site and remains low through the Winter and Spring but then nearly disappears during the summer when bioturbation rates are extremely high, leaving a post‐depositional RM which is statistically indistinguishable from the earth's geomagnetic field inclination at the site. Since only very high bioturbation rates appear to produce a post‐depositional RM in this experiment, it is inferred that post‐depositional RM development in some low bioturbation rate natural sediments, e.g. locally in the deep‐sea, may be dominated by physical rather than biological depositional or post‐depositio

ISSN:0094-8276    

DOI:10.1029/GL011i007p00653

年代:1984

数据来源: WILEY

摘要:

While in the lobes of the distant magnetotail, ISEE‐3 encountered regions of compressed magnetic field, δB/Bo=0.3‐0.1, at a rate of several per day. The duration of these events was 5 to 20 minutes and they were observed 10 to 30 minutes following the onset of substorm activity near the earth. During each event, the lobe magnetic field tilted first northward and then southward with the inflection point near the time of peak field strength. Following the compression events, the lobe field weakened and retained a southward component for 20 to 40 minutes. It is suggested that these traveling compression regions (TCR’s) are the lobe signatures of plasmoids moving rapidly down the tail in the plasma sheet. Comparison of ISEE‐3 compression event times with substorm onset times yielded propagation speeds of 350 to

ISSN:0094-8276    

DOI:10.1029/GL011i007p00657

年代:1984

数据来源: WILEY

摘要:

ISEE‐3 electric field measurements are used to examine the properties of electromagnetic continuum radiation in the distant geomagnetic tail. Continuum is observed in all the tail's plasma regions and in the magnetosheath. The power spectrum at 210 REis nearly identical to that at 40 RE, indicating that the tail cavity forms a reasonably loss‐free waveguide. The angular distribution exhibits both anisotropy, which is similar to that observed nearer the earth, and isotropy for high frequencies (f>31.6 kHz) in the magnetosheath and for low frequencies (f ≤ 17.8 kHz) in the tail lobes and boundary layer. Isotropic radiation suggests that, in addition to the near earth source, continuum is also generated over a large spatial region in the tail. Electrostatic electron plasma oscillations are also detected in the in the distant tail, and these could represent the local source of the cont

ISSN:0094-8276    

DOI:10.1029/GL011i007p00661

年代:1984

数据来源: WILEY