摘要:

The sound speed of a two‐phase fluid, such as a magma‐gas, water‐air, or water‐steam mixture, is dramatically different from the sound speed of either pure component. In numerous geologic situations the sound speed of such two‐phase systems may be of interest: in the search for magma reservoirs, in seismic exploration of geothermal areas, in prediction ofPwave velocity decreases prior to earthquakes, and in inversion of crustal and upper mantle seismic records. Probably most dramatically, fluid flow characteristics during eruptions of volcanoes and geysers are strongly dependent on the sound speed of erupting two‐phase (or multiphase) fluids. In this paper the sound speeds of water, air, steam, water‐air mixtures, and water‐steam mixtures are calculated. It is demonstrated that sound speeds calculated from classical acoustic and fluid dynamics analyses agree with results obtained from finite amplitude ‘vaporization wave’ theory. To the extent that air and steam are represented as perfect gases with an adiabatic exponent γ, independent of temperature, their sound speeds vary in a simple manner directly with the square root of the absolute temperature. The sound speed of pure liquid water is a complex function of pressure and temperature and is given here to 8 kbar, 900°C. In pure water at all pressures the sound speed attains a maximum value near 100°C and decreases at higher temperatures; at high pressures the decrease is continuous, but at pressures below 1 kbar the sound speed reaches a minimum value in the vicinity of 500°–600°C, above which it again increases. The sound speed of a water‐air mixture depends on the pressure, the void or mass fraction of air, the frequency of the sound wave, and, if surface tension effects are included, on bubble radius. The admixture of small volume fractions of air causes a dramatic lowering of the sound speed by nearly 3 orders of magnitude. The sound speeds of the pure liquid and gas end‐members are nearly independent of pressure, but the sound speed of a mixture is highly dependent on pressure. Calculated values for water‐air mixtures are in good agreement with measured values. The sound speed in a single‐component two‐phase system, such as a water‐steam mixture, depends on whether or not equilibrium between the phases on the saturation curve is maintained. Heat and mass transfer which occur when equilibrium is maintained cause the sound speed to be much lower than under non‐equilibrium conditions in which heat and mass transfer are absent. The sound speed in a wate

ISSN:0148-0227    

DOI:10.1029/JB082i020p02895

年代:1977

数据来源: WILEY

摘要:

We present a study of the active tectonics of China based on an interpretation of Landsat (satellite) imagery and supplemented with seismic data. Several important fault systems can be identified, and most are located in regions of high historical seismicity. We deduce the type and sense of faulting from adjacent features seen on these photos, from fault plane solutions of earthquakes, and from existing field reports. In central China the three major east‐west trending fault systems are left lateral strike slip faults. Movement on these faults appears to displace much of China eastward, out of the way of the converging Indian and Eurasian subcontinents. Moreover, these large eastward displacements are a key to the understanding of the tectonics both of southern China, where normal faulting and right lateral strike slip faulting occur in Yunnan in contrast with thrust faulting in Szechwan, and of northeastern China, where extensional tectonics and basaltic volcanism dominate. Hence we relate all of the recent tectonics of China to the convergence of India and Eurasia during the Cenozoic at a rate of about 5 cm/year. The continental lithosphere of Asia appears to behave like a rigid plastic medium indented by India. In this context the large strike slip faults are analogous to slip lines in the indented plastic materia

ISSN:0148-0227    

DOI:10.1029/JB082i020p02905

年代:1977

数据来源: WILEY

摘要:

Very high frequencies (5–10 Hz) are recorded at Garm, Tadzhikistan (Δ∼200 km), near Toktogul, Kirgizia (Δ∼600 km) and at other close stations from intermediate depth earthquakes in the Pamir‐Hindu Kush region. The seismic phase, Sn, is recorded with large amplitudes and high frequencies at stations in Pakistan and India from intermediate depth earthquakes. Such high frequencies require either extremely high average values of Q(several thousand) or very high stress drops of earthquakes (kilobars) or possibly both. Regardless of the stress drops, we infer that Q ≳ 1000 along paths through a portion of the mantle where Q is usually low. Thus these data indicate a discontinuity, or marked thinning, of the asthenosphere. Even if Q were infinite, the spectral content of the signals at Garm, in addition, could also be interpreted as evidence for relatively high stress drops for the intermediate depth earthquakes. If Q is equal to 3000, the highest calculated stress drops are for events with depths between about 50 and l80 km and range from several tens to several hundred bars.Tables 1 to 3 are available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document JTT‐OOl; $1.00. Payment must ac

ISSN:0148-0227    

DOI:10.1029/JB082i020p02931

年代:1977

数据来源: WILEY

摘要:

