Studies of aftershocks and earthquake swarms in the Nevada region



Because of its high seismicity and generally low level of microseismic noise, the Nevada area has in recent years begun to play an important role in studies connected with the occurrence of microearthquakes and small earthquakes. During the last two years, a considerable effort by the Mackay School of Mines Seismological Laboratory has gone into field investigations of small earthquakes and development of analysis techniques for the handling of field recordings.

Techniques for analyzing microearthquake data have evolved primarily as a function of the conditions under which the data were collected. For the most part, these techniques represent attempts to make the maximum possible use of data collected at a small number of nearby recording locations. As a rule, field experiments have been carried out using a tripartite array 1 or 1 km in radius, supplemented by recordings on one or two single-component systems. In recent studies, the supplementary recorders were operated for only a short time, and were used primarily to derive site corrections to arrival times of seismic waves at elements of the tripartite array. These corrections, designed to eliminate, insofar as possible without a detailed explosion study of the area, the effects of near-surface structure on arrival times, permit the determinations of focal coordinates of nearby earthquakes using only P-wave arrival times, S-P times, and the geometrical properties of the array.