摘要:

Seismic techniques provide the highest‐resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high‐pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pnvelocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5‐km depth intervals, both in the form of histograms and as an average velocity‐depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5‐km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low‐grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2conte

ISSN:0148-0227    

DOI:10.1029/95JB00259

年代:1995

数据来源: WILEY

摘要:

Elastic waves velocities were measured in the laboratory on homogeneous limestones in three different frequency ranges with varying water saturations (Sw). Measurements at frequencies of approximately 1 kHz were obtained using large (1‐m length) resonant bars. The conventional pulse transmission technique was used to measure the samples at 50 kHz and 500 kHz. In order to study the influence of the fluid distribution we used two different saturation methods: drying and depressurization. Using a computerized tomography scan to image fluid distribution, we found that during depressurization, saturation was highly homogeneous at the millimetric scale at all saturations. In contrast, during the drying, heterogeneous saturation was observed at high water saturation levels. Results from elastic wave propagation show that compressional and extensional velocities are sensitive to the saturation technique at high water saturation level. Furthermore, the dependence of velocity upon saturation technique is frequency dependent, and dispersion is greatest at high saturations. We attribute this behavior to a scale effect that varies with the size of heterogeneities of the rock and with the wavelength. In addition to global and local flow mechanisms it seems necessary to include a “path dispersion” effect to explain the amount of velocity dispersion observed. For shear waves the experimental results show little variation of the velocity with frequency and very little with fluid distrib

ISSN:0148-0227    

DOI:10.1029/95JB00757

年代:1995

数据来源: WILEY

摘要:

Both body wave (PKP) travel times (Creager, 1992; Song and Helmberger, 1993a; McSweeney and Creager, 1993; Shearer, 1994) and fits to the splitting of core modes (Tromp, 1993) show general agreement that the top 300 km of inner core is very anisotropic. The anisotropy displays axial symmetry around the Earth's spin axis, with the polar direction 3% faster than the equatorial direction. One key problem now is the depth dependence of the inner core anisotropy. Here we extend our polar path studies to include both long‐period and short‐period modeling for thePKPphases at ranges 120° to 173°. Arrivals from the top of the inner core (PKIKP) and reflections from the inner core boundary (PKiKP) can be observed distinctly in short‐period records at ranges 130° to 140° and as waveform distortions in the long‐period records at ranges 130° to 146°. These waveforms provide a new set of data for examining the topmost 150 km of the inner core, which is not well sampled by the previous body wave travel times. Record sections of waves traversing the inner core nearly parallel to the Earth's spin axis (polar paths) from three events, two beneath the South Sandwich Islands and one along the Macquarie Ridge, recorded at World Wide Standardized Seismograph Network, Canadian Network, and Long Range Seismic Measurements stations are analyzed. Our results suggest that the top 150 km of the inner core is only weakly anisotropic (less than 1%), with strong evidence indicating that the top 60 km is not aniso

ISSN:0148-0227    

DOI:10.1029/95JB00244

年代:1995

数据来源: WILEY

摘要:

Present Earth core models derived from the retrieval of global Earth structure are based on absolute travel times, mostly from the International Seismological Centre (ISC), and/or free‐oscillation eigenfrequencies. Many core phase data are left out of these constructions, e.g.,PKPdifferential travel times, amplitude ratios, and waveforms. This study is an attempt to utilize this additional information to construct a model of corePwave velocity which is consistent with the different types of core phase data available. In conjunction with our waveform modeling we used 150 differential time measurements and 87 amplitude ratio measurements, which were the highest‐quality observations chosen from a large population of Global Digital Seismograph Network (GDSN) records. As a result of fitting these various data sets, a one‐dimensionalPwave velocity model of the core, PREM2, is proposed. This model, modified from the Preliminary Reference Earth Model (PREM) (Dziewonski and Anderson, 1981), shows a better fit to the combined data set than any of the existing core models. Major features of the model include a sharp velocity discontinuity at the inner core boundary (ICB), with a large jump (0.78 km/s), and a low velocity gradient at the base of the fluid core. The velocity is nearly constant over the lower 100 km of the outer core. The model features a depth‐dependentQαstructure in the inner core such that a constantt* for the inner core fits the amplitude ratios and waveforms of short‐period waves moderately well. This means the top of the inner core is more attenuating than the deeper part of the inner core. In addition, thePvelocity in the lowermost mantle is reduced from that of PREM as a baseline adjustment for the observed separations of theDFandABbranches ofPKPat large

ISSN:0148-0227    

DOI:10.1029/94JB03135

年代:1995

数据来源: WILEY

摘要:

