For most folks, quake played a different tune

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Think of last Wednesday's earthquake as a very loud moment in a musical performance, like the cannon blast in Tchaikovsky's "1812 Overture."

Now think of the cannon sitting on the southern end of Puget Sound, at the magnitude-6.8 quake's epicenter near the Nisqually River Delta.

This map linked below is a seating chart for those of us who took in the performance.

Scientists will now spend the following weeks, months and even years trying to figure out why the big shakeup was particularly earth-moving for some but relatively light for others.

The map shows how shaking was particularly intense around the epicenter but then had varying effects farther away. In yellow areas, pictures and books fell and masonry cracked. In orange areas, heavy furniture moved, waves disturbed otherwise calm ponds, and people fell to the ground. In red areas, houses moved on their foundations and masonry was more severely damaged.

In theory, shaking should decrease the farther one is from the quake's source. In reality, judging from early numbers being analyzed at the University of Washington seismology lab and elsewhere, some seats in this audience farther from the source were shakier than others closer in.

One measure of this is the horizontal acceleration felt by 44 strong-motion seismometers, not all of which appear on the map. Represented by the blue dots, many of these seismometers were installed just last summer to gauge the lateral and vertical forces of an earthquake.

As the numbers show, Boeing Field was pushed sideways by an acceleration that was 27 percent of the force of gravity, a standard unit of earthquake energy. Seward Park, at the south end of Lake Washington, was pushed at more than 30 percent of gravity.

Yet sites in Tacoma, which sits significantly closer to the epicenter, were shaken by less than 7 percent of the force of gravity.

To account for the discrepancy, scientists will need to get a better understanding of how the region's geology, topography and soil conditions might focus or absorb quake energy, said Ruth Ludwin, a UW seismologist. They will also need to look at the full range of the forces, including varying wave frequencies at different times during the quake, said Tom Pratt, a U.S. Geological Survey geophysicist.

In other words, they will be looking at several measures of music filled with many notes.

--Eric Sorensen