U.S. Hikes Quakes’ Magnitude
The U.S. Geological Survey on Thursday upgraded the magnitude of the two earthquakes that rocked the Imperial Valley last week, explaining that Richter scale readings are not always the most accurate.
In a statement from its Reston, Va., headquarters, the survey upgraded the size of the Nov. 24 quake from 6.3 to 6.6 and that of the temblor the previous afternoon from 6.0 to 6.2.
The larger quake, centered near the Imperial County town of Westmorland, injured at least 94 people in California and Mexico and caused about $4 million in damages.
Tom Heaton, head of the survey’s Pasadena office, said the upgrading was based on averages of readings from more seismographs than were used for the original estimates. In fact, he explained, the revised figures were not even related to Richter scale magnitudes, which through common usage have