The large features of Pacific seismicity, including association of shallow and deep earthquakes with arc structures, almost exclusively shallow earthquakes with block structures, and earthquakes at intermediate depth with volcanic belts, together with distribution in a complicated and branching pattern of narrow zones, are well known.

Broad generalizations about this seismicity and its relation to geologic structures, involving specific interpretations of critical areas, have sometimes been supported by appeals to mapping on an extremely small scale, which allows confounding structures and epicenter perhaps hundreds of miles apart. The briefness of our adequate seismic history on the geologic time scale is often insufficiently considered, and conclusions drawn from present relative lack of seismicity in areas where field evidence, or even historical records, indicate greater activity in the immediate past.

Distribution in depth, even within the crust, is sometimes not considered, and earthquakes are discussed with respect to surface structure as if the latter could be expected to continue vertically down without appreciable dip or other complication in three dimensions.

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