Identification Of Seismic Events Research Articles

THE CORRELATION OF SEISMIC VELOCITIES WITH FORMATIONS IN THE SOUTH‐WEST OF SCOTLAND *

ABSTRaCTIn seismic studies being carried out to elucidate the structure at depth of the Midland Valley rift and the Tertiary igneous province in the West of Scotland, a lack of deep boreholes makes the collection of velocity data imperative for identification of seismic events and for determinations of depth. Three methods are used to investigate the correlation of seismic velocities and geological formations. The results show as strong a dependence of velocity on method of measurement as on lithology and the wide spread of values within a given formation makes the attribution of discrete velocity ranges to specific formations impossible. Changes of velocity are more significant than absolute values. The variations in both are discussed.

An application of statistical discrimination to classify seismic events

Abstract A basic statistical method is presented for the classification of observations into one of two multivariate, normal populations. This theory is applied to the identification of seismic events by considering one of the “populations” to consist of measurements on seismic recordings of earthquakes, the other of explosions. The measurements, or parameters, consist of ratios of the “energies” contained within predetermined “velocity windows” on the seismograms. The choice of velocity windows is guided by the assumption that earthquake source mechanism is extended both in time and space and generates a larger fraction of energy in shear waves as compared to explosion source mechanism. The best separation of twenty earthquakes and twenty-seven explosions is achieved when only seven of the nine ratios calculated are used in the statistical discriminator. Based on this data we have about 85% probability of correctly classifying a given event either as an explosion or as an earthquake.

SEISMIC PROPAGATION PATHS, REGIONAL TRAVELTIMES, AND CRUSTAL STRUCTURE IN THE WESTERN UNITED STATES

The U. S. Geological Survey, with VELA UNIFORM support, has recorded a network of about two thousand long‐offset refraction seismograms to provide information on the detection, location, and identification of seismic events. The measured velocities of compressional waves in upper‐mantle rocks, [Formula: see text], varies from at least 7.75 to 8.25 km/sec in the western United States. There seems to be a systematic variation of the velocity of [Formula: see text] with geologic environment and also with crustal thickness. Where the crust is thick (>35 km), measured values of the velocity of [Formula: see text] tend to be high (>8km/sec), and, where the crust is thin (<35 km), measured values of the velocity of [Formula: see text] tend to be low (<8 km/sec). The velocity of [Formula: see text] is generally low in the high mountains of the far west and generally high in the stable continental interior. The close relation between rock density and seismic velocity implies that, on a regional scale, a large part of isostatic compensation is achieved by density variations in upper‐mantle rocks.