PHYSICAL PHENOMENA CONTROLLING HIGH-FREQUENCY SEISMIC WAVE GENERATION IN EARTHQUAKES

Several physical parameters may contribute to control the amount of high frequency energy in the spectrum of strong ground motions. These phenomena include the total amount of geological slip on the fault causing the earthquake, the slip rate (or repeat time) of earthquakes on the fault, dynamics of fault rupture, lithology of the fault zone, whether the fault is in an extensional or compressional environment, and geometrical factors including whether or not the rupture reaches the surface. In this paper, we review evidence that points towards the significance of these parameters, and present examples where they may play a role. These considerations are particularly relevant when considering the significance of ground motions from the three largest earthquakes that are well-recorded on strong motion accelerographs, and for all of which the peak accelerations were surprisingly low. The first was the Michoacan, Mexico earthquake of Sept. 19, 1985 (Mw=8.0), where peak accelerations at four stations on rock directly above the fault had peak accelerations of 13-17 percent of gravity. The second is th