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Defining an Earthquake's Magnitude

Story by
Nancy Joseph

The first reports following the Nisqually earthquake estimated it as a magnitude 6.2. Before long, the quake was described as a 7.0 magnitude. Then the figure dropped to 6.7, and finally 6.8.

Why the conflicting magnitude estimates?

Each estimate was based on different data, explains William Steele, coordinator and director of information services for the UW Seismology Lab.

The first estimate of 6.2 came from the Alaska Tsunami Wave Canter, which has seismometers located all over the world. Then scientists at the UW Seismology Laboratory ran a new program they had developed, which uses strong-motion seismometers in the Puget Sound region to estimate magnitude. That program led to a 6.7 estimate.

Next the National Earthquake Information Center in Colorado came up with its own estimate—7.0—based on data from its 60 stations located throughout the U.S.

“At that point, the UW Seismology Lab started consulting with the National Earthquake Information Center to come up with a definitive magnitude,” says Steele. “Together we concluded that 6.8 was most accurate. That became the final figure.”