Seismic moments for 12 major earthquakes (M ≥ 7.6) in central Asia from 1911 to 1967 were calculated from long‐period Rayleigh and Love wave spectral densities. With fault lengths estimated from geological field observations of surface faulting, intensity distributions, or master event relocations of aftershocks, the calculated moments place bounds on the average slip and fault widths. The following table summarizes the calculated moments, estimated fault lengths, and inferred possible average displaceme

ISSN:0148-0227    

DOI:10.1029/JB082i020p02945

年代:1977

数据来源: WILEY

摘要:

Amplitudes ofPwaves recorded by World‐Wide Standard Seismograph Network seismographs from deep focus earthquakes around the world were analyzed to determine long‐ and short‐period amplitude‐distance curves. Data from deep focus earthquakes were chosen to remove the effect of near source heterogeneity in the upper mantle. Effects of near receiver heterogeneity have been eliminated by estimating the station anomaly terms. The amplitudes were corrected for source radiation pattern also. The short‐period amplitude‐distance curve has a low (‘minimum’) at a distance of 60° and a peak at a distance of 75°. The long‐period amplitude‐distance curve is smoother than the short‐period curve. Both curves show a rapid decrease of amplitudes beyond a distance of 85° due to the effects of the core. On visual inspection the short‐period amplitude‐distance curve is found to be similar to that of Carpenter et al. (1967) and the long‐period one to that of Nuttli (1972), both of which are derived from explosion data. Ray theoretical amplitudes using theQmodel of Archambeau et al. (1969) fit our amplitude data well, a result suggesting that averageQin the lower mantle is greater than about 1000. Body wave magnitudes determined separately from short‐period and long‐period amplitude‐distance curves approximately agree with each other, a finding suggesting that at the source, thePwave amplitude/period ratio remains constant for deep focus earthquakes at least for the periods of l s and 10s and for the range of m

ISSN:0148-0227    

DOI:10.1029/JB082i020p02971

年代:1977

数据来源: WILEY

摘要:

The conventional magnitude scaleMsuffers saturation when the rupture dimension of the earthquake exceeds the wavelength of the seismic waves used for the magnitude determination (usually 5–50 km). This saturation leads to an inaccurate estimate of energy released in great earthquakes. To circumvent this problem the strain energy dropW(difference in strain energy before and after an earthquake) in great earthquakes is estimated from the seismic momentM0. If the stress drop Δσ is complete,W=W0= (Δσ/2μ)M0∼M0/(2×104), where μ is the rigidity; if it is partial,W0gives the minimum estimate of the strain energy drop. Furthermore, if Orowan's condition, i.e., that frictional stress equal final stress, is met,W0represents the seismic wave energy. A new magnitude scaleMwis defined in terms ofW0through the standard energy‐magnitude relation logW0= 1.5Mw+ 11.8.Mwis as large as 9.5 for the 1960 Chilean earthquake and connects smoothly toMs(surface wave magnitude) for earthquakes with a rupture dimension of about 100 km or less. TheMwscale does not suffer saturation and is a more adequate magnitude scale for great earthquakes. The seismic energy release curve defined byW0is entirely different from that previously estimated fromMs. During the 15‐year period from 1950 to 1965 the annual average ofW0is more than 1 order of magnitude larger than that during the periods from 1920 to 1950 and from 1965 to 1976. The temporal variation of the amplitude of the Chandler wobble correlates very well with the variation ofW0, with a slight indication of the former preceding the latter. In contrast, the numberNof moderate to large earthquakes increased very sharply as the Chandler wobble amplitude increased but decreased very sharply during the period from 1945 to 1965, whenW0was largest. One possible explanation for these correlations is that the increase in the wobble amplitude triggers worldwide seismic activity and accelerates plate motion which eventually leads to great decoupling earthquakes. This decoupling causes the decline of moderate to large earthquake activity. Changes in the rotation rate of the earth may be an important element in

ISSN:0148-0227    

DOI:10.1029/JB082i020p02981

年代:1977

数据来源: WILEY

摘要:

A paleomagrtetic study of sediments from Deep‐Sea Drilling Project (DSDP) sites 290, 292, and 294 suggests that the West Philippine Basin originated between 5° and 10°S. Results for site 292 are particularly consistent; a group of 20 closely spaced samples at the sediment‐basalt interface has a mean absolute paleolatitude and 95% confidence interval of 4.9°±2.0°. An equatorial crossing is implied by the trend of paleoinclinations as a function of age for 73 additional samples from site 292 and concurrent results from sites 290 and 294. Origin of the West Philippine Basin in southern latitudes is also consistent with the phase shifting of marine magnetic anomalies, which must be inverted to be matched with a set of worldwide Eocene reversals. Two well‐defined paleomagnetic pole positions are determined if the low amplitude of these east‐west anomalies, in comparison to similarly oriented anomalies in the Pacific and Indian oceans, is due to a rotation of this plate. One of these is consistent with paleopoles calculated from the ‘absolute’ motion of the Pacific and directions of relative motion between the Philippine and the Pacific plate. This suggests a clockwise rotation of 60° between the Philippine plate and the magnetic pole and is similar to results from the study of Miocene rocks on Guam. This may imply that both Guam and the West Philippine Basin have undergone the same past plate motion and that the rotation of Guam is not due to a simple bending of

