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Seismological method yields information similar to structural information [Byerly (1926)]. The procedure in general is called first-motion study and the results, which can be interpreted in terms of fault movements, are known as focal mechanism solutions. This technique complements geological methods because it permits interpretations of present-day movements on deeply buried or otherwise concealed faults. First motion studies have played a central role in the development of plate tectonics and provide an important method for determining the relative motions of litho spheric plates (see Section 10.2). Summaries of the method are given by Honda (1962), Stauder (1962), and Khattri (1973).

Suppose an earthquake originates by sudden shear displacement over part of a fault plane as shown in the Figure. Elastic waves are radiated in all directions from the source region or focus, but they will have different characteristics in different directions: Notice, for example, that the first compressional wave disturbance orfiTst motion of P arriving at point I or point 3 will be a compression but that the first motion at point 2 or point 4 will be a dilation.

The boundaries between regions close to the focus that receive compressional or dilational first motions ofP are defined by the fault plane itself and by a second plane (the auxiliary Plane) which is normal to the fault plane and to the slip direction. The fault plane and auxiliary plane divide the space near the focus into four quadrants. In alternate quadrants the first motion of P is

compressional or dilationai. This pattern is known as a quadrantal distribution natural of most earthquakes. A quadrantal distribution of the first motion of P does not, however, prove that an earthquake originated by faulting because other focal mechanisms can give the same pattern (for example, a sudden volume change at the focus accompanied by a pure shear strain). Because the actual mechanism is usually unknown, it is preferable to describe the result of a first-motion study as afoeal mechanism solution rather than afault plane solution. However, any pattern with quadrantal character can be analyzed to suggest the orientations of possible fault planes and displacement directions.