ISSN:0094-8276    

DOI:10.1029/GL014i009p00889

年代:1987

数据来源: WILEY

摘要:

The hypothesis of S‐wave scattering by three‐dimensional heterogeneity in the lowermost mantle is shown to provide a plausible alternative to the spherically stratified models recently proposed by Lay and Helmberger to explain certain observed complexities in long period S‐wave data. A result of central importance is that, contrary to the conclusions of Lay and Helmberger and, most recently, Young and Lay, sharp increases of S velocity with depth are not required to explain their

ISSN:0094-8276    

DOI:10.1029/GL014i009p00891

年代:1987

数据来源: WILEY

摘要:

In a frequency domain analysis of long‐period seismic data, observations of free oscillation excitation amplitudes are required in addition to frequency shifts to constrain odd‐degree aspherical earth structure. To determine the feasibility of incorporating such data into large‐scale inversions, we have undertaken a preliminary analysis of the observed amplitudes of fundamental spheroidal mode multipletsoSlwith 26 ≤l≤ 43 using seismograms recorded by the IDA network. For many records, the amplitudes may be significantly better fit by allowing for a systematic shift of the theoretical spherical earth nodal lines either toward or away from the epicenter. In the great‐circular geometrical optics approximation, this spatial phase shift of the amplitude pattern can be asymptotically represented in terms of the minor arc integral of δωlocal— along the source‐receiver path. Results from common source‐receiver paths indicate consistent sampling of the structure underlying these paths. For a number of paths, the results differ significantly from the theoretical predictions of model M84A of Woodh

ISSN:0094-8276    

DOI:10.1029/GL014i009p00895

年代:1987

数据来源: WILEY

摘要:

We derive the single‐mode, mean‐field equations for spherical‐shell convection with variable viscosity and compressibility included. Core‐mantle boundary (CMB) topography produced by whole mantle convection is computed for various rheologies and different thermal boundary conditions at CMB. Results for long‐wavelength undulations show the extreme sensitivity of these signatures to rheological and equation of state parameters. In order to obtain excess ellipticity of about 1 km, the activation energy and volume in the lower mantle should be low and the temperatures at CMB around 4,500 K, in accord with recent experimental

ISSN:0094-8276    

DOI:10.1029/GL014i009p00899

年代:1987

数据来源: WILEY

摘要:

A procedure for the joint inversion of regional and teleseismic long‐period waveforms for source parameters is presented. Three test cases demonstrate that the inclusion of regional distance seismograms to a data set of teleseismic observations stabilizes the inversion process and adds valuable constraints to the source parameter estimates, particularly when the teleseismic Green's functions are inadequate. The added stability and constraints from regional waveforms can be attributed to the robust nature of regional distance Green's functions, as well as the added coverage of the focal sphere that regional data provide. Singular value decomposition is a useful inversion method that is well suited for this study because it provides a measure of the solution dependence on individual observation

ISSN:0094-8276    

DOI:10.1029/GL014i009p00903

年代:1987

数据来源: WILEY

摘要:

Paleomagnetic data from western Anatolia and from the island of Lesbos demonstrate significant counterclockwise, clockwise or null rotations of coherent blocks which occurred during the neotectonic period. These results suggest that the brittle carapace of the lithosphere may not everywhere follow the motion of the ductile lower part but that its movement arises from adjustment to the extensional tectonic regime.

ISSN:0094-8276    

DOI:10.1029/GL014i009p00907

年代:1987

数据来源: WILEY

摘要:

For the past 20 years, much effort has been directed to determining the present‐day relative motion of the Pacific and North American plates using two independent approaches. One uses geologic observations and geodetic measurements along the San Andreas Fault and other faults in the plate boundary zone. The other is based on plate motion models that incorporate spreading rates from marine magnetic anomalies, transform azimuths, and earthquake slip vectors. Geologic and geodetic studies find two principal elements of deformation: slip along the San Andreas of ∼34 mm/yr directed N41°W, and extension across the Basin and Range province of about 10 mm/yr directed N56°W. In contrast, plate motion studies find 56‐60 mm/yr directed N35°W. The discrepancy between these estimates, a vector of about 15 mm/yr oriented nearly due north, is often attributed to a combination of slip along faults parallel to the San Andreas and shortening normal to it. Here we revise the estimate of Pacific‐North America motion by analyzing marine magnetic profiles from the Gulf of California. Since 3 Ma, spreading has averaged 48 mm/yr, 10 mm/yr slower than estimated before, and consistent with the 49 mm/yr spreading predicted by a new global plate motion model derived without any data along the Pacific‐North America boundary. The discrepancy with geodetic and geologic estimates is thus reduced to only 5 mm/yr parallel to the San Andreas, 60% less than estimated before, and 7 mm/yr of shortening across the San Andreas, similar to prior estimates. These results suggest that strike‐slip motion on faults west of the San Andreas is less than thought before, a conclusion consistent with geodetic, seismological, and other geologi

ISSN:0094-8276    

DOI:10.1029/GL014i009p00911

年代:1987

数据来源: WILEY

摘要:

Estimates of the Rayleigh number are necessary for determining the thermal and dynamic states of convecting magma. Calculation of a temperature based Rayleigh number,RaT, is complicated by the thermal coupling that exists between an intrusion and its host. However, the typically greater thermal resistance of the host regime compared to that of convecting magma can allow the host to control heat flux from a magma chamber in a manner somewhat independent of the state of convection in the chamber. In a system where heat flux is controlled rather than temperature, a heat flux based Rayleigh number,RaF, is a more natural parameter to employ, although it has not been used in most magmatic convection models. While the insulating properties of the host regime tend to reduce the value of the thermal Rayleigh number, whether temperature or flux based, this value can be very large compared to any critical value of the Rayleigh number that might exist. The quantityRaFis used to obtain useful relationships for estimating the thermal and kinematic regime in magma. Conservative models of a cooling lava lake, which include temperature dependent viscosity, show that convection will be vigorous enough to circulate phenocrysts. However, the time dependent nature of magmatic convection and the tendency of whole body convection to vanish before crystallization is complete severely complicate any attempt to infer convective amplitudes from the observed distribution of crystals “frozen” into the interior of the b

ISSN:0094-8276    

DOI:10.1029/GL014i009p00915

年代:1987

数据来源: WILEY

摘要:

Microearthquakes induced by hydraulic fracturing in crystalline rock at a depth of 3.5 km were located with a precision of better than 30 m to obtain information about the geometry and dimensions of the fracture system produced. The induced microseismicity was monitored by a network of five borehole seismic stations; a total of about 800 induced events were reliably located from arrival times. Event locations show a tabular distribution that strikes 350° and dips 65° east, subparallel to the injection well. The injection was intended to produce a fracture system that would hydraulically connect two subparallel wells. A lack of fluid communication between them is consistent with a lack of induced microearthquakes near the target wellbore. The 150 m thickness of the zone of seismicity far exceeds the relative locational uncertainties. The injected fluid appears to have stimulated a zone of rock, rather than simply a single fracture. The distribution of microseismic events presumably envelops the zone of fluid paths created by the fracture experimen

ISSN:0094-8276    

DOI:10.1029/GL014i009p00919

年代:1987

数据来源: WILEY

摘要:

A previous empirical analysis of rainfall to January 1983 at Rotorua led to a deduction that periods of low rainfall preceded temporary cessations of geothermal springs in 1924 and 1943 by about 8 years. That analysis is shown here to approximate flow from a spring fed by a leaky reservoir. Upon adding recent rainfall an additional minimum of pressure in such a reservoir is seen to have occurred in 1983‐85. With the deduced 8‐year response, a natural decline in the spring flow would thus be expected in 1991‐93. Using this spring‐reservoir model, minima of flow are indeed apparent in 1924 and 1943, and there is a downward trend to the

ISSN:0094-8276    

DOI:10.1029/GL014i009p00923

年代:1987

数据来源: WILEY