摘要:

This volume of theJournal of Geophysical Researchis devoted to Anton L. Hales. It contains many of the papers presented at the Anton L. Hales Symposium on ‘Some Recent Advances in Geophysics’ held at The University of Texas at Dallas (UTD) on October 5–6, 1981, to honor Professor Hales on his retirement from UTD at the age of 70. Many former students, colleagues, and friends attended the symposium to honor Anton and to share their experiences with a man who had touched their lives and left his mark on the field of geophysics. Professor Hales' career in geophysics spans 50 years beginning with his first course in geophysics in 1932 at Cambridge Unive

ISSN:0148-0227    

DOI:10.1029/JB088iB04p03245

年代:1983

数据来源: WILEY

摘要:

Data from the Global Digital Seismograph Network were used to obtain ‘centroid‐moment tensor’ solutions using the method of Dziewonski et al. (1981). Results were obtained for 201 earthquakes ranging in seismic moment from 7×1023to 3×1027dyne‐cm. The wide dynamic range of the SRO/ASRO stations allows us to investigate, using the same algorithm, series of events among which the smallest and the largest may differ in moment by a factor as large as 1000. Among the events studied is a particularly interesting series, in January 1981, off the island of Honshu. The main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis. There is an indication that the source mechanism changes with the size of the event. Statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1.6×10−8M01/3. The shifts in origin time for deep‐focus earthquakes are indicative of the general complexity of these events, which depends only weakly on the seismic moment. The extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth. While for shallow earthquakes these departures decrease, from a maximum at about 1025dyne‐cm, with increasing moment, the reverse appears to be true with respect to deep‐focus earthquakes. Shallow earthquakes north of New Guinea and along the Solomon Islands show systematic and substantial departures from the doubl

ISSN:0148-0227    

DOI:10.1029/JB088iB04p03247

年代:1983

数据来源: WILEY

摘要:

Total‐moment spectraMT(ω) = ‖M(ω)‖/ , where M is the moment rate tensor, are computed for 14 large earthquakes recorded by the International Deployment of Accelerometers (IDA) network using the scalar‐moment retrieval method proposed bySilver and Jordan[1982]. For each event we obtain estimates ofMTaveraged over the 10 disjunct, 1‐mHz intervals in the low frequency band 1–11 mHz; typical IDA record sets from events withMT≳ 0.2 A (1 A = 1027dyne · cm) yield standard errors on the 1‐mHz averages that are generally less than 20%. Our multiple‐band estimates ofMTare usually consistent with comparable single‐band values found by other investigators. From the total‐moment spectra we derive the zero‐frequency (static) moment ≡MT(0) and the characteristic source duration τc≡ 2 (Var [f(t)])½, wheref(t) is the time function of the moment rate tensor, assumed to be the same for all components. The parameter τCformally depends on the second‐degree temporal, spatial, and mixed spatial‐temporal moments of an extended source, but calculations with realistic source geometries indicate that the interpretation ofTCstrictly in terms of the second temporal moment leads to very little error. A plot of τcagainst shows considerable scatter; some events lie significantly above the empirical scaling curve ofKanamori and Given[1981] (‘slow earthquakes’) and some below (‘fast earthquakes’). Examples of the former include all three deep‐focus events analyzed here as well as the 1970 Colombia and 1963 Peru‐Bolivia deep‐focus earthquakes, whose total‐moment spectra are calculated from the moment tensor solutions ofGilbert and Dziewonski[1975]. An example of the latter is the great Sumbawa earthquake of August 9, 1977 (= 24±3 A), which is inferred to have a relatively shallow spatial centroid (<20 km) and a nearly flat moment spectrum, suggesting that the faulting was confined to the upper, more brittle portion of the oceanic lithosphere. Three other large earthquakes—Tonga (June 22, 1977; = 23±2 A), Kuril Islands (December 6, 1978;= 3.6±0.2 A), and Tumaco (December 12, 1979; = 25±2 A)—exhibit moment spectra which significantly decrease toward higher frequencies, indicating larger values of τc. The aftershocks of both the Tonga and Kuril Islands events are distributed over a considerable range of depths, consistent with rupture into the lower, more ductile portions of the oceanic lithosphere. We speculate that the characteristic source duration may be causally rel

ISSN:0148-0227    

DOI:10.1029/JB088iB04p03273

年代:1983

数据来源: WILEY

摘要:

Approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of P wave travel times and station residuals, including azimuthal effects. The events were selected from a catalog containing 160,000 earthquakes, and those having uniform distance and azimuthal coverage were systematically relocated and used to refine P wave travel times and station corrections. Station corrections are provided for 994 seismic stations. The station corrections involve three terms: the static effect and two cosine terms with appropriate phase shifts. They exhibit general consistency over broad geographic areas and, where coverage is dense, often show abrupt changes from one geological province to another. The cos 2θ terms appear to be due to upper mantle anisotropy, and they correlate with the stress direction in the crust

