Bulletin of the Seismological Society of America. Vol. 61, No. 2, pp. 343-356. April, 1971 RESPONSE ENVELOPE SPECTRUM AND INTERPRETATION OF STRONG EARTHQUAKE Gt~OUND ~/[OTION BY M. D. TRIFUNAC* ABSTRACT A multiple filter technique is developed using a single-degree-of-freedom lightly-damped oscillator as a narrow band-pass filter. It provides for a physi- cally simple approach to the analysis of accelerograph records from the point of view of structural engineering. The analysis of several typical accelerograph records indicates that a signifi- cant portion of strong earthquake ground motion consists of surface waves. It is concluded that the duration of intense shaking will be determined predominantly by the dispersion properties of the ground. INTRODUCTION An understanding of the phenomena associated with strong earthquake motion is important for many areas of research and application. This motion of destructive in- tensity endangers the safety of all man-made structures and has, on several occasions, led to complete or partial devastation of whole cities. The recording and interpreting of strong-motion data is also essential for various studies of earthquake mechanism, for high resolution of the patterns of earthquake energy release and, in general, for better understanding of the close field of earthquake ground motion. The basic information related to this phenomenon comes from strong-motion ac- celerographs. The first instruments of this kind were installed in the field some 40 years ago, and the first strong-motion record was obtained during the 1933 Long Beach, California, earthquake. Since that time, several hundred earthquakes have been re- corded, predominantly in California and Japan. Although the history of modern recording of teleseismic earthquake waves is not much older than the measurements of strong, near-field, ground motion, the nature of teleseismic waves and the great number of recorded shocks resulted in early interpre- tations of the different wave forms displayed in those records. This was not the case with the strong-motion records. The inhomogeneities and discontinuities in the Earth's crust lead to a wide variety of wave velocities. For distant recordings different frequency-wave components are dispersed and cause natural separation of various wave types. This separation, which helps in the record interpretation, is further enhanced by the great distances and hence the long travel times which allow dispersion effects to take place. Strong earthquake ground motion close to the source of energy release is less ob- vious to interpret. In principle, it is composed of essentially the same types of waves, but often they are not clearly separated in time because of the source proximity. In addition, high frequency motions, whose amplitudes decay rapidly with distance and so cannot be observed on most teleseismic records, are also present. Also source size, relative to the distance to the recording station, and the spatial and temporal distribu- tion of energy release become important. All of these complexities are probably the * Present address: Lamont-Doherty Geological Observatory, of Columbia University, Palisades, New York. 343