摘要:

In 1977 a wide‐aperture seismic network of land and ocean bottom stations (OBS) was operated for 6 weeks in the southern New Hebrides island arc. Data on the spatial distribution and mechanisms of small events recorded by this local network were integrated with worldwide observations of moderate to large size New Hebrides earthquakes of the past 17 years to study the structure and deformational processes in the New Hebrides subduction zone. Surprisingly, small differences (less than about 10 km) were found between the locations of shallow‐ and intermediate‐depth earthquakes as located by the temporary local network and by the International Seismological Centre using worldwide stations. Analysis of thePwave travel time data from the OBS station closest to the trench shows limited evidence for a high‐velocity zone (about 5% higher than the surrounding mantle) associated with the descending plate. The thrust zone between the descending and upper plates is defined mainly by the spatial distribution and focal mechanisms of moderate to large size events and their aftershocks, but it is not very well defined by the spatial distribution of small size events located in this study. Shallow seismic activity located beneath the trench and beneath the interplate thrust zone indicates a lower limit of about 40 km for the thickness of the seismically active part of the descending plate beneath the thrust zone. Focal mechanisms of moderate to large size earthquakes located within the upper plate and geological observations on the islands suggest that the upper plate in the region of Erromango and Tanna is divided into a series of blocks that are differentially uplifted along mainly NW‐SE striking faults. During the OBS experiment several shallow events were well located beneath the Coriolis trough, a riftlike feature located to the east of the volcanic arc. Well‐determined depths of these events are between 11 and 22 km. The well‐located intermediate‐depth events define a 20‐km‐thick Benioff zone that has a dip of about 70°. The spatial distribution of events within the descending slab is very strongly clustered in a persistent pattern that is seen in both the short‐term sample of small events determined by the local network and the long‐term sample of locations based on worldwide data. Several focal mechanisms of moderate to large size intermediate‐depth events show components of lateral extension along the strike of the arc and some show components of lateral compression, both of which could be interpreted as the result of a lateral bending of the descending plate in the part of the southern New Hebrides arc where the trench begins t

ISSN:0148-0227    

DOI:10.1029/JB086iB07p05905

年代:1981

数据来源: WILEY

摘要:

The techniques of modeling upper mantle structure by matching long‐period waveforms with synthetic seismograms have been applied to observations from the tectonically stable part of North America and from Europe. The consistent differences, which can be resolved by the long‐period data between Europe and North America, can be interpreted in terms of variations in the crust, lid, and low‐velocity zone. At epicentral ranges less than 15° the effects of shallow lateral variations are strong and body wave propagation is regionally dependent. Between ranges of 15° and 20°, regional effects are still observed, but they can be explained in terms of variations above 250 km. Beyond 20°, wave propagation appears to be stable and independent of region. Most of the observed long‐periodPwaves from 20° to 30° are consistent with a single model. This indicates that the relative depths and sizes of the major discontinuities do not vary substantially. A comparison of the upper mantle models of this study with those of other studies indicates that the shape of thePvelocity profile is fairly uniform through the transition region (300–700km) though there may be differences in the absolute depth to the discontinuities. These differences could, however, be the result of systematic errors in t

ISSN:0148-0227    

DOI:10.1029/JB086iB07p05926

年代:1981

数据来源: WILEY

摘要:

We combine observations of group and phase velocity dispersion of Rayleigh waves, of the waveform of a long‐period PLphase, of Pnand Snvelocities from unreversed refraction profiles using earthquakes, and of teleseismic S‐P travel time residuals to place bounds on the seismic wave velocity structure of the crust and upper mantle under Tibet. From surface wave measurements alone, the Tibetan crustal thickness can be from 55 to 85 km, with corresponding uppermost mantle shear wave velocities of about 4.4 to 4.9 km/s, respectively. The Pnand Snvelocities were determined to be 8.12±0.06 and 4.8±0.1 km/s, respectively, using travel time data at Lhasa from earthquakes in and on the margins of Tibet. Combining these results, the crustal thickness is most likely to be between 65–80 km with an average shear wave velocity in the upper crust less than 3.5 km/s. A synthesis of one PLwaveform does not provide an additional constraint on the velocity structure but is compatible with the range of models given above. In contrast to observations obtained for eight earthquakes in the Himalaya, measurements of both teleseismic S and P wave arrival times for nine earthquakes within Tibet show unusually large intervals between P and S compared with the Jeffreys‐Bullen Tables. Thus the Pnand Snvelocities apparently do not reflect high velocities in the mantle to a great depth beneath Tibet. From the dependence of the seismic velocities of olivine on pressure and temperature and from the similarity of the measured Pnand Snvelocities beneath Tibet and beneath shields and platforms, the velocities at the Moho beneath Tibet are compatible with the temperature being 250°–300° higher than beneath shields and platforms, i.e., 750°C if the temperature beneath the platforms is close to 500°C. Such a temperature could reach or exceed the solidus of the lower crust. Simple one‐dimensional heat conduction calculations suggest that the volcanic activity could be explained by the recovery of the geotherm maintained by a mantle heat flux of about 0.9 HFU at the base of the crust. If the distribution of radioactive heat production elements were not concentrated at the top of the crust, radioactive heating could also contribute significantly to the recovery of the geotherm and thus lower the required mantle heat flow. Thus the idea of a thickened crust in response to horizontal shortening is compatible both with these data and with these calculations.Appendices are available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐004; $1.00. Payment m

