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More than 250 strong-motion accelerograph stations were triggered by the Whittier Narrows, California, earthquake of 1 October 1987. Considering the number of multichannel structural stations in the area of strong shaking, this set of records is one of the more significant in history. Three networks, operated by the U.S. Geological Survey, the California Division of Mines and Geology, and the University of Southern California produced the majority of the records. The excellent performance of the instruments in these and the smaller arrays is attributable to the quality of the maintenance programs and their funding and personnel requirements. Readiness for a magnitude 8 event is directly related to these maintenance programs. Prior to computer analysis of the analog film records, a number of important structural resonant modes can be identified, and frequencies and simple mode shapes have been scaled. The structural records form a basic performance measurement for comparison with larger earthquake response in the future.

The strong-motion instrumentation network operated by the U.S. Geological Survey (USGS) in cooperation with other agencies in the central and eastern United States consists of approx imately 140 triaxial accelerographs located at 68 stations in 25 states east of the Rocky Mountains. This network is largely the result of programs of the U. S. Army Corps of Engineers (COE) to monitor strong ground shaking and structure response at COE dams and the Veterans Administration (VA) to measure base motions of large hospital buildings. In addition, the National Science Foundation (NSF) is supporting the USGS in development of accelerograph networks to obtain ground motion measurements in seismically active areas. Other organizations, including the Lamon-Doherty Geo- logical Observatory (LDGO) and numerous power utilities with nuclear generating units, have established small strong-motion arrays in this region. More than a dozen accelerograms have been recovered from instrumentation in the central and eastern United States since LDGO obtained the first of several records from a series of northern New York earthquakes in July-August 1973. The maximum reported acceleration (0.076.g.) for this region was recorded during the New Madrid earthquake of June 13, 1975 (magnitude 4.3) at an epicentral distance of about 10 km. As a result of recent studies of the recurrence of strong ground motion, the USGS will install about 36 accelerographs in the New Madrid seismic zone; this instrumentation will comprise a grid network with station spacings ranging from 10 to 25 km. The primary objective of the network, which is supported by NSF, is to obtain data for use in the investigation of the spectral characteristics and attenuation of strong ground motion.

The Whittier Narrows earthquake sequence (local magnitude, M_L = 5.9), which caused over $358-million damage, indicates that assessments of earthquake hazards in the Los Angeles metropolitan area may be underestimated. The sequence ruptured a previously unidentified thrust fault that may be part of a large system of thrust faults that extends across the entire east-west length of the northern margin of the Los Angeles basin. Peak horizontal accelerations from the main shock, which were measured at ground level and in structures, were as high as 0.6g (where g is the acceleration of gravity at sea level) within 50 kilometers of the epicenter. The distribution of the modified Mercalli intensity VII reflects a broad north-south elongated zone of damage that is approximately centered on the main shock epicenter.