Study of small-magnitude, delayed multiple events and interactive three-dimensional visualization of seismic data

This dissertation addresses two interrelated topics: interpretation of data obtained by a broadband phased three-component seismic array and development of new techniques to process and visualize these data. First, I present two techniques I developed for interactive, three-dimensional visualization of seismic array data. The first technique allows one to simultaneously analyze three-component seismic array data by displaying three-dimensional particle motion at the array stations as computer animation. It provides fundamental new insights into the nature of ground motion within the scale length of a given array. The second technique is used to analyze slowness measurements derived from seismic array observations. It displays the results of single-component slowness analysis calculations utilizing three-dimensional isosurfaces, volume slicing planes, and a volume rendered using transparency. The technique allows one to detect, and to simultaneously visualize, analyze, and compare multiple phase arrivals through the whole seismogram. The described techniques were used to analyze both body and surface waves recorded by broadband arrays with a wide range of apertures and to study spatial variability of seismic wave fields. Second, I present an extensive study of a new type of compound microearthquakes I have discovered processing data from an experimental, broadband, three-component array deployed in the Republic of Turkmenistan in 1993--1994. I demonstrate unique data showing records of small local events that exhibit prominent secondary P and S arrivals and use coherent waveform processing and the visualization technique described in the first part of the thesis to ascertain that the records correspond to small, very closely spaced earthquakes that are separated by times on the order of one second. This study provides important insights on earthquake dynamics at time scales longer than dynamic rupture processes, but significantly less than the processes that load a fault to a critical state. Finally, I present a technique for joint visualization of earthquake focal mechanism and tectonic data in three dimensions. The technique provides intuitive representation of three-dimensional orientation of focal mechanisms and makes comparison of these data with three-dimensional geometry of the surrounding tectonic structures much easier.