A technique was developed in the present study to determine fault-plane solutions for small earthquakes. The method uses the direction and amplitude of initial P-wave motions recorded at a small number of seismic stations for epicentral distances less than 2000 km. Seismic arrivals recorded on short-period seismograms were identified as p, P or Pn waves for crustal shocks and P waves for subcrustal shocks. Source amplitudes were converted from station amplitudes using known theoretical methods, based on determining angles of incidence at the surface of the earth and straight ray paths in experimenta1 crustal models. Source amplitudes were calculated for three stations and were then projected back to the earthquake source. The source amplitudes were compared to amplitudes that correspond to more than 6000 theoretical amplitude patterns. The pattern which most nearly fitted the first motions was taken as the fault-plane solution. P-wave amplitudes, velocity structures, focal depth and wave attenuation were varied to show the relative deviations of the dip and strike in a fault-plane solution. When the S-wave was identified, it was found that polarization could be determined for epicentral distances less than 20°. Thirty-three earthquakes in the Pacific Northwest region were analyzed, and twenty-two fault-plane solutions were determined by the method described in this paper. Seven additional fault-plane solutions were determined using the well-known Byerly method. The fault-plane solutions generally showed large dip-slip components. This was particularly evident in fault-plane solutions for earthquakes occurring off the coast of Oregon and northern California, and west of the Cascade Mountains. The solutions for earthquakes east of the Cascade Range and off the coast of British Columbia have either dip-slip or strike-slip components. The solutions obtained by the present technique were compared with solutions for generally larger earthquakes in western North America as previously determined by other investigators, using the Byerly method. Satisfactory agreement was found between the two methods. Two general tectonic hypotheses are proposed from the study of earthquake stresses in the Pacific Northwest region. / Graduation date: 1969

The vertical and radial components of a seismic P wave can be decomposed by a Fourier transform into two sets of nonterminating sinusoidal waves with one set for each component. The tangent of the vertical transform divided by the radial transform gives by definition he apparent angle of emergence for that frequency. The actual angle of emergence can be calculated from the apparent angle. The change of the angle with frequency can be obtained by determining the angle over the entire frequency spectrum of the pulse. The angle of emergence is only defined for a pure pulse. Just the short length of uncontaminated signal can be used to calculate the angle U the signal is interfered with by other signals. The actual angle of emergence was calculated as a function of frequency for stations near nuclear explosions. In all cases, the angle varied with frequency. / Graduation date: 1967

Graduation date: 1970

No description available.

Fourier transformed amplitudes of the compressional waves recorded between 9° and 91° (about 1000 to 10,000 km) epicentral distance from Gnome, Shoal, Haymaker,and Bilby underground nuclear explosions have been compared with Fourier transformed amplitudes of P waves measured at distances between 9.0 and 1 3. 0 km from the explosions. Using a formula of the type A= ε AoF(ξ)e - πfr/QV apparent Qs have been computed for frequency ranges of 0.5 to 1.0 cps and 0.7 to 1.0 cps at the various epicentral distances. The geometrical spreading factor F(ξ) and interface loss parameter ε drop out of the computations when assumed to be frequency independent in the narrow frequency pass-bands considered. The computed apparent Q values indicate that the mantle is inhomogeneous with respect to the absorption of seismic energy. Relatively low Q (high absorption) regions are centered around 25°, 42° and 74° and high Q (low absorption) regions are centered around 20°, 31°, 71°, and 78° epicentral distances. These are superimposed on a gradually increasing Q with epicentral distance trend. The average apparent Q value for the upper mantle is found to be 286 ± 38. The highest Q value of this study occurs between 78° and 80° epicentral distance where it is greater than 3000. The high and low Q values obtained in this investigation are of the same order of magnitude as those reported by other investigators from body wave studies and the general variation of the absorption properties with depth agrees reasonably well with those obtained from surface wave studies. The depth of great increase in electrical conductivity and the observed maximum depth of earthquake foci both agree roughly with a region of large increase in Q in the upper mantle. A relatively high absorption zone is found to occur below this depth. / Graduation date: 1967