ISSN:0148-0227    

DOI:10.1029/JB086iB07p05937

年代:1981

数据来源: WILEY

摘要:

The moment tensor inversion method for long‐period Rayleigh waves is discussed from the point of view of event‐depth resolution. In this method, depth is usually determined by finding the minima in appropriate least squares residuals versus depth curves. We show that depth cannot be resolved if the data are sampled only at one frequency and a choice of frequencies spanning a wide range of values yields better resolution. We also show that the variance reduction at the minimum depends strongly on biases in the data, provided they are not simple π‐periodic functions in azimuth from the source. Since such biases are introduced, in particular, when the propagation corrections are inaccurate, this provides a means by which to study regional trends of phase velocities. This method is then applied to obtain an estimate of lateral heterogeneity in Central Asia. Starting with a reference point in the Pamir mountains and a set of accurately known phase velocities on paths to Eurasian WWSSN stations, we study the Rayleigh waves at these stations, in the period range 30 to 90 s, for eight events located in or near the Tibet Plateau, at stances from the reference point, heterogeneity in phase velocity is thus found to be about 1000 to 1500 km in this period range. Constraining the source parameters of the most distant events by body wave modeling, we determine new phase velocities on paths to WWSSN stations from the center of the Tibet Plateau. The variation of phase velocity with source location reaches 1 to 2%, and the low velocity within Tibet clearly has a dominant effect. We then discuss the regionalization of phase velocities as a possible way to determine a calibration curve for a given source‐to‐receiver path. We find that although the regionalized phase velocities in Eurasia are stable to first order, they do not seem accurate enough to be used for moment tensor inversions with arbitrary source locations. This may be due to non ray theoretical effects causing the breakdown of phase velocity additivity when higher order accuracy is needed. On the other hand, reference phase velocities on source‐receiver paths can be obtained reasonably fast for events large enough so that body wave modeling is possible to constrain depth and source mechanism. We demonstrate that these phase‐velocity curves are accurate enough for the calibration of propagation effects for another event located as much as 400 to 500 km away. This provides an encouraging approach to study both structure and source mechanisms of smaller events within a highly heterogeneous continental region. The only constraint is to find several events with the right magnitude of both long‐period body waves a

ISSN:0148-0227    

DOI:10.1029/JB086iB07p05963

年代:1981

数据来源: WILEY

摘要:

We present a study of the motion of the ground in the near field of a fluid‐driven tensile crack embedded in a layered half space. The source that we consider is the jerky opening of a channel connecting two fluid‐filled cracks, and the cause of this opening is the excess pressure of fluid in one of the cracks. We make a complete representation of the three components of ground motion in the space, time, and frequency domains and analyze the effects of fluid compressibility, source depth, and medium structure on the ground response. The calculations show the presence of a dominant frequency of motion which depends not only on the source geometry and bulk modulus of the fluid but also on medium characteristics, receiver position, and the component of motion being considered. Using this source model, we view an episode of volcanic tremor as a continuous sequence produced by numerous jerky openings of channels occurring randomly in time along a chain of cracks. Our results are applied to the October 5–6, 1963, east rift eruption of Kilauea volcano, Hawaii, and found to be compatible with available seismic data, suggesting that magma is transported through an ensemble of cracks with the area of 1 × 1 km, each pair of cracks constituting an individual dike segment which opens in discrete increments at a rate of 1/s. We obtain the following parameters for each dike‐segment opening: increase in cavity volume, 40 m3; stress drop, 0.004 bar; seismic moment, 1019dyn cm; and force applied by the fluid to open the dike, 1014dyn. The total moment integrated over the entire duration of tremor for the eruption is 0.5 · 1024dyn cm, roughly equivalent to a single magnitudeM= 5 earthquake. The seismic source parameters and other field observations offer constraints on the process of mass transport and are compatible with the excess pressure and viscosity of magma of about 40 bars and 102P, res

ISSN:0148-0227    

DOI:10.1029/JB086iB07p05985

年代:1981

数据来源: WILEY

摘要:

