Potentials and displacements for two theoretical seismic sources

Theoretical, P, SV, and SH displacement potentials and displacements for a double couple or point shear dislocation source and for a ‘mixed quadrupole’ source at any arbitrary orientation in an isotropic homogeneous elastic space are expressed as multiple integral and derivative operations on the source history in the time domain and their algebraic equivalent in the frequency domain. These sources have the same angle orientation functions, which are given explicitly. The double couple and ‘mixed quadrupole’ are both quadrupole sources but, unlike the double couple, the P and S waves from a ‘mixed quadrupole’ have different source histories. Analytic displacements are obtained using as examples the Ohnaka shear dislocation history for a double couple and the Randall and Archambeau tectonic release histories for ‘mixed quadrupole’ sources. The displacement fields are investigated numerically, in order to establish a criterion for estimating the minimum range for applying far-field theory results to the total displacement field. The chosen criterion is the ratio of the far-field peak amplitude, which is a function of source rise or duration time, to the static displacement, which is a near-field phenomenon. The proposed criterion is found to be conservative as to the minimum range for the farfield, predicted (1/R) dependence of the total field peak amplitude, but quite satisfactory for time domain estimates of moment and corner frequency based on far-field theory

Reply [to “Comments on paper by David G. Harkrider, ‘Theoretical and observed acoustic-gravity waves from explosive sources in the atmosphere’”]

I acknowledge and agree with the comments made on Harkrider [1964] by A.D. Pierce

The perturbation of Love wave spectra

The equations governing the variational principles for Love wave spectra are investigated. It is shown that assumptions used by earlier authors are not necessary to the validity of the variational techniques. Moreover it is demonstrated that except for a homogeneous plate, these assumptions are false for plane multilayered media and lead to incorrect expressions for group-velocity perturbations. The correct expressions are determined and examples of their use are given

Elastic relaxation coefficients for a spherical cavity in a prestressed medium of arbitrary orientation

Archambeau gave elastic relaxation coefficients for a spherical cavity introduced into a pure shear prestress field. The technique is generalized to a stress field for which only the trace of σ_(ij)^(0) is zero. The coefficients are given for a general deviatoric prestress field of arbitrary orientation. They are then specialized to the case of a pure shear stress expressed in terms of the orientation angles commonly used in fault plane descriptions, i.e. dip and slip angle. The extension of this technique to an arbitrary homogeneous prestress field and its limitations are discussed

Surface waves in multilayered elastic media. Part II. Higher mode spectra and spectral ratios from point sources in plane layered Earth models

Phase and amplitude spectra of Rayleigh and Love waves are presented for two Earth models, one oceanic and one continental shield. The spectra of the first three Rayleigh modes and the first four Love modes are tabulated for point sources at selected depths. These tables along with computer algorithms described here allow one to estimate the amplitude spectra at nontabulated source depths. The use of spectral ratios as a means of determining source depth is investigated. A source depth of 20 km is obtained for the Fallon earthquake of July 20 1962. This depth agrees with previous estimates but the technique requires a fault-plane orientation which differs from radiation pattern solutions

Theoretical and observed acoustic-gravity waves from explosive sources in the atmosphere

A matrix formulation is used to derive the pressure variation for acoustic-gravity waves from an explosive source in an atmosphere modeled by a large number of isothermal layers. Comparison of theoretical and observed barograms from large thermonuclear explosions leads to the following conclusions: (1) The major features on the barogram can be explained by the superposition of four modes, (2) different parts of the vertical temperature structure of the atmosphere control the relative excitation of these modes, (3) a scaled point source is sufficient to model thermonuclear explosions, (4) the observed shift in dominance of certain frequencies with yield and altitude can be explained by means of the empirical scaling laws derived from the direct wave near the explosion, and (5) out to 50° from the source, the observed variation of amplitude with distance can be accounted for by geometrical spreading over a spherical surface

The early years of computational seismology at Caltech

Early this year I was asked to make some "remarks" at the 100th anniversary of the Berkeley station as President of the SSA. Not knowing exactly what was expected, I decided to do some background research on the history of the Berkeley station as found in papers, letters, and reports to the Bulletin. I found these very informative and some very entertaining. Even though a lot of the information was obviously hearsay and written well after the events, I felt that there was a place for this type of anecdotal "history" in the Bulletin and probably no better place for it than in the Presidential Address