D. W. Vasco and L. R. Johnson
A study was completed in which the arrival times of seismic body waves were used to estimate the velocity structure of the entire earth. A solution was obtained for laterally varying velocities in the mantle, outer core, and inner core in addition to boundary topography for the 410 km, 660 km, and mantle-core discontinuities. Anisotropy in the inner core was also allowed.
D. W. Vasco, O. Marques, and L. R. Johnson
The problem of solving for velocity structure in the earth using seismic travel time data was investigated with regard to the resolution and uncertainty of the results. The iterative Lanczos algorithm was used to obtain a partial singular value decomposition of the problem. A total of 5,000 Lanczos values and vectors were used to construct model parameter estimates as well as measures of model parameter resolution and covariance. The Lanczos resolution estimates provide lower bounds on conventional SVD-based measures.
R. M. Nadeau and L. R. Johnson
Waveform data from a bore hole network of broad band seismographic stations have been used to study microearthquakes along the Parkfield segment of the San Andreas fault. Analysis of almost ten years of such data demonstrates that much of the seismicity in this region consists of repeating sequences, quasi-periodic sequences of earthquakes that are essentially identical in terms of waveform, size, and location. Scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the Stone Canyon section of the fault and the main Parkfield sequence. These estimates show that seismic moment is being released as a function of time in a very regular manner. Measurements of the moment release rate, combined with an assumed tectonic loading rate, lead to estimates of the seismic parameters source area, slip, and recurrence interval. Such parameters exhibit a systematic dependence upon source size over a range of 1010 in seismic moment which can be described by three simple scaling relationships. Several implications of these scaling relationships are explored, including the repeat time of earthquakes, average stress drop, strength of the fault, and heat generated by earthquakes. What emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1% of the fault area. This means that the fault is highly heterogeneous with respect to stress, strength, and heat generation. Such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes, such as why faults appear weak, why significant heat flow is not observed, how significant high frequencies can be generated by large earthquakes, and how various geologic features such as pseudotachylytes might form.
P. K. Seifert, J. T. Geller, and L. R. Johnson
Laboratory experiments were conducted to investigate elastic wave propagation in unconsolidated granular material. The results were compared with a number of different numerical models, including a one-dimensional propagator solution, a two-dimensional boundary integral equation method, a local flow theory, the combined Biot and squirt flow theory, and a dynamic composite elastic medium theory. The combined experimental and theoretical analysis yields a better understanding of how wave propagation in unconsolidated materials is affected by homogeneous phase distribution, inhomogeneous phase distribution, pore fluids of different viscosity, and wettability of a porous medium.
B. Kaelin and L. R. Johnson
Elastic wave propagation in highly heterogeneous media was investigated using both theoretical calculations and field experiments. A dynamic composite medium elastic medium theory was developed and results were obtained for both the one-dimensional and three-dimensional cases. The method is self-consistent and agrees with the Reuss average at low frequencies and with the ray theory average at high frequencies. This theory was used to explain observations of travel times and waveforms in partially saturated tuffs and in fractured limestone.
P. B. Parker and L. R. Johnson
A set of algorithms which mimic Darwinian evolution have been developed for the global optimization problems that are encountered in non-linear geophysical inverse problems. The basic algorithms, as well as more sophisticated options involving various selection schemes, crossover operators, niching methods, and parallel models, were tested on the geophysical problems of interpreting crustal receiver functions and inverting observed gravity data.