Global Positioning System (GPS) data from eight sites on the Caribbean plate and five sites on the South American plate were inverted to derive an angular velocity vector describing present-day relative plate motion. Both the Caribbean and South American velocity data fit rigid-plate models to within ؎1-2 mm/yr, the GPS velocity uncertainty. The Caribbean plate moves approximately due east relative to South America at a rate of ϳ20 mm/yr along most of the plate boundary, significantly faster than the NUVEL-1A model prediction, but with similar azimuth. Pure wrenching is concentrated along the approximately east-striking, seismic, El Pilar fault in Venezuela. In contrast, transpression occurs along the 068؇-trending Central Range (Warm Springs) fault in Trinidad, which is aseismic, possibly locked, and oblique to local plate motion.

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Based on 13 new fault plane solutions and published seismological, geological, and geophysical data, we interpret the deformation along the Pacific‐North American plate margin in the eastern Gulf of Alaska. Three major tectonic units can be distinguished: (1) the North American plate, (2) the Pacific plate, and (3) a belt of mobile borderland terranes. The Pacific plate moves in a NNW direction at rates of about 6 cm/yr in relation to the North American plate. That motion results in mostly right‐lateral strike slip at the Queen Charlotte‐Fairweather fault system, a well‐known observation. A new finding,however, is that a small component (∼1 cm/yr) of convergence may also be present which results in minor subduction of the oceanic plate beneath portions of the continental margin. Heretofore the Queen Charlotte‐Fairweather fault zone and associated continental margin was interpreted as a classical, pure transform boundary. The Yakutat block, a borderland terrane about 400 km long and 100 to 200 km wide, is carried passively by the Pacific plate except that the block slowly overrides this plate at about 1 cm/yr. This motion is taken up by almost pure thrust faulting in a southwesterly direction along a 400‐km long SE striking shelf edge structure. At its NW edge the Yakutat block is in turn being thrust beneath the North American plate along the Pamplona zone‐Icy Bay lineament. The underthrusting of the Yakutat block results in a major orogeny, crustal shortening and uplift of the Chugach‐St. Elias range. The effects of this collision may extend as far as 500 km inland and cause some deformation at the Denali fault in the central Alaska Range. Subduction of the Pacific plate beneath the colliding margin appears responsible for development of an active volcanic arc up to 300 km inland which trends SE from the Wrangell Mountains to Yukon Territory, Canada, and perhaps to Mt. Edgecumbe volcano in southeast Alaska. The tectonic model proposed implies a high seismic hazard for the Queen Charlotte, Fairweather, and Chugach‐St. Elias fault systems. At these fault zones we estimate recurrence times for great events of about 100 years, but they may vary between 50 and 200 years. A temporarily very high potential for a great earthquake has been determined for the ‘Yakataga seismic gap’ located between Icy Bay and Kayak Island. Large or great thrust earthquakes on the detachment fault underlying the entire Yakutat wedge also appear possible but may only occur infrequently. Their recurrence times are estimated to be several

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Four telemetered seismic arrays were operated in northeastern Venezuela during the summer of 1979. An analysis of the data collected has resulted in accurate locations for about 100 microearthquakes and four new focal mechanism solutions. On the basis of these new data, an in‐depth analysis of teleseismic data, and geologic evidence, we propose a new plate tectonic framework for the southeastern Caribbean. In this model, underthrusting of the Atlantic seafloor along the Lesser Antilles is extended to the southeast of Trinidad. The subducted slab is shown to dip northwesterly beneath Trinidad and the Caribbean Sea, penetrating depths of at least 150 km. This subduction appears to terminate abruptly in the vicinity of the Los Bajos‐El Soldado fault zone, which trends WNW‐NW and is located in the Gulf of Paria west of Trinidad. Geologic evidence shows right‐lateral strike slip (RLSS) motion in this fault zone, probably initiated in the late Pliocene. This WNW‐NW trending fault zone joins up with the E‐W trending El Pilar‐Casanay fault system in northeastern Venezuela, which also moves right laterally. The El Pilar fault is apparently offset by NNW trending faults that exhibit RLSS motion as well as normal faulting. This complex motion on an en echelon series of faults is interpreted to be the result of WNW motion of South America relative to the Caribbean plate. In our interpretation, the E‐W component of relative motion is accommodated by RLSS motion on E‐W faults and results in normal faulting on NNW‐SSE faults. The smaller component of N‐S convergence is reflected by RLSS motion on NNW trending faults and is accommodated by some internal deformation. The timing of the initiation of RLSS motion on the Los Bajos fault zone and the known total amount of displacement on it as well as other geologic indicators lead us to suggest that this complex mode of strike slip motion began only a few m.y. ago, and that the average rate of relative plate motion in this region is probably ≥0.9 ± 0.2 cm/yr.

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A 250-kilometer-long seismic gap in southern Alaska, which is situated along the boundary between the North American and Pacific plates, ruptured in two great earthquakes in 1899. Within the gap, earthquakes of moderate size form a ring of activity around a region of very low seismicity. The number of shocks of magnitude 6 or larger in this ring appears to have increased significantly since the 1958 earthquake, which occurred on the adjacent part of the plate boundary. This space-time pattern is similar to long-term patterns that preceded several large earthquakes in Japan. A shock of magnitude 7.7 on 28 February 1979 ruptured only a small part of the seismic gap. The remaining part, which already may have stored sufficient strain to generate a great shock, warrants intensive study to evaluate its potential for such an event.