ISSN:0148-0227    

DOI:10.1029/JB088iB04p03295

年代:1983

数据来源: WILEY

摘要:

The multimode method of Cheng and Mitchell [1981] is used to study the attenuation properties of the crust throughout the Barents shelf. The sensitivity of the spectra to various source factors and to propagation factors which might characterize a shelf region are thoroughly studied. For plane‐layered models, the method is very sensitive to Qβin the upper crust and to Qβof sedimentary layers if those layers are thick and highly attenuating. It is far less sensitive to Qβin the lower crust and to Qα. The spectra, for shallow strike slip earthquakes, are very insensitive to changes in strike, slip, and dip angles of the fault throughout the entire period range of interest but are very sensitive to variations in focal depth except in the long‐period range of the fundamental mode and the short‐period range of the higher modes. The fundamental mode, but not the higher mode, spectra for one path across the Barents shelf are, apparently, distorted at short periods by lateral heterogeneities along the travel path. Simple two‐layer, trial‐and‐error inversions yield a model of the Barents shelf with an upper crustal Qβvalue of about 80. Three‐layer inversions with a sedimentary layer, however, yield an upper crustal Qβof about 250, if the sediments are 1 km thick and are characterized by Qβvalues of 40. Shear velocity models for the Barents shelf show variations between about 3.0 and 3.6 km/s in the upper crust along different paths. The lower crust shows smaller regional differences, but velocities vary between 3.5 and 4.0 km/s through the depth range 25–35 km. The variation of shear velocities in the upper mantle may correlate with those of the upper crust and vary regionally between

ISSN:0148-0227    

DOI:10.1029/JB088iB04p03315

年代:1983

数据来源: WILEY

摘要:

This is a speculative paper on the continental crust, a major frontier of modern earth science. Seismic reflection profiling data from the COCORP project, coupled with other kinds of geophysical and geological observations, suggest some fresh perspectives on the crust. For example, there is evidence for great differences within the basement at a single site and from one COCORP site to another. A simple layered model is surely not an adequate representation of the crust in most areas. There is, however, the puzzling and rather common occurrence of limited zones of parallel, though not always flat, layered features at various depths in certain parts of the crust. Are the corresponding rocks metasedimentary? Or igneous? Or both? There are transparent zones from which little or no coherent reflected seismic energy is detected. Are the corresponding rocks intrusions? Or highly deformed metasediments? Can degree of deformation suggest whether rocks were deformed in the hanging wall or footwall of a thrust? There is evidence for large‐scale thrusting and related features that emphasizes the great importance of the Wilson cycle to continental tectonics and that raises the possibility that water and rocks of sedimentary origin are trapped deep in the crust by the thrusting process and remain there for long periods with a variety of possible rheological and petrological consequences. In some places the modern Moho and adjacent rocks may correspond to parts of former oceanic or continental margin sections that have been emplaced at depth as a result of thrusting. The widely held concept of the Moho as a consistent boundary that is everywhere similar and that always separates the same kinds of rocks may be a major barrier to further progress in understanding the continental crus

ISSN:0148-0227    

DOI:10.1029/JB088iB04p03329

年代:1983

数据来源: WILEY

摘要:

Although the thermal mode of lithosphere cooling can be used to predict the East Pacific Rise topography reasonably well, it does not adequately predict the observed gravity when the zero temperature boundary condition is at the seafloor. The misfit arises primarily because this model predicts thermally lowered densities in the crust at the rise axis which counteract the gravitational effect of the topography, generating a theoretical gravity anomaly smaller than that observed. It is shown, using data at 12°N as an example, that improved agreement with the thermal model is obtained if, to first order, the crust is treated as an isothermal, constant thickness, constant density layer with variable depth. This result is consistent with seismic data and observations of convective cooling of the crust by seawater circulation. It implies that convective cooling of the whole crust is very rapid, even at the axis of fast spreading ridges. The oceanic Moho may represent the depth to which cracking and convective cooling extends

ISSN:0148-0227    

DOI:10.1029/JB088iB04p03348

年代:1983

数据来源: WILEY

摘要:

Studies, using a variety of techniques, of the subduction beneath western South America and the southwest Honshu‐Kyushu region of Japan indicate that volcano‐free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere. The subduction is initiated at normal dip angles, but the plate deforms at some depth (100 km under central Peru) to travel horizontally immediately beneath continental lithosphere. The most plausible reason for this geometry is that the subducted plate is buoyant. A model is developed constrained by age of the plate, bathymetry, and heat flow. Estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts. If the high‐density tranformation of crustal basalt to eclogite is retarded by low temperatures, the plate can remain buoyant to considerable depth for long periods of time. Heat flow data from western South America are consistent with this

