Some Characteristics of the Tectonic Stress Pattern in Alaska

Summary Horizontal azimuths of tectonic ‘pressure’ are plotted for 38 small and intermediate earthquakes recorded in Central Alaska by a six station network during the period October 1967‐September 1968. Two potential mechanisms were considered in evaluating P ‐wave data from the shocks. The first assumed lateral motion on vertical fault planes, while the second considered the possibility of normal faulting on vertical, or near vertical planes. Although many deeper‐than‐normal shocks occur in the region under investigation, they do not appear to be consistent with either of these simplified mechanisms. Of the 38 events considered, only one was at a depth significantly in excess of crustal thickness in the region. The results of this study suggest that maximum tectonic pressure is being exerted in a direction normal to the continental margin in the area of Cook Inlet, while block‐faulting is occurring in the northernmost portions of the Alaska Range. Between these two areas, on the inside of the sharp bend formed by the Alaska Range, maximum compressive stress is being exerted in a direction parallel to the mountain front, implying that further ‘bending’ of the range may be occurring. This is the condition which would be expected if this portion of the Alaska Range is performing in the manner of a hinge as is stated in Carey's (1956) ‘Alaskan Orocline’ theory of continental drift.