1999 “Dry” SHIPS Experiment

Serendipity plays a role in science, as long as we are prepared. In September of 1999, scientists from the USGS, the University of Washington, and the University of Texas, El Paso, installed 1008 seismometers in an east-west line across the Seattle Basin and detonated 38 underground explosions to measure the shape of the Seattle basin. The computers in most of these seismometers were programmed to record ground motions for only short time intervals when we detonated the explosions, but we left 29 of the recorders running continuously for all 4 days “just in case”. This foresight paid off when the magnitude 7.6 Chi-Chi, Taiwan, earthquake sent strong seismic waves into our recorders, giving us the first good look at how the Seattle basin amplifies seismic waves.

Looking at the seismic records, which are graphs of ground motion versus time (graph at left), we see that the ground shaking is much larger at seismometer sites located on the soft layers of the Seattle sedimentary basin. Also, the strongest shaking lasted much longer on the basin than at the surrounding bedrock areas.

When we examine the ground shaking at different frequencies, it becomes clear that the Seattle basin increases low frequency shaking during earthquakes. The strongest amplification occurs at 0.33 Hz (3 second periods), where the shaking is as much as 16 times stronger than at rock sites. The amount of amplification decreases at lower frequencies, although it is still amplified 4 times at 0.11 Hz (9 second periods). Amplification also decreases at higher 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)