ISSN:0148-0227    

DOI:10.1029/JB088iB04p03355

年代:1983

数据来源: WILEY

摘要:

Lateral variations of electrical conductivity in the lithosphere and below it can be informative concerning both contemporary and ancient global tectonics. The methods of investigation include classical four‐electrode resistivity depth probing, which is of special value in continental shields; magnetotellurics, long used on land and recently in the Pacific to show that in electrical terms the lithosphere is thicker, near an oceanic ridge, than had been expected; and magnetometer arrays, which are useful primarily in mapping highly conductive structures. Examples of the use of all three methods will be reviewed, with emphasis on the last. In conjunction with other geophysical, geological, and geochemical data, magnetometer arrays have been useful in mapping partial melt under the western United States, in discovering an extension of the southern end of the African rift system to the Atlantic coast, in detecting a major fracture zone in central North America which may be a Proterozoic interplate collision feature, and in discovering a conductive belt under southernmost Africa which may result from accumulation of serpentinized marine crustal rocks at the top of a subduction under Proterozoic Gondwanaland. Inherent difficulties of quantitative modeling limit the utility of magnetometer arrays largely to discovery and mapping of two‐ and three‐dimensional conductive structures. This remains valuable, however, particularly when other physical properties can be used to limit the range of possible interpretations of the conductive structures. Recent array studies now in progress in western Canada have defined two new conductivity anomalies. One of these may represent a local partial melt concentration under the Rocky Mountains Trench, the other a Precambrian rift valley in the lower crust previously suggested by others on the basis of seismological, gravity, and static magnetic data. Both are being investigated by more concentrated a

ISSN:0148-0227    

DOI:10.1029/JB088iB04p03367

年代:1983

数据来源: WILEY

摘要:

Mesozoic and Tertiary granitic rocks in and adjacent to the northern Great Basin (NGB) in Nevada and Utah display a wide range of initial143Nd/144Nd (εND) and87Sr/86Sr (εSr) values which vary regularly with geographic position. From the Klamath Mountains inland 500 km to central Nevada, granite εNdvalues decrease regularly from +8 to −6 and correlate with εSrvalues that increase from −20 to +60. In east–central Nevada, near the trace of the Roberts Mountains Thrust (RMT), the εNDvalues decrease from −6 to an average of −18, while εSrbecomes highly variable with values generally greater than +100. These isotopic discontinuities correspond to the west‐to‐east facies transition from pelagic clastic sedimentary rocks to shelf carbonates and to the shift in the dominant granite bulk composition from metaluminous to peraluminous. In the easternmost NGB a second discontinuity in esr occurs with values dropping to ∼+60; average εNDremains at −18. Combined with known aspects of NGB geology the isotopic data suggest that west of the RMT, granites formed via interaction of magma derived from a LREE‐depleted mantle reservoir with an inland increasing proportion of assimilated continentally derived pelagic sedimentary rock. Variations in87Rb/86Sr with Sr, and147Sm/144Ndwith εND, indicate that crystal fractionation accompanied assimilation, but that plagioclase was not an important fractionating phase. East of the RMT, granites appear to be primarily derived from Precambrian continental basement with little mantle input. The isotopic discontinuities near the RMT mark the western edge of Precambrian basement and occur 100–200 km east of the87Sr/86Sr (=0.7060) line ofKistler and Peterman[1973]. The εSrdiscontinuity in the eastern NGB marks a boundary between Rb‐depleted (granulite?) lower continental crust to the east, and basement that has no ‘depleted’ lower crust to the west. The difference may be a result of lateral variations in metamorphic grade in the orogenic episode at ∼1.7 b.y. ago or a result of ductile attenuation of the lower crust during the late Precambrian continental rifting. The basement age appears to be intermediate between that of Wyoming (2.6 b.y.) and Colorado (1.8 b.y.) but cannot be precisely determined from the granite Nd model ages due to changes in Sm/Nd during granite magma generation. In contrast to the eastern NGB granites, Mesozoic batholith granites of the western United States have a lowest εNDof only −8 even where thought to have been emplaced in crystalline basement, a difference which could be related to an inland decreasing flux of mantle‐derived magma into the continental crust in the Mesozoic. A model for subduction‐induced flow in the upper mantle beneath the continent, resulting in perturbations in the thermal structure of the continental lithosphere, is presented to account, in part, for the spatial difference in mantle magma flux and the distribution of silicic magmatism in the western United States. Overall, the NGB granite data support the concept that mantle magmas are added to continents mainly at the margin, with little mantle magma generation, or continental growth, in the interior, and demonstrate the potential of Nd and Sr isotopic data from granites in discerning f

ISSN:0148-0227    

DOI:10.1029/JB088iB04p03379

年代:1983

数据来源: WILEY