摘要:

The low Reynolds number dynamics of a thin layer of fluid bounded below by a flat horizontal boundary and moving buoyantly through a fluid of another viscosity and density is observed by means of model experiments and is described theoretically. Three distinct stages of growth were observed. The first stage is described by a linearized Rayleigh‐Taylor instability, for which previous literature, present theory, and experiments exhibit close agreement. In this stage, disturbances of one specific wave number are found to grow most rapidly. In the second stage, distortion of the interface is large enough to invalidate the linearized analysis. It is found experimentally that the fluid moves upward as circular columns surrounded by relatively broad regions of descending material. A theory is advanced that attributes an accelerated growth to a structure of this kind through a resonant triad interaction. In the third stage, fully matured structures are formed. If the upwelling material has greater viscosity than the surrounding material, the structure is a long vertical column with gradually decreasing diameter; if the upwelling material has less viscosity than the surrounding material, the structure develops a rim syncline and a pronounced overhang and eventually ascends as a spherical pocket of fluid fed by a pipe. Dynamic explanations for these features are advanced, and some possible implications for geological and geophysical processes are discusse

ISSN:0148-0227    

DOI:10.1029/JB080i005p00705

年代:1975

数据来源: WILEY

摘要:

Renewed earthquake activity at Blue Mountain Lake (BML), New York, in July 1973 provided an excellent opportunity to monitor the travel time ratio ofStoPwaves (ts/tp) in real time and to test thets/tptechnique as a predictive tool. From a mean value of 1.73 on July 30, 1973,ts/tpdecreased to about 1.5 over the next 2–3 days. On August 1 a prediction was made that an earthquake of magnitude 2.5–3 would occur in a few days. Upper limits of the magnitude and the time of occurrence of the expected earthquake were inferred from the spatial extent of the seismic anomaly. As a result of the prediction an additional strong motion accelerograph (SMA) was installed in the source region. At 2310 UT on August 3,1973, a magnitude 2.6 earthquake occurred at BML and triggered two SMA's. In addition to the seismic activity a number of explosions were recorded from a variety of azimuths. The P wave arrivals from distant quarry blasts, refracted from a high‐velocity layer at 4 km beneath BML, showed late arrivals at five stations during the premonitory low ints/tp. ThePwave delays were maximum (0.13 s) in the hypocentral region of the earthquake and decreased away from it along two profiles. These results indicate that changes ints/tpare caused by changes in the material properties of the earthquake source region. The anomalous zone (region of lowPvelocity) for the August 3 earthquake (aftershock length 1 km, depth 1±0.1 km) was about 3–5 km in radius and was wholly or largely limited to the layer above the interface at 4‐km depth. In contrast to thePdelays observed from distant quarry blasts,PandSarrivals from local construction blasts (Δ<10 km) show no large premonitory changes in eitherPorStravel times. This observation suggests that either no significant velocity anomalies occurred in the upper 0.5 km of the BML region or that the dilatant cracks were predominantly horizontal, the result being strong velocity anisotropy. This conclusion is supported by data from seismic sources; events located at shallow depths show normal (1.75) to high (1.85)ts/tpvalues even when values as low as 1.5 are observed from deeper sources. This finding places constraints on the use of artificial sources to monitor changes in velocity, since the effects of anisotropy may have to be taken into account in addition to ensuring that ray paths to the recording stations penetrate the zone of anomalous velocity. A maximum likelihood method was used to invert data from individual small earthquakes to determinePandSvelocities in the anomalous zone. The results, which are consistent with the explosion data, indicate that the premonitory decreases inPandSvelocities were much more pronounced for sources at depths of 1–2 km than for those near the surface. The inferred low values ofPandSvelocities were, respectively, about 22 and 12% below normal. This study also shows thatts/tpinferred from a Wadati plot is a function of the distribution of stations relative to the anomalous zone. To optimize the use ofts/tpas a predictive tool, this study suggests that not only temporal but spatial variations of velocity anomalies should be monitored. This emphasizes the need for multistation coverage for reliable earthqua

ISSN:0148-0227    

DOI:10.1029/JB080i005p00718

年代:1975

数据来源: WILEY

摘要:

Detailed seismic monitoring of the Hebgen Lake‐western Yellowstone Park region of Wyoming and Montana has delineated a zone of earthquakes extending 80 km in a direction N80°W from the northwest edge of the Yellowstone calderas to the Madison Valley. The active zone includes the epicentral area of the 1959 Hebgen Lake earthquake (magnitude 7.1). Focal depths ranged from near surface to 15 km in the Hebgen Lake region, whereas near the caldera boundaries the maximum focal depths decreased to 5 km. The abrupt change to shallow focal depths over and near the calderas may be related to increased temperature and pore pressure sufficient to inhibit brittle fracture. Six composite and four single‐event fault plane solutions indicate north‐south regional extension. Three composite fault plane solutions for earthquakes along the northwest boundary of the Yellowstone calderas indicate northwest‐southeast crustal shortening, possibly from uplift on concentric fractures around the

ISSN:0148-0227    

DOI:10.1029/JB080i005p00733

年代:1975

数据来源: WILEY

摘要:

