The focal mechanisms for 61 earthquakes occurring in northern and central Chile during the years 1962–1970 indicate underthrusting of the oceanic plate for earthquakes with focal depth 30–60 km. The axis of tension for intermediate‐depth earthquakes is parallel to the direction of dip of the plate. For deep‐focus earthquakes the axis of compression is parallel to the axis of the plate. Together with the seismicity of the region, the focal mechanisms indicate that subduction of the oceanic plate under this part of the coast of South America takes place in discrete and localized episodes and that the lithospheric slab itself is broken into a series of tongues that are absorbed independently and quite differently from one latitude zone to the next or even at one depth as opposed to another. Near the Chile‐Peru corner, 18°S–25°S, the more principal present day activity is at intermediate focal depths. The motion of the plate is in the azimuth N85°E at shallow depths, veering to N65°E at intermediate depths. The zone 25°S–27°S is at present a silent zone at intermediate depths. The zone 27°S–34°S corresponds to underthrust of the oceanic plate but such that at depths of about 120 km this segment of the plate moves horizontally under the continent. The deep‐focus zone, 19°S–28°S, overlaps the three zones just mentioned and is discontinuous with them. It more probably corresponds to an independent and earlier epoch of plate absorption. Under central Chile the plate motion appears to correspond to a current episode of subduction of relatively recent initiation. The motion is in the azimuth N80°E.
The focal mechanisms of 40 earthquakes in Peru and Ecuador, together with the seismicity of the region, indicate particular features of the subduction of the oceanic plate beneath this portion of South America. At shallow depths near the coast and at foci along the contact between the subduction zone and the continental plate the focal mechanisms indicate an underthrust of the continent by the oceanic plate on a thrust plane dipping 10°–15° beneath the continent. Near this same depth but at foci within the oceanic plate, normal faults occur that correspond both to flexure of the plate and to downdip axial tension. At intermediate depths the plate continues to act as a stress guide, the axis of tension being down about 30° from the horizontal and trending to the ENE. The dip of the Benioff zone steepens notably in southern Peru near the Peru‐Chile corner, and the motion of the descending slab relative to the continental plate is in a direction N40°E. Deep‐focus earthquakes indicate a vertical segment of plate under axial compression at depths of 550–600 km. Numerous earthquakes also occur interior to the continent and within the continental lithosphere at depths down to 90 km. Both strike slip and reverse‐type faults are found, but in either case the stress system corresponds to an E‐W horizontal compression. Comparison with the seismicity is consistent with the model of an oceanic plate moving almost horizontally under the continental lithosphere in northern and central Peru and a separate, more steeply plunging segment of plate moving normal to the coast under southern Peru. Equal‐area projections are available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D. C. 20006. Document J75‐003; $1.00. Payment must accompany order.
Mechanisms of the main shock and of fourteen larger aftershocks of the Rat Island earthquake of February 4, 1965, indicate two classes of foci. The first, including the main shock and nine aftershocks all located within the island arc proper, corresponds either to an overthrust of the island block relative to the oceanic block or to reverse faulting oblique to the trend of the arc. The second, consisting of foci under the trench, represents fractures arising from a horizontal tensional stress. The entire motion is conformable to an oceanic plate moving under the island arc. The sequence is terminated by a transform fault at the eastern end of the active zone.
The relation between the hypothesis of sea‐floor spreading and features noted in previous studies of the Alaska earthquakes of 1964 and the Rat Islands earthquake of 1965 has prompted a further investigation of the larger earthquakes of the entire Aleutian chain. Sixty‐six earthquakes which occurred in the decade 1957–1966 are examined. The hypocenters of these shocks are found to be separated spatially into two distinct groups. Twelve earthquakes occurred along a narrow line immediately below the axis of the Aleutian trench or under the seaward slope of the trench. The remaining hypocenters were distributed over a broader tabular region beginning midway between the trench and the islands proper and extending under the islands. Foci of the first group are uniformly extensional in character with axes of tension aligned normal to the local axis of the trench. Foci of the second group are conformable to an underthrust of the island arc, and the direction of underthrust at the individual foci is in agreement with a rotation of the oceanic plate about a pole located at 85°W, 50°N. Certain foci represent related effects such as an arc‐arc transform fault, or a strike‐slip shear at the eastern extremity of the arc. All features could be predicted by the hypothesis of sea‐floor spreading and are presented as evidence in support of island arcs as the locale of a sink mechanism required by the hypothesis.
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.
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