Frictional forces were measured during sliding between saw cut cylinders of Barre granite deformed in a servo‐controlled triaxial loading machine. Two different effects occur, depending on the type of loading. When steady shortening of a stably sliding sample is halted suddenly, slip continues at a diminishing rate, which depends on the logarithm of the initial sliding velocity. Thus, the frictional strength of the surface is decreasing with time. When shortening at a constant rate resumes after the holding period, however, the frictional resistance temporarily rises to a peak proportional to the logarithm of time of no driving, showing a well‐known increase in friction with time. The subsequent decrease in resistance from this peak is continuous. The observed frictional resistance of a surface thus results from the interaction of several processes and is not a constant, in general, but depends on the immediate and preceding deformation. When shortening is resumed after a period of holding, slip rate across the friction surface temporarily accelerates above the driving rate. The slip rate can then either decrease to the driving rate and remain stable or accelerate further and lead to stick slip motion, at stress levels below the peak value. There is no apparent difference in the frictional resistance, until the unloading rate of the loading machine is exceeded, between slip episodes which settle stably to the constant driving rate and those which become unstable and result in stick slip. The stick slip failure criterion is not a peak stress but rather loss of strength more rapidly that the unloading rate of the loading machine. The evidence that accelerating motion before stick slip is due to the time variable strength of rock suggests that accelerating slip will occur before earthquakes. Unfortunately for easy observation, experiments indicate that slip need not be uniform over a fault surface. Also to be established is whether slip will be sufficiently large in the earth to be detectable. Nevertheless, a plausible mechanism for generating observable phenomena prior to earthquakes is suggested by the laboratory observati

ISSN:0148-0227    

DOI:10.1029/JB086iB07p06017

年代:1981

数据来源: WILEY

摘要:

A reversed seismic refraction profile extending 260 km between Globe, Arizona and Tyrone, New Mexico has been recorded using quarry blasts from open pit copper mines as energy sources. Interpretation of these data suggest a 28‐km‐thick crust for the Basin and Range in east‐central Arizona and a 32‐km‐thick crust for the Transition Zone in eastern Arizona and western New Mexico. Delays in Pn arrivals have been interpreted as evidence for approximately 4 km of abrupt crustal thickening near Morenci, Arizona. The area of abrupt Moho offset corresponds to rapid changes in tectonic style, late Quaternary faulting, Quaternary and late Tertiary volcanism, high heat flow and evidence for partial melting of the lower crust. Uniformity of layer thicknesses along the profile can be interpreted as upward displacement of the Basin and Range relative to the Transition Zone. The above evidence suggests that the Transition Zone of eastern Arizona and western New Mexico may be experiencing active tectonic rea

ISSN:0148-0227    

DOI:10.1029/JB086iB07p06029

年代:1981

数据来源: WILEY

摘要:

Using the concepts of fracture mechanics, we develop a theory of the earthquake mechanism which includes the phenomenon of subcritical crack growth. The theory specifically predicts the following phenomena: slow earthquakes, multiple events, delayed multiple events (doublets), postseismic rupture growth and afterslip, foreshocks, and aftershocks. The theory also predicts that there must be a nucleation stage prior to an earthquake and suggests a physical mechanism by which one earthquake may ‘trigger’ another. These predictions are obtained by combining two fundamental concepts. The first is that, and the second that, wherekis the stress intensity factor, Δτ is stress drop,Xis rupture length,is rupture velocity,Cis a geometrical factor, andK0,V0, andnare material constants. The first is a fundamental result of fracture mechanics; the second describes stress corrosion cracking, a well‐established physical process that results in subcritical crack growth. We investigate in detail two phenomena of special interest and which are not predicted by ordinary fracture mechanics: nucleation and delayed multiple events. In the first case we find that all earthquakes must be preceded by quasi‐static slip over a portion of their rupture surfaces, but it may be difficult to detect in practice. In the second case we studied two pairs of delayed multiple events that were separated by the same ‘barrier’ in order to calculaten. We find that the stress corrosion

ISSN:0148-0227    

DOI:10.1029/JB086iB07p06039

年代:1981

数据来源: WILEY

摘要:

It is shown that the shear modulus and density profiles of a layered, isotropic elastic earth can be uniquely inferred from the displacement of the surface due to an impulsive SH line source. An inversion algorithm is presented which constructs these profiles from the surface data. Two illustrative examples are given in which the shear modulus and density profiles are reconstructed analytically from an analytic surface data.

ISSN:0148-0227    

DOI:10.1029/JB086iB07p06052

年代:1981

数据来源: WILEY

摘要:

The analysis of travel time delays from 94 teleseisms recorded at The Geysers‐Clear Lake, California, region reveals a broad area of delay centered at Mount Hannah and encompassing The Geysers geothermal field. A three‐dimensional velocity inversion of the delays indicates that the source of the delays is a low‐velocity body largely within the top 30 km of the lithosphere. Velocity decreases of as much as 20% are observed in the top 15 km near Mount Hannah and diminish with depth. No significant velocity fluctuations are observed below 60 km. The block model of velocity perturbations resulting from the teleseismic residual inversion is used to predict the observed Bouguer gravity field by solving for the relation between velocity and density perturbation. Comparison between the predicted gravity and observed gravity indicates that the same body responsible for the teleseismic delays accounts for the long‐wave‐length 30‐mGal Bouguer gravity minima at Mount Hannah. The results of the velocity‐density modeling suggest that the material beneath Mount Hannah and The Geysers may be partially molten and somewhat more silicic than the surrounding medium.Supplemental table is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐006; $1.00. Payment must

ISSN:0148-0227    

DOI:10.1029/JB086iB07p06057

年代:1981

数据来源: WILEY