Two‐dimensional numerical models of steady state convection show that convection cells of aspect ratio as large as 8.6 are possible for variable viscosity convection in the upper mantle. Our models include the effects of variable viscosity, viscous dissipation, internal heating, heat flow through the bottom, and the adiabatic gradient. The large aspect ratio of the convection cells is primarily due to the large viscosity contrast between the lithosphere and the asthenosphere. It appears possible for multiple convection cells to occur in a low‐viscosity zone while the surface velocities give the appearance of a single cell. The details of the viscosity law relevant to mantle materials and conditions are presently uncertain but are of crucial importance; temperature, viscosity, and flow patterns are inextricably entwined. Convection decreases the overall temperature gradient; consequently, generally accepted temperatures for most of the mantle are too high. The controversies over plate‐mantle decoupling and passive versus active plates are probably due to oversimplifications that disregard hydrodynamic con

ISSN:0148-0227    

DOI:10.1029/JB080i005p00742

年代:1975

数据来源: WILEY

摘要:

Permeability and volumetric strain were measured under constant confining pressure and pore pressure as a function of increasing and decreasing differential stress. Permeability was found to increase appreciably during dilatancy. Our results have also shown that permeability and dilatant volume changes are not unique functions of differential stress and that permeability changes with differential stress are not uniquely dependent upon dilatant volume changes. Most significant, however, is that if dilatancy‐fluid diffusion occurs in situ, our results indicate that microcrack dilatancy is not a reasonable physical mechanism to account for such a phenomeno

ISSN:0148-0227    

DOI:10.1029/JB080i005p00752

年代:1975

数据来源: WILEY

摘要:

The vibrational spectra of spherical samples of two granites, a diabase, an optical glass, and a ball bearing steel were observed in the frequency range 11–200 kHz. The frequencies were reduced to give mean velocities of compressional and shear waves, and the widths of resonance curves to give estimates of damping coefficients (Q). Close agreement with calculated frequencies for an isotropic homogeneous sphere is found for the glass, with rms deviations of the order of a few parts in 10,000 and aQof about 6000, nearly independent of mode. For the diabase the rms deviation was about 3 parts per 1000, with aQof about 1000; for Barre granite the rms deviation was 3 parts per 1000, with aQof about 200; for Quincy granite the rms deviation was 14 parts per thousand, andQwas about 200. The steel ball showed an effect of hardening, with a decrease of velocity of a few parts per thousand as frequency increased from 30 to 200 kHz;Qwas notably dependent on mode of vibration, about 8000–9000 for the torsional modes and most fundamental spheroidal modes but several times as high for the purely radial mode and several overtones. The observations of damping in the steel are roughly consistent with a simple theory in which all damping is attributed to shear, with a single, frequency‐independent shearQ, whereas pure compression is und

ISSN:0148-0227    

DOI:10.1029/JB080i005p00756

年代:1975

数据来源: WILEY

摘要:

A seismic refraction technique for interpreting the subsurface shape and velocity distribution of an anomalous surface feature such as an impact crater is described. The method requires the existence of a relatively deep refracting horizon and combines data obtained from both standard shallow refraction spreads and distant offset shots by using the deep refractor as a source of initial arrivals. Results obtained from applying the technique to Meteor crater generally agree with the known structure of the crater deduced by other investigators and provide new data on an extensive fractured zone surrounding the crater. The breccia lens is computed to extend roughly 190 m below the crater floor, about 30 m less than the value deduced from early drilling data. Rocks around the crater are fractured as distant as 900 m from the rim crest and to a depth of at least 800 m beneath the crater floor.

ISSN:0148-0227    

DOI:10.1029/JB080i005p00765

年代:1975

数据来源: WILEY

摘要:

The residual gravity anomaly associated with Meteor Crater, Arizona, which attains a maximum negative amplitude of 0.6 mGal, is attributed to a breccia lens 225 m thick beneath the center of the apparent crater floor. The lens is in general radially symmetric with only minor irregularities, particularly in the southern half of the crater. No definitive evidence was found for a dense meteoritic body in the crater vicinity. The crater is surrounded by a negative gravity anomaly due to low‐density rim debris, uplifted and fractured bedrock, and possibly an underlying fractured zone extending about one crater radius out from the rim crest and dipping into the crater. A magnetic minimum with a maximum negative amplitude of approximately 20 γ is coincident with Meteor Crater. The most probable source of this anomaly is the alteration, by impact, of the rémanent magnetization of the underlying formations, particularly the Supai, within and beneath the breccia l

ISSN:0148-0227    

DOI:10.1029/JB080i005p00776

年代:1975

数据来源: WILEY

摘要:

The Washougal howardite is alleged to have struck the earth from a retrograde orbit, with a preatmospheric velocity of 55 ± 3 km/s. To check this report, we determined the ablation loss of this 0.2‐kg meteorite from the angular distribution of cosmic ray tracks, using a new method based on the model of Maurette et al. (1969). The preatmospheric mass was 660 ± 220 kg, which according to meteor theory implies a geocentric velocity of ≤35 km/s. This upper limit is well below the minimum value for a retrograde orbit, 48

ISSN:0148-0227    

DOI:10.1029/JB080i005p00789

年代:1975

数据来源: WILEY

摘要:

A subset of Pogo satellite magnetometer data has been formed that is suitable for analysis of crustal magnetic anomalies. Through the use of a thirteenth‐order field model fit to these data, magnetic residuals have been calculated over the world to latitude limits of ±50°. These residuals, averaged over 1° latitude‐longitude blocks, represent a detailed global magnetic anomaly map derived solely from satellite data. The occurrence of these anomalies on all individual satellite passes independent of local time and their decay as altitude increases imply a definite internal origin. Their wavelength structure and their correlation with known tectonic features further suggest that these anomalies are primarily of geologic origin and have their sources in the litho

ISSN:0148-0227    

DOI:10.1029/JB080i005p00794

年代:1975

数据来源: WILEY