ISSN:0148-0227    

DOI:10.1029/JB082i020p02989

年代:1977

数据来源: WILEY

摘要:

New measurements of interval compressional wave velocities were made in the first sediment layer using the sonobuoy technique during two expeditions in the Bay of Bengal, in the Andaman Sea, and over the Nicobar Fan and Sunda Trench. Sediment interval velocities from these areas were added to those previously reported, and revised diagrams and regression equations of instantaneous and mean velocity versus one‐way travel time are furnished for four areas of the Bengal Fan, and for the Andaman Basin, Nicobar Fan, and Sunda Trench. The velocity gradients directly below the sea floor were used to separate the Bengal Fan into four geoacoustic provinces. In the north and west the velocity gradients are 0.86 and 1.28 s−1, respectively, whereas in the central part of the fan the gradient is 1.87 s−1. These variations indicate lesser increases of velocity with depth in the sea floor in the north and west, and they are probably due to more rapid deposition, less consolidation, and less lithification near the riverine source areas of the sediments. The near‐surface velocity gradients in the other areas are the Andaman Basin, 1.53 s−1; the Nicobar Fan, 1.63 s−1; and the Sunda Trench, 1.41 s−1. The linear velocity gradients (from the sediment surface to a given travel time) in 17 areas of the Indian Ocean, Pacific area, Atlantic Ocean, and Gulf of Mexico were averaged at each 0.1 s from 0 to 0.5 s of one‐way travel time. These averaged gradients ranged from 1.32 s−1att= 0 to 0.76 s−1att= 0.5 s. The regression equation for the velocity gradient a, in s−1, as a function of one‐way travel timet, in seconds, isa= 1.316–1.117t(for use fromt= 0 to 0.5 s). These average velocity gradients can be used with sediment surface velocities and one‐way travel times (measured from reflection records) to compute sediment layer thicknesses in areas of turbidites lacking interval velocity measurements

ISSN:0148-0227    

DOI:10.1029/JB082i020p03003

年代:1977

数据来源: WILEY

摘要:

The concept of pressure solution, that a stressed portion of a solid is more soluble than an unstressed portion, is not new, but the relationship between stress and solution has not been experimentally established. The solution rate was examined around a fluid‐filled circular hole in a slab loaded by σ0perpendicular to the axis of the hole. The known circumferential stress around the hole varied from a tension of −σ0to a compression of +3σ0. A variety of geologic materials, including single‐crystal halite, limestone, dolomitic limestone, marble, quartzite, and novaculite, was used. Although the details of the solution kinetics for these materials differed considerably, and the pore fluid, sample material, axial load, and temperature differed from experiment to experiment, the same result was obtained whenever the solution rate was limited by the dissociation of the mineral at the solid‐liquid interface. The shape of the hole after solution occurred was approximately an ellipse and differed markedly from the shape that it would have attained if the rate of solution were proportional to strain energy density. The solution rate at a point seems to be directly proportional to lo

ISSN:0148-0227    

DOI:10.1029/JB082i020p03013

年代:1977

数据来源: WILEY

摘要:

The thermoluminescence (TL) properties of plagioclase separates from 10 independently dated alkalic basalts from the Hawaiian Islands were studied for the purpose of developing a TL dating method for young volcanic rocks. The samples range in composition from alkalic olivine basalt to trachyte and in age from 4500 years to 3.3 m.y. The TL process in basaltic plagioclase appears to involve the production of structural defects by radioactive decay particles, as well as the filling of traps associated with these defects with excited electrons. The ratios of natural to artificial TL for the samples up to 200,000 years old increase at a uniform exponential rate; the ratios for samples greater than 1 m.y. old appear to be attenuated, probably due to saturation effects. The equation for the best fit line through a plot of the TL ratios versus known age for the younger samples constitutes a TL age equation suitable for assigning TL ages to other samples of similar origin and chemical composition. The probable age range of the method is about 2500 to at least 250,000 years. The precision of the ages is approximately ±5% for samples older than 50,000 years; concordance with K‐Ar ages is tentatively estimated to be approximately ±

ISSN:0148-0227    

DOI:10.1029/JB082i020p03023

年代:1977

数据来源: WILEY