Recent developments in cone penetration testing have resulted in the addition of both pore pressure measurements and seismometers. The seismometers allow shear wave velocity testing to be performed at designated intervals. Both of these additions were researched to improve their application and interpretation. The significant factors effecting the pore pressure generated during cone penetration tests are discussed. The importance of various factors is especially dependent upon permeability, strength, and stiffness. For all sands tested, pore pressures lower than static were recorded behind the tip and higher than static were recorded on the face of the cone. It is believed that the large compressive stresses on the cone face result in positive pore pressures. As the cone tip passes a soil element unloading and continued shearing generate pore pressures lower than static in all sands. The sign of this pore pressure (higher or lower than static) was therefore considered primarily a function of the test equipment. Pore pressure response and the rate of dissipation of excess pore pressures were found useful in distinguishing fine granular soils and explaining soil stratigraphy. In cohesive soils the details of pore pressure measurement were found to be important only in stiff soils. Pore pressures at all measurement locations were found to increase with soil strength in soft to firm clays but may be negative of static in very stiff clays. Pore pressures behind the cone tip were often negative of static in stiff clays. Measurement techniques were refined to improve the accuracy of downhole shear wave velocity measurements. Comparisons of downhole and crosshole measurements were made at three well documented sites validating the technique. At several sites it was found useful to consider the Gmax values determined from shear wave velocity and density to distinguish soil type. Gmax to cone resistance ratios were shown to vary systematically with cone resistance values in sands. A wide range in Gmax to cone resistance was observed in clays. The dependence of both cone penetration resistance and Gmax to increased stress level or overburden stress is discussed. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

Thesis (Ph.D.)--University of the Witwatersrand, Science Faculty (Geophysics), 1980 / The physical processes at the source of mine tremors at the East Rand Proprietary Mines (E.R.P.M.) near Johannesburg were investigated. [Abbreviated Abstract. Open document to view full version] / AC2017

To date, most shear velocity heterogeneity models in the lower mantle have been derived using long period data. Comparatively little use has been made of the vast ISC database of shear wave arrival times, which covers the years 1964 to 1991. The aim of this study is to use this database to construct global models of shear wave heterogeneity in the lower mantle and compare it with existing P models using similar period data in order to evaluate the hypothesis that P and S heterogeneity are proportional. In order to reproduce the resolution operator inherent in these existing models, the same parameterization has been employed; lateral dependence is in terms of spherical harmonics up to degree and order 6, and radial dependence in terms of a quartic polynomial. The inversion uses data from over 4500 events selected according to criteria which minimize the bias introduced by uneven source receiver coverage. Despite having approximately one sixth of the amount of data the model correlates well with a P wave model computed using data from the same events, and this in turn is almost identical to models computed by other authors using similar inversion methods. Assuming proportionality of P and S heterogeneity, a joint inversion of the P and S data has been performed. The resultant model correlates extremely well with the P model of this study, and similar variance reductions are obtained for both the P and S datasets as were found in the original inversions. Several model parameterizations have been used to try to investigate the dependence of the model expansion on the results. Consequently, taking into account results from synthetic experiments and from experiments into trade-offs with other inversion parameters, the average ratio of relative S to P heterogeneity ^{d ln v_s}andfrasl;_{d ln v_p} throughout the lower mantle is constrained to be in the interval (1.8,2.1) with emphasis on higher values within these bounds. This value is in agreement with other studies using similar period data, and the upper bound is close to the optimum value of 2.27 obtained by Li et al., 1991 from normal mode data, suggesting that the ratio is at least only weakly dependent on frequency. In addition evidence is presented that the ratio is increasing with depth although attempts to extract specific gradients have failed because of the very poor distribution of S rays in the deepest mantle.

No description available.

No description available.