Areas of tectonic activity in the United States as indicated by earthquake epicenters

Abstract

A study based on earthquake occurrences in the United States shows several zones of continuing seismic activity. Over the past 30 years there is a suggestion of systematic variations in seismic activity following what appears to be a three‐year cycle which is superimposed on a longer period. Areas of activity can not be related to either geologic age or type of structure. In California transcurrent movement on known faults is associated with seismic activity. In the other western states activity is associated with the boundaries of areas of epeirogenic uplift and subsidence and appears to involve predominantly vertical crustal displacement. In the Midcontinent area activity for the most part is associated with positive crystalline rock tectonic elements and their borders. In the Appalachian area activity is associated with the zones of overthrusts and shear. A conspicuous zone of earthquakes paralleling the Appalachians from Missouri to the St. Lawrence River valley while not related to known geologic controls does follow a zone of better‐than‐normal seismic transmissibility. The depth of focus varies from area to area and appears to be related directly to crustal thickness and possibly also changes in crustal rigidity with depth. No direct relation is noted between areas of isostatic unbalance and seismic activity or variations in degree of seismic activity with changes in crustal thickness. For the most part, it appears local stress conditions are dominant in causing earthquakes.