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Data for local earthquakes recorded by a network of stations in northeastern United States and adjacent Canada were analyzed to study the seismicity, the relationship between earthquakes and known faults, the state of stress, and crustal and upper mantle velocity structure. In addition, portable seismographs were deployed in the field to study aftershocks. As a result, accurate locations for about 364 local earthquakes (2 ≤ mb ≤ 5) and 22 focal mechanism solutions were determined. A comparison of the spatial distribution of these events (1970–1979) with historical earthquakes (1534–1959) reveals that seismic activity in the northeast is relatively stationary in space: those areas that have had little or no seismicity historically are relatively aseismic today, whereas the historically active areas are also active today. The instrumental locations, historical seismicity, and focal mechanism solutions show an internal consistency that help us distinguish two distinct seismogenic provinces. (1) The Adirondack‐western Quebec province is a northwesterly trending zone of seismic activity, about 200 km wide and at least 500 km long, extending from the SE Adirondacks into western Quebec, Canada. Thrust faulting on planes striking NNW to NW appears to predominate, and the inferred axis of maximum horizontal compression is largely uniform and trends WSW, nearly parallel to the calculated absolute plate motion of North America. Little or no seismicity is found where anorthosite outcrops at the surface. Correlations between gravity anomalies and earthquake locations suggest that seismic activity in this zone is localized to regions of steep NE or SW gradient in Bouguer anomalies. This zone does not appear to extend southeastward to Boston, as proposed by some workers. (2) The Appalachian province is a northeasterly trending zone of seismic activity extending from northern Virginia to New Brunswick, Canada. Highangle reverse or thrust faulting on N to NE trending planes appears to predominate. The western margin of this province, however, appears to be relatively aseismic. We attribute this relative lack of activity to one or more of the following: the presence of igneous activity postdating rifting of Africa from North America, the occurrence of intense metamorphism during the Acadian orogeny which may have annealed preexisting faults, and the predominance of ductile as opposed to brittle deformation in the geologic past. The inferred axis of maximum horizontal compression along the eastern margin of the Appalachians is rather uniform and trends W‐WNW, almost perpendicular to the magnetic lineations offshore. We suggest that this W‐WNW compression reflects the gravitational force arising from horizontal density variations in the oceanic lithosphere as it cools and moves away from spreading centers.

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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.

Seismic activity in the greater New York City area is concentrated along several northeast-trending faults of which the Ramapo fault appears to be the most active. Three nuclear power plants at Indian Point, New York, are situated close to the Ramapo fault. For a reactor site in use for 40 years, the probability that the site will experience an intensity equal to or in excess of the design (safe shutdown) earthquake is estimated to be about 5 to 11 percent.

