ABSTRACT
The USA is located on the Nor th American Plate which is adjacent to the Pacific Plate on the West Coast , and the Euras ian and African Plates at the Mid-At lant ic Ridge (Figure 7.1). Mos t ear thquakes have occurred in the vicinity of the bounda ry between the Pacific and N o r t h American Plates. The well-known San Andreas fault zone is a por t ion of this bounda ry where the Pacific Plate is sliding nor thwest past the N o r t h American Plate at a ra te of abou t 5-8 cm per year. Ear thquakes on the San Andreas fault typically occur a t relatively shallow depths of 5-40 k m below the surface, with varying rup ture lengths. In the 1906 San Francisco and 1857 Fo r t Tejon ear thquakes , the rup ture lengths were over 400 km. The Aleut ian Subduct ion Zone is another por t ion of this bounda ry where the Pacific Plate is being subducted under the N o r t h American Plate, creating ear th quakes a long the zone of contact (called the Benioff Zone) . Ea r thquakes a t the subduct ion zone have usually occurred at great depths . Ea r thquakes have also occurred in the interior of the plates, most likely caused by the bui ld-up of strains from pressures developed at the plate boundar ies . The mos t notable ones are the New Madr id ea r thquakes of 1811-1812 in the Mississippi River Valley and the Char les ton, South Carol ina, ea r thquake of 1886. Figure 7.2 shows the locations of the notable historic ear thquakes in the USA.