The purpose of this paper is the investigation of cylindrical anisotropy in the inner core based on the travel time anomalies of thePKIKPphase. We use the arrival times reported in the International Seismological Centre Bulletins for years 1964–1990. We select only earthquakes which have a good azimuthal coverage and a sufficiently large number of reporting stations. The earthquakes are relocated using corrections for lateral heterogeneity computed for our most recent three‐dimensional mantle model. We use a total of 313,422 observations of theDFbranch ofPKPtravel time anomalies within the epicentral distance ranges of 120°–140° and 150°–180° reported by 2335 stations for 26,377 earthquakes. We process the data using an averaging procedure, which we call cylindrical anisotropy stacking, that enhances the effects of anisotropy, but is expected to suppress those due to lateral heterogeneity and random errors. The processed residuals show a remarkably consistent pattern. This confirms the dominance of the cylindrical elastic anisotropy in the inner core with, approximately, a constant axis of symmetry. This axis of symmetry is found to be tilted 10.5°±1° from the Earth's rotation axis in the direction 160°E±5° in the northern hemisphere. In this new coordinate system we determine a four‐layer axisymmetric model of transverse anisotropy with each layer approximately 300 km thick. The model shows that the anisotropy is strongest (>3%) within the innermost part of the core. The travel time anomalies show significant (±1.5 s) longitudinal variations, even when the tilt of the axis of symmetry is considered. There is a substantial increase in the amplitude of the longitudinal variations, which are dominated by the second and fourth harmonics, for rays with bottoming depths exceeding 400 km below the inner core boundary. The measurable tilt of the axis of symmetry and the presence of significant nonaxisymmetric signal may provide important clues with respect to the possible causes of anisotropy. The increase in the anisotropy in the innermost part of the core departs from earlier inferences that anisotropy may be limited to the outermost 200–300 km of

ISSN:0148-0227    

DOI:10.1029/95JB00746

年代:1995

数据来源: WILEY

摘要:

Source parameters of moderate to large size (Mw>4.5) earthquakes in the western United States from 1992 to 1994 are determined by point source moment tensor inversion of complete long‐period (T>35–50 s) ground motions recorded at regional distances (1°–12°). Stable long‐period signals are obtained by low‐pass filtering records from the very broadband seismometers recently deployed in several networks in the western United States. These signals are dominated by fundamental mode Rayleigh and Love waves, which have very simple waveforms due to the limited dispersion on the short paths to regional stations. Since long‐period motions are relatively insensitive to the attenuation model and crustal structure used in the inversion, they provide robust constraints on the seismic moment and faulting geometry as long as adequate azimuthal coverage is available. Comparisons of solutions for 21 events with results of other regional and teleseismic wave inversions are made to assess the model dependence and uncertainties of our regional centroid moment tensor (RCMT) solutions. RCMT inversion has limited source depth resolution for shallow crustal events, but the focal mechanism and seismic moment determinations prove quite stable over a range of source depths in the crust, as well as over a range of crustal propagation models. Simultaneous waveform inversion of shorter‐period body wave signals can improve the source depth resolution. By applying path corrections for heterogeneous crustal structure, shorter‐period surface wave energy can also be inverted, allowing the methodology to be extended to lower‐magnitude regional events as well. The RCMT procedure requires minimal signal processing, only a sparse broadband network, and a simple laterally homogeneous propagation model; thus it can readily be automated and applied in near real time to events in the magnitude range from 4.5 to 7.5 distributed over an area as large as the we

ISSN:0148-0227    

DOI:10.1029/95JB00238

年代:1995

数据来源: WILEY

摘要:

The June 28, 1992, Landers, California, earthquake (Mw= 7.3) was preceded for about 7 hours by a foreshock sequence consisting of at least 28 events. In this study we examine the geometry and temporal development of the foreshocks using high‐precision locations based on cross correlation of waveforms recorded at nearby stations. By aligning waveforms, rather than trying to obtain travel time picks for each event independently, we are able to improve the timing accuracy greatly and to make very accurate travel time picks even for emergent arrivals. We perform a joint relocation using the improved travel times and reduce the relative location errors to less than 100 m horizontally and less than 200 m vertically. With the improved locations the geometry of the foreshock sequence becomes clear. The Landers foreshocks occurred at a right step of about 500 m in the mainshock fault plane. The nucleation zone as defined by the foreshock sequence is southeast trending to the south and nearly north trending to the north of the right step. This geometry is confirmed by the focal mechanisms of the foreshock sequence, which are right‐lateral and follow the trend as determined by the foreshock locations on the two straight segments of the fault, and are rotated clockwise for foreshocks that occur within the step. The extent of the foreshock sequence is approximately 1 km both vertically and horizontally. Modeling of the Coulomb stress changes due to all previous foreshocks indicates that the foreshocks probably did not trigger each other. This result is particularly clear for theMw= 4.4 immediate foreshock. Since stress transfer in the sequence appears not to have played a significant role in its development, we infer an underlying aseismic nucleation process, probably aseismic creep. Other studies have shown that earthquake nucleation may be controlled by fault zone irregularities. This appears to be true in the case of the Landers earthquake, although the size of the irregularity is so small that it is not detectable by standard location techniq

ISSN:0148-0227    

DOI:10.1029/95JB00871

年代:1995

数据来源: WILEY

摘要:

