One of the surprising results from the SHIPS99 experiment is how the Seattle basin affects ground shaking at high frequencies (greater than 7 Hz). As discussed earlier, the Seattle basin amplifies seismic waves by factors of 12 to 16 at low frequencies (0.33 Hz; 3 sec periods). This amplification is caused by the waves being amplified in the soft sediments, plus focusing and trapping of energy. These effects continue to amplify seismic waves at 1 Hz and 3 Hz, though to a lesser dgree than at the lower frequencies (graph below).
At higher frequencies, like the waves emitted in small, local earthquakes, the seismometers over the Seattle basin showed smaller amplitudes than at the bedrock sites at the ends of the line of seismometers (plot to left). When these ground motions are plotted by frequency, we found that the Seattle basin decreases the amplitude of seismic waves above about 7 Hz.
Attenuation is the reason shaking decreases at higher frequencies at sites over the basins. The seismic waves lose energy because they are expanding (like waves from a pebble thrown in a pond) and because the waves cause motion, which is decreased by friction. The friction is greater for higher frequencies (the wave motion is faster), so the attenuation is also greater at higher frequencies. Thus, the basins absorb high frequencies more than low frequencies. Above about 7 Hz, this absorption counteracts the amplification caused by other factors described earlier (shaking), resulting in little or no effect from the basin at high frequencies.
Results from the 1999 SHIPS experiment were published in the Bulletin of the Seismological Society of America:
Pratt, T.L., Brocher, T.M., Weaver, C.S., Miller, K.C., Tréhu, A.M., Creager, K.C., and Crosson, R.S., 2003, Amplification of seismic waves by the Seattle basin, Washington State, Bulletin of the Seismological Society of America, v. 93, p. 533-545. (pdf)
