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UNITED STATES COAST AND GEODETIC SURVEY, WASHINGTON, D.C.
Abstract
The following empirical law governing the azimuth of S-wave impulses is advanced: (1) the azimuth of the horizontal S-wave impulse at the focus (the horizontal component of the initial earthquake displacement) is equal to (2) the azimuth from station to epicenter, plus (3) the azimuth from epicenter to station, or back azimuth, minus (4) the azimuth of the S-wave displacement at any station; all azimuths being counted from north around by east through 360°. It states that a common direction of horizontal impulse or thrust at a seismic focus can be calculated from S-wave azimuths observed at two or more distant stations. These focal thrusts appear to be either parallel or normal to fault lines.
A theory is later developed which appears to substantiate the above-named law in so far as it concerns simple horizontal focal displacements. S-wave impulses, due to a simple vertical displacement at a focus, apparently follow the laws of longitudinal motion, but arguments are advanced to question the existence of this type of activity. The displacements are longitudinal only in the sense that they occur in the planes of the rays; they are not compressional waves.
Three distinct types of activity are found in the S-wave group.
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