During subduction, the downgoing slab interacts with a strong overriding plate at shallow depths and with the surrounding asthenosphere at intermediate depths. Thus the transition from earthquakes representing interplate slip to those reflecting intraplate deformation within subducting lithosphere is important for understanding how the mechanics of subduction varies with depth. To characterize this transition, we studied 44 large to moderate sized earthquakes that occurred between 1964 and 1990 in two regions along the central portion of the Kuril‐Kamchatka arc. Precise earthquake source parameters are determined by analyzing body waveforms recorded at teleseismic distances. At depths of 30–50 km, our results delineate a transitional thrust zone, characterized by earthquakes showing thrust faulting on moderately dipping nodal planes. The northwesterly dipping nodal plane dips approximately 40°, some 10°–15° steeper than the dip of low‐angle earthquake faulting along the interplate thrust zone at depths shallower than 30 km. For earthquakes along the transitional thrust zone, secondary features in observed waveforms suggest that either a thick layer of sediments is present in the forearc region or the sources are embedded in materials with crustal seismic wave speeds. In the former case, the transitional thrust zone is interpreted as the deep‐seated portion of the plate interface whose dip increases with depth. The alternative implies that the transitional thrust zone occurrs below the plate interface within subducted crust of the Pacific plate and that the occurrence of earthquakes along the transitional zone may indicate a locked plate interface between depths of 30 and 50 km. The inference of a locked plate interface is consistent with the occurrence of compressional earthquakes beneath the outer rise region. However, a locked interface does not seem to affect the state of strain in the double seismic zone. The upper, compressional layer of the double seismic zone abuts the transitional thrust zone at a depth of approximately 50 km. At this depth, the two layers of seismicity in the double seismic zone are separated by as much as 30–40 km, too large a distance to be accommodated by models of seismogenesis invoking either the gabbro‐eclogite transition or dehydration of subduct

ISSN:0148-0227    

DOI:10.1029/95JB00239

年代:1995

数据来源: WILEY

摘要:

Travel time inversion and amplitude forward modeling have been applied to two seismic refraction profiles from the northern Yukon‐Mackenzie Delta region of northwestern Canada. The two‐dimensional crustalPwave velocity models feature a near‐surface layer which is 1–7 km thick and has an average velocity of 4 km/s; this overlies three crustal units, each having an average thickness of 11–15 km and with average velocities of 5.9, 6.1, and 7.1 km/s. The Moho is at ∼37 km with little relief and overlies an upper mantle with a poorly constrained velocity. Tectonically, the study area lies between cratonic and Cordilleran North America and adjacent Mesozoic polar continental margin. The velocity models clearly illustrate a domainal crustal structure in the study area. A cratonic domain is characterized by a middle and lower crust with homogeneous velocities of 6.6–6.8 km/s. The other domain (“Yukon domain”) is characterized by midcrustal velocities near 6 km/s and a lower crustal layer with velocities near 7.1 km/s. The transition zone between these domains is well‐defined and is interpreted as a Proterozoic paleocontinental margin, supporting previous interpretations based on geological trends and potential field data. Lateral homogeneity of the crustal velocity structure within Yukon domain supports interpretations that Arctic Alaska was not emplaced into its present position on strike‐slip faults. Local variations in lower crustal thickness, together with clear wide‐angle Moho reflections, suggest a lower crustal and Moho signature possibly related to rifting, crustal extension, and magmatic intrusion and underplating during the Jura‐Cretaceous development of the Arctic Ocean and

ISSN:0148-0227    

DOI:10.1029/95JB00673

年代:1995

数据来源: WILEY

摘要:

Analysis of Global Positioning System (GPS) data from two sites separated by a horizontal distance of only ∼2.2 m yielded phase residuals exhibiting a systematic elevation angle dependence. One of the two GPS antennas was mounted on an ∼1‐m‐high concrete pillar, and the other was mounted on a standard wooden tripod. We performed elevation angle cutoff tests with these data and established that the estimate of the vertical coordinate of site position was sensitive to the minimum elevation angle (elevation cutoff) of the data analyzed. For example, the estimate of the vertical coordinate of site position changed by 9.7±0.8 mm when the minimum elevation angle was increased from 10° to 25°. We performed simulations based on a simple (ray tracing) multipath model with a single horizontal reflector which demonstrated that the results from the elevation angle cutoff tests and the pattern of the residuals versus elevation angle could be qualitatively reproduced if the reflector were located 0.1–0.2 m beneath the antenna phase center. We therefore hypothesized that the elevation‐angle‐dependent error was caused by scattering from the horizontal surface of the pillar, located a distance of ∼0.2 m beneath the antenna phase center. We tested this hypothesis by placing microwave absorbing material between the antenna and the pillar in a number of configurations and by analyzing the changes in apparent position of the antenna. The results indicate that (1) the horizontal surface of the pillar is indeed the main scatterer, (2) both the concrete and the metal plate embedded in the pillar are significant sources of scattering, and (3) the scattering can be reduced greatly by the use of microwave absorbing materials. These results have significant implications for the accuracy of global GPS geodetic tracking networks which use pillar‐antenna configurations identical or similar to the one used for this study at the We

ISSN:0148-0227    

DOI:10.1029/95JB00868

年代:1995

数据来源: WILEY