The focal mechanism of the Alaska earthquake of March 28, 1964, and of its aftershock sequence

Abstract

Focal mechanisms have been determined for one preshock, for the main shock, and for more than 25 aftershocks of the Alaska earthquake of March 28, 1964. For the main shock a single nodal plane with a strike azimuth of 66°, dip 85° southeast, is determinable from the polarity of the P wave. This plane may be taken either as a plane normal to the fault motion (thrust faulting) or as the fault plane (dip‐slip motion on a near‐vertical plane). A combination of P wave first motion and S wave polarization data make possible the determination of both nodal planes in each shock studied of the aftershock sequence. One of these planes is near vertical and closely resembles the nodal plane of the main shock; the other dips 5° to 15° to the northwest or north. For earthquakes of the Kodiak Island region, the near‐vertical plane has the same orientation as that of the main shock. For earthquakes to the east of Prince William Sound, this plane shows a systematic change in orientation corresponding to the change in trend of the tectonic features. Three earthquakes that have foci at increasing depths along the line of greatest flexure of the tectonic features differ from the others. The difference in character of these foci probably provides an important clue to the right interpretation of the motion in the main shock. Although the focal mechanism solutions for the shocks are subject to the same ambiguity of interpretation as that of the main shock, criteria which favor a thrust hypothesis are advanced from the interrelation of the foci. From dislocation theory it is shown that differential slip and/or a dipping thrust plane explain satisfactorily the observed vertical displacements at the surface.

Eos, Transactions, American Geophysical Union Volume 95, Number 17, 29 April 2014

“…In a landmark 1965 paper, U.S. Geological Survey (USGS) geologist George Plafker [Plafker, 1965] convincingly showed that the only fault configuration consistent with the pattern of deformation was a "megathrust" on the low-angle fault plane. Analysis of the aftershock sequence supported this interpretation [Stauder and Bollinger, 1966]. Plafker's work provided the geologic field evidence that helped to explain where oceanic crust, initially created at mid-ocean ridges, is eventually consumed.…”

Tectonic Summary of Leg 18

“…The absence of a Benioff zone beneath the continental margin off Oregon and Washington seems to indicate that subduction is not taking place, yet some observations are enticingly suggestive of subduction, such as the folded Pliocene and Pleistocene sediments of the Oregon lower continental slope. Similarly, off Alaska, the deformation during the great 1964 earthquake and the results of focal-mechanism studies are most easily explained by underthrusting (Plafker, 1969;Stauder and Bollinger, 1966). Thus some field observations support the theory and others conflict with it.…”

“…ago, the section that was depressed to form it should contain a former continental-rise type of sedimentary wedge. However, no such feature appears in seismic records in the depressed section below the trench fill (von Huene, 1972), and any previous rise or trench must now be incorporated into the continental slope. Since the Aleutian Trench is one component of a tectonic system that includes the Kenai and Chugach mountains, the trench may have begun to form at the same time as these mountains, in the early or middle Miocene.…”

“…mechanisms operating across the Aleutian Trench ( Figure 4). A geophysical study prior to Leg 18 (von Huene, 1972) provided the basis for selecting drill sites and the structural framework for interpreting core data. The interpretation of Site 178 and its associated seismic records sets the broad framework of geologic history for the area.…”

“…As with other developing geologic theories, there is a need to check the theory with field observations, where discrepancies between theory and observation seem to occur. For instance, the 6 cm/yr subduction under western Alaska required by present north Pacific models is difficult to reconcile with the lack of large compressional structures in sediments filling the Aleutian Trench and the steep faults and open folds of Neogene age on the continental shelf (von Huene et al, 1972). The absence of a Benioff zone beneath the continental margin off Oregon and Washington seems to indicate that subduction is not taking place, yet some observations are enticingly suggestive of subduction, such as the folded Pliocene and Pleistocene sediments of the Oregon lower continental slope.…”