PNSN Strong-Motion Recordings: 2002 Denali Earthquake

As part of the Advanced National Seismic System (ANSS), we are starting to install large numbers of strong-motion seismograph (accelerograph) stations to map ground amplifications from earthquakes that are too large to be recorded on our normal seismographs. The strong-motion instruments require relatively strong ground shaking for it to be properly recorded, but the Denali earthquake produced seismic waves that moved the ground 20 cm in Seattle. We recorded these waves on 46 strong-motion instruments throughout the Puget Lowland, but concentrated primarily in the Seattle area. Most of this energy was in the 0.05 to 0.3 Hz frequency range (20 to 3 second periods).

Comparing the ground motions to that at bedrock sites, we see that the Seattle basin amplifies these long-period seismic waves, resulting in much larger ground motions over the basin. The shaking also lasted longer over the basin, with shaking lasting nearly 6 minutes (300 sec).

One of the effects of the amplification of the seismic waves by the Seattle basin was the initiation of water waves that were large enough to damage houseboats along the shores of Lake Union. These waves were likely seiches, or sloshing, in which waves generated by the motion of the sides of the lake bounced back and forth across the lake, creating stronger waves than might otherwise be expected.

Results from our analysis of the strong-motion recordings of the Denali earthquake were published in Geophysical Research Letters:

Barberopoulou, A., Qamar, A., Pratt, T.L., Creager, K.C., and Steele, W.P., 2004, Local amplification of seismic waves from the Denali Earthquake and damaging seiches in Lake Union, Seattle, Washington, Geophysical Research Letters, v. 31, L03607, doi:10.1029/2003GL018569. (pdf)