Renewed earthquake activity at Blue Mountain Lake (BML), New York, in July 1973 provided an excellent opportunity to monitor the travel time ratio of $ to P waves (ts/t•,) in real time and to test the ts/t•, technique as a predictive tool. From a mean value of 1.73 on July 30, 1973, ts/t•, decreased to about 1.5 over the next 2-3 days. On August 1 a prediction was made that an earthquake of magnitude 2.5-3 would occur in a few days. Upper limits of the magnitude and the time of occurrence of the expected earthquake were inferred from the spatial extent of the seismic anomaly. As a result of the prediction an additional strong motion accelerograph (SMA) was installed in the source region. At 2310 UT on August 3, '1973, a magnitude 2.6 earthquake occurred at BML and triggered two SMA's. In addition to the seismic activity a number of explosions were recorded from a variety of azimuths. The P wave arrivals from distant quarry blasts, refracted from a high-velocity layer at 4 km beneath BML, showed late arrivals at five stations during the premonitory low in t•/t•,. The P wave delays were maximum (0.13 s) in the hypocentral region of the earthquake and decreased away from it along two profiles. These results indicate that changes in ts/t•, are caused by changes in the material properties of the earthquake source region. The anomalous zone (region of low P velocity) for the August 3 earthquake (aftershock length I km, depth I + 0.1 km) was about 3-5 km in radius and was wholly or largely limited to the layer above the interface at 4-km depth. In contrast to the P delays observed from distant quarry blasts, P and $ arrivals from local construction blasts (A < 10 km) show no large premonitory changes in either P or $ travel times. This observation suggests that either no significant velocity anomalies occurred in the upper 0.5 km of the BML region or that the dilatant cracks were predominantly horizontal, the result being strong velocity anisotropy. This conclusion is supported by data from seismic sources; events located at shallow depths show normal (1.75) to high (1.85) t•/t•, values even when values as low as 1.5 are observed from deeper sources. This finding places constraints on the use of artificial sources to monitor changes in velocity, since the effects of anisotropy may have to be taken into account in addition to ensuring that ray paths to the recording stations penetrate the zone of anomalous velocity. A maximum likelihood method was used to invert data from individual small earthquakes to determine P and $ velocities in the anomalous zone. The results, which are consistent with the explosion data, indicate that the premonitory decreases in P and $ velocities were much more pronounced for sources at depths of 1-2 km than for those near the surface. The inferred low values of P and $ velocities were, respectively, about 22 and 12% below normal. This study also shows that t•/t•, inferred from a Wadati plot is a function of the distribution of stations relative to the anomalous zone. To optimize the use of t•/...

Seismic and volcanic activity in the Philippine Islands was examined in an attempt to decipher the tectonics of this region. Several new fault plane solutions for shallow, intermediate, and deep focus earthquakes were determined. This study has revealed the presence of a zone of eastward underthrusting in the western Philippines which is well developed near Negros Island. Fault plane solutions of several shallow earthquakes in the western Philippines show thrust faulting with slip vectors toward east or northeast. As active eastward subduction of the Eurasian plate is also taking place along the Manila trench near west central Luzon, it suggests that the underthrusting of the Eurasian plate may have occurred at one time along the western Philippines from Taiwan to Sulawesi in the Molucca Sea. Subduction has ceased along sections where continental crust is present. This interpretation is consistent with the geology and gravity anomalies in the area. Near the eastern Philippines the westward subduction of the Philippine Sea plate occurs (1) along the Philippine trench and (2) in a localized zone near the western edge of the Benham rise. The Philippine Islands are therefore flanked in the east and west by active but disjointed subduction systems. Left lateral strike slip faulting has been deduced near one end of several of these active trench systems and suggests movement on transverse features. Seismic activity on the Philippine fault is concentrated in the zone between 10°N and 15°N and appears to be due to stresses generated by opposing movements of the Philippine and Eurasian plates which are not released in underthrusting. Fault plane solutions of shallow earthquakes associated with the Philppine fault show left lateral strike slip motion consistent with field observations. Our study suggests that the extent and magnitude of earthquake activity on the Philippine fault forms one component of movement between the Eurasian plate and the Philippine Sea plate.

The principal purpose of this study is to report the existence of a zone of extremely low compressional‐wave velocities in the uppermost mantle beneath most of the Lau basin, an interarc basin located west of or behind the Tonga Island arc. Velocities beneath the basin appear to be as low as 7.1 km/sec. In contrast, times of P and S waves traveling beneath and parallel to the Tonga‐Kermadec ridge indicate velocities of 8.45 and 4.75 km/sec, respectively. Although the lateral boundaries of the zone of low velocity beneath the Lau basin are not well defined, they coincide approximately with the boundaries of the zone of high seismic‐wave attenuation existing beneath the Lau basin. The large difference (up to 15%) between P wave velocities beneath the Lau basin and those in areas adjacent to it probably requires partial melting in the upper mantle beneath the Lau basin. The P and S velocities measured parallel and approximately perpendicular to the Tonga trench do not differ significantly and hence provide no evidence for anisotropy in the Pacific lithosphere.