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Engineering seismology consists of those branches of seismology that have application to the aseismic design of structures. The expression "earthquake engineering" applies primarily to the design problem itself, but the two terms are sometimes used interchangeably.

Engineering semismology experienced its most rapid development in Japan, and present practices in aseismic design are based largely on Japanese research. In recent years the Coast and Geodetic Survey filled a serious void in all such programs by establishing a network of strong-motion seismograph stations in the Pacific Coast area to register damaging ground and building vibrations. This yielded new information on the nature and magnitude of such motions and inaugurated new approaches to the aseismic-design problem.

Former efforts to associate various grades of earthquake intensity with acceleration alone are giving way to more logical relationships involving the frequencies and energies of seismic waves. The wide ranges of intensity and ground motion measured on different types of geological formations (under otherwise similar conditions) are more than substantiating earlier findings pointing to the importance of the foundation factor in earthquake-engineering problems. New interpretations are being made of intensity-distribution maps, and isoseismal lines may be replaced by efforts to evaluate basement rock and surface intensities quantitatively using both instrumental and noninstrumental data. Other phases of the seismic-risk problem are also discussed.

Steady progress is being made in determining the maximum lateral forces that specific earthquake motions impress on structures and in overcoming the mathematical bottlenecks associated with the problem. The so-called "earthquake spectrum" appears to be the engineer's answer to the former problem, but effort is still required to derive the maximum benefit from this analytical tool. A spectrum shows the maximum theoretical responses of multi-period oscillators to a recorded earthquake motion. Some examples are cited of the use of seismological techniques in solving other types of vibration problems including blast vibrations.

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