摘要:

Modern geodetic and atmospheric data sets have demonstrated that changes in the solid earth's rotation rate are closely coupled to changes in the atmosphere's angular momentum over a range of subseasonal through interannual time scales. Nevertheless, limits to the correspondence between changes in the momenta of the two bodies exist at both ends of the spectrum resolvable by the data. At high frequencies we show that changes in the length of day as short as a fortnight can now be attributed almost entirely to atmospheric forcing; more rapid fluctuations in the length of day, however, cannot be shown to coincide with atmospheric behavior, a result we blame mostly on errors remaining in both the geodetic and atmospheric series rather than on the neglect of some other component of the momentum budget. On the decadal time scale, nonatmospheric processes, believed primarily to involve coremantle coupling, dominate the global momentum budget. Our analysis of the low‐frequency component of the difference between the momenta of the solid earth and atmosphere during 1976–1988 shows it to consist of a general trend interrupted by three major changes in slope. Whether this result indicates that coremantle coupling is a nonsteady phenomenon or that unmodeled oceanic processes are responsible for the intermittency of the difference series is presently uncert

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00265

年代:1990

数据来源: WILEY

摘要:

The 0.3‐ to 2.6‐μm reflectance spectra of meteoritic enstatite (nearly pure MgSiO3), iron meteorite metal, magnetite and amorphous carbon, and various mixtures of these materials with mafic silicates have been measured in order to search for diagnostic spectral parameters which can be used to differentiate among the different materials and to provide information on the detection limits for mafic silicates. Enstatite and metal can be discriminated on the basis of albedo and spectral slopes. Metal, magnetite, and amorphous carbon are variably effective at suppressing mafic silicate absorption bands. Metal‐silicate spectra suggest that>25 wt% olivine must be present to be spectrally resolvable, while the lower limit for orthopyroxene is ∼ 10 wt%. Magnetite‐silicate spectra indicate that magnetite is not an effective suppressor of mafic silicate absorption bands and that fine‐grained magnetite imparts a less red overall slope to mafic silicate spectra than coarse‐grained magnetite. Carbon‐silicate spectra indicate that suppression of olivine absorption bands is enhanced when the olivine is finely comminuted and is less effective at larger grain sizes. Carbon and magnetite both seem to be required to impart a blue overall slope to mafic silicate spectra and to suppress Fe2+absorption bands. The small amount of carbon present in ordinary chondrites is probably not the cause of the flat reflectance spectra of metal‐rich ordinary chondrite separates. The observational data for a representative M‐class asteroid, (16) Psyche, are largely consistent with a fine‐grained metal‐rich surface assemblage, although an enstatite chondrite‐like assemblage cannot be ruled out. The observational data for the E‐class asteroid (44) Nysa indicate that its surface is composed of fine‐grained material similar to enstatite achondrites (aubrites), with a small amount of material comparable to the chondritic inclusions found i

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00281

年代:1990

数据来源: WILEY

摘要:

Two temporary three‐station seismic networks, deploying surface and 100‐m borehole high‐frequency seismometers, of the order of 200 km from the Kazakh test site in the USSR and the Nevada test site in the United States are discussed, with emphasis on chemical explosion experiments. Seismograms attained from the detonation of three buried explosions (10 t, 20 t, 10 t) in eastern Kazakhstan at distances between 156 and 637 km are examined in the frequency band of 1–80 Hz. Observed signal‐to‐noise (S/N) ratios were high, reaching a maximum of 400 forPgwaves and 200 forLgwaves. Good signal‐to‐noise levels persisted to high frequencies; S/N = 2 at about 50 Hz forLgwaves about 250 km from the source, and at about 14 Hz at 680 km distance. ForPgwaves, S/N = 2 at about 50 Hz 270 km from the source. Shapes of displacement amplitude spectra were similar, characterized by a broad maximum in signal‐to‐noise levels between 4–8 Hz, and a decay at higher frequencies (e.g. above 10 Hz) of aboutf−3.5−f−4.1forLgwaves, andf−3.1−f−4.5forPg, unconnected for distance. Magnitudes estimated fromLgtime domain amplitudes for the 10 t explosion are between 2.8 and 3.3, depending on the magnitude relation used. Spectral characteristics are used to put some constraint onLgQ.PgQis poorly constrained by the data. A similar experiment in southern Nevada showed much lowerPgandLgsignal‐to‐noise levels above 1 Hz, although Kazakh and Nevada

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00295

年代:1990

数据来源: WILEY

摘要:

The coda waves of 21 Parkfield earthquakes were recorded simultaneously at the surface and at a depth of 198 m. We characterize these codas by computing (1) the time dependence of the integral of squared particle velocity, (2) their frequency content, (3) time‐frequency coda decay planes, (4) the frequency dependence of root mean square ellipticity, and (5) the difference in their normalized decay rates. These five measures reveal significant differences in the early portion of the uphole versus downhole codas. Our analysis solves for codaQas an explicit function of both time and frequency. Average apparent codaQfor the 21 events observed both at the surface and downhole isQa(f, τ) = 39(f/f0)0.43(τ/τ0)0.37, with the reference frequency and time defined at 1 Hz and 1 s, respectively. CodaQexhibits a trend in its value over the 1‐month period of the study. The trend correlates with a spatial variation in source locations and is not produced by temporal variations of theQof the medium. The spectral ratio between the uphole signal and the downhole signal varies with time through the coda. The early portion of the uphole coda experiences a pulse of low‐frequency energy not seen in the downhole coda. To explain this observation, we propose a model of the coda that includes body wave to surface wave scattering near the site. In this model, the near‐surface acts as both a filter and a scattering waveguide. The coda at the surface is the sum of a coda produced by scattering deep in the lithosphere and a coda produced by scattering in the near‐surface. The scattering in the near‐surface is made up of near‐vertically traveling multiples and horizontally travelling surface waves. The guided waves are recorded in the surface coda but are absent from the coda recorded at depth. The time‐frequency structure of the surface waves in the uphole coda is determined by the velocity, scattering, and attenuation characteristics of the site. By fitting the model to our data, we have found that the fractional conversion factor of body waves to surface waves travelling toward the receiver is 0.03 at the Vineyard Canyon site. The relatively rapid damping of these trapped waves can be modelled by assuming a quality factor ofQT= 50. The principles of this model are general enough to be of use

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00309

年代:1990

数据来源: WILEY

摘要:

Some of the gross characteristics of the rupture processes for the 1966 Parkfield, 1979 Imperial Valley, and 1980 Mexicali Valley earthquakes are determined by waveform inversion of near‐source data employing a steady state dislocation model in a layered half‐space. The forward model involves a piecewise‐linear rupture front moving with a constant horizontal rupture velocity on a fault of infinite length and finite width. The inferred shapes for the rupture front and for the distribution of slip as a function of depth are consistent with previous results obtained by use of more general models. The results obtained show that a strong velocity pulse observed in the nearsource region can be modeled as the passage of the rupture front phase and that the supershear propagation of the rupture front in the sedimentary layers of the medium provides a mechanism for the generation of the observed large ampli

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00327

年代:1990

数据来源: WILEY

摘要:

The hydrostatic compaction behavior of a suite of porous sandstones was investigated at confining pressures up to 600 MPa and constant pore pressures ranging up to 50 MPa. These five sandstones (Boise, Kayenta, St. Peter, Berea, and Weber) were selected because of their wide range of porosity (5–35%) and grain size (60–460 μm). We tested the law of effective stress for the porosity change as a function of pressure. Except for Weber sandstone (which has the lowest porosity and smallest grain size), the hydrostat of each sandstone shows an inflection point corresponding to a critical effective pressure beyond which an accelerated, irrecoverable compaction occurs. Our microstructural observations show that brittle grain crushing initiates at this critical pressure. We also observed distributed cleavage cracking in calcite and intensive kinking in mica. The critical pressures for grain crushing in our sandstones range from 75 to 380 MPa. In general, a sandstone with higher porosity and larger grain size has a critical pressure which is lower than that of a sandstone with lower porosity and smaller grain size. We formulate a Hertzian fracture model to analyze the micromechanics of grain crushing. Assuming that the solid grains have preexisting microcracks with dimensions which scale with grain size, we derive an expression for the critical pressure which depends on the porosity, grain size, and fracture toughness of the solid matrix. The theoretical prediction is in reasonable agreement with our experimental data as well as other data from soil and rock mechanics studies for which the critical pressures range over 3 orders of magni

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00341

年代:1990

数据来源: WILEY

摘要:

Anomalous high heat flow previously reported for the Great Plains is inconsistent with the tectonic setting and requires reexamination. Forty‐six new heat flow measurements, 12 revised heat flow values, and several hundred geothermal gradient measurements indicate extensive geothermal anomalies with heat flows ranging from 80 to 140 mW m−2in the northern and central Great Plains. Heat flow in the Great Plains outside the geothermally anomalous regions ranges from 40 − 60 mW m−2. The heat flow anomalies result from the thermal effects of regional groundwater flow where it moves upward either within a dipping aquifer or by cross‐formational flow through fractures. The gravitational driving force for the groundwater flow derives from the eastward sloping surface of the Great Plains, and the locations of the geothermal amonalies are determined by the structures of the aquifers and the crystalline basement rocks. The most widespread and largest‐amplitude geothermal anomaly occurs in southern South Dakota and northern Nebraska. Another large anomaly occurs on the eastern flank of the Denver Basin, and small anomalies occur on structures such as the Billings and Nesson anticlines in the Williston Basin. Previous reports of high heat flow in the Great Plains generally are supported by the results of this study. However, the source of anomalous heat is shown to be nontectonic, and theoretical arguments for normal continental heat flow in the Great Plains are supported. Another difference from the results of previous heat flow studies is that the thermal conductivities of shales in the Mesozoic strata in the Great Plains are about 40% lower than the conductivities that commonly have been used for shales. This observation and recent studies which have suggested lower thermal conductivities for shales in the Great Plains are the reasons for revision of some previous heat flow calculations. A significant result of revising some of the previous heat flow values is that the high heat flow zone that previously has been shown to extend through North Dakota into Canada is a region of normal continenta

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00353

年代:1990

数据来源: WILEY

摘要:

Geosat geoid undulations over four Pacific fracture zones have been analyzed. After correcting for the isostatic thermal edge effect, the amplitudes of the residuals are shown to be proportional to the age offset. The shape of the residuals seems to broaden with increasing age. Both geoid anomalies and available ship bathymetry data suggest that slip must sometimes occur on the main fracture zone or secondary faults. Existing models for flexure at fracture zones cannot explain the observed anomalies. A combination model accounting for slip and including flexure from thermal stresses and differential subsidence is presented. Our model accounts for lateral variations in flexural rigidity from brittle and ductile yielding due to both thermal and flexural stresses and explains both the amplitudes and the shape of the anomalies along each fracture zone. The best fitting models have mechanical plate thicknesses that are described by the depth to the 600°–700°C isothe

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00375

年代:1990

数据来源: WILEY

摘要:

The recognition and dating of corals that have been killed by tectonic uplift allow us to date paleoseismic uplifts in the Vanuatu island arc. We recognize corals that record paleouplifts by their similarity to those known to have died during contemporary sudden uplifts and date them (1) by counting annual coral growth bands (only if part of the coral is alive at the time of collection) or (2) by newly developed techniques for obtaining230Th ages by mass spectrometry. The mass spectrometric method produces isotopic ages with precisions of ±3 to ±9 years (2σ) in the 0–1000 years B.P. time range. The230Th ages in this time range appear to be accurate. Samples whose ages are known by counting coral growth bands give230Th ages that are indistinguishable from their growth band ages. By dividing the average increment of uplift for the latest Holocene uplifts by the mean Holocene uplift rate, we can estimate average seismic uplift recurrence intervals for the past 6000 years. The results for each of four central Vanuatu arc segments are (1) North Santo emerged 1.2 m in 1866 A.D. and 0.6 m 107 years later in 1973 A.D. The average coseismic uplift of 0.9 m and mean Holocene uplift rate of 4.3 mm yr−1suggest a longer recurrence interval of 212 years. (2) South Santo emerged 0.29 m in 1946 and 0.26 m 19 years later in 1965, including the related 1971 event. Here the mean Holocene uplift rate is 5.5 mm yr−1. The uplift data suggest a longer average recurrence interval of about 51 years. (3) North Malekula emerged 1.23 m near 1729 A. D. and 1.05 m 236 years later in 1965. The mean Holocene uplift rate of 2.7 mm yr−1and mean coseismic uplift of 1.14 m for dated events suggest a longer recurrence interval of 422 years. (4) Part of southernmost Malekula has uplifted continuously or episodically by about 0.35 m from about 1957 until at least mid‐1983 A.D. The maximum uplift of 2.7 mm yr−1occurs near a nest of small earthquakes. Both the earthquakes and rapid uplift suggest that interplate slip beneath south Malekula may be continuous, rather than episodic. Episodes of 0.35 m uplift would have to recur every 130 years to maintain the 2.7 mm yr−1uplift rate. In contrast, we find no evidence of interseismic vertical movements for the other three blocks. The most reasonable interpretation of these results is that the seismic recurrence intervals and processes for accommodation of slip are quite different on adjacent arc segments. We have used the most widely accepted momentmagnitude relationship to evaluate the accumulated seismic slip caused by large earthquakes occurring since 1920. In all four arc segments this analysis suggests that the seismically radiated moments account for less than one‐third to one‐tenth of the slip associated with plate convergence. The similarity between the paleoseismic record of uplifts and the contemporary record of coseismic uplifts suggests that this analysis can be generalized to times before 1920. For the northern three segments of central Vanuatu, aseismic slip probably occurs in the same years as large earthquakes because the contemporary coral record records uplifts only in years having large historic earthquakes. This suggests that aseismic slip is not continuous and does not occur at rates which vary slowly over the course of the earthquake cycle. The south Santo segment may have the highest proportion of seismic slip because the mean recurrence interval of 51 years is shortest and the mean Holocene uplift rate of at least 5.5 m

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00393

年代:1990

数据来源: WILEY

摘要:

A linear perturbation analysis is made on convective instability of the oceanic lithosphere to understand the occurrence of subduction on the Earth. The oceanic lithosphere is modeled by the upper part of top thermal boundary layer in a convecting Newtonian temperature‐dependent viscosity fluid. Thickening of oceanic lithosphere with its age due to cooling at the surface is modeled by thickening of the thermal boundary layer with time; its thickening rateL˙is a measure of the rate with which temperature in the thermal boundary layer decreases with time and hence is a measure of the rate with which viscosity in the thermal boundary layer increases with time. The linear perturbation analysis shows the possible excitation of plural eigenmodes to give rise to an instability in the thermal boundary layer depending on a threshold valueRcof the local Rayleigh numberRof the thermal boundary layer. Further,Rcis shown to be a linearly increasing function ofL˙, which is treated as a free parameter. From estimates ofRandL˙appropriate for the major oceanic lithosphere (denoted asRobsandL˙obs, respectively), I find thatRc(L˙= 0)

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00409

年代:1990

数据来源: WILEY