Abstract

The high‐frequency spectral rolloff of recorded ground acceleration during earthquakes is commonly modeled by either of the two ad hoc high‐cut filters, fmax and kappa, which have been used interchangeably. The physical origin of the cutoff they produce is still debated, although it is more often attributed to the site effect. A natural extension of the earthquake source time function, radiating an omega‐n spectrum into the far field, to noninteger n through the gamma function allows the spectral fall‐off to be intermediate between the omega‐square and omega‐cube shapes, providing natural high‐cut filtering exclusively as a source effect. The difference between applying the omega‐2.5 source spectrum and the κ filter to simulate the high‐frequency decay is no greater than the difference between applying the κ and its equivalent fmax filters. The conceptual advantage of employing the unfiltered omega‐n source spectrum, with real n>1, to represent the spectral decay of strong ground motions, is clear. This approach attributes the high‐frequency behavior and its variability to a well‐defined form of slip on an earthquake fault, making it unnecessary to introduce an artificial additional filter of unclear physical nature.

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