2002 Seattle SHIPS Experiment

The recordings of the Nisqually and Chi-Chi earthquakes led us to undertake more systematic measurements of ground motions over the Seattle basin during earthquakes. In 2002, we installed 87 seismographs across the Seattle basin along east-west and north-south lines, plus a grid in the city of Seattle. We kept the seismographs working from January to May, during which time we recorded strong arrivals from large earthquakes in Afghanistan (M7.4), the Marianas (M7.1), Taiwan (M7.1), and the Philippines (M7.5). We analyzed the S-wave arrivals from these earthquakes.

The maps below show the strength of ground shaking at different frequencies, with the sizes of the gray dots equal to the amplification at the site. Comparing the recordings we made at our sites with recordings from stations we had in the Olympic Mountains and the Cascade Range (note small dots), we again see that the ground shaking was largest over the east half of the Seattle basin. The largest amplifications are again at about 0.3 to 0.5 Hz, as we saw in the 1999 SHIPS results, with maximum amplifications greater than 13. The amplifications decrease in areas where bedrock is shallow to the north and south of the Seattle basin. The data show an abrupt decrease in amplification south of the Seattle fault.

At 0.09 Hz (11 second periods), amplifications are moderate with a maximum of about 7. The largest amplifications are again over the east half of the Seattle basin. Most sites have amplifications of less than 3.

At higher frequencies (0.5 to 0.7 Hz; 2 to 1.4 sec periods), amplifications are only slightly less than at 0.3 Hz. The largest amplifications were again over the east side of the basin, with maximum amplifications of about 12. Amplifications to the south of the basin are generally smaller, but the decrease is gradual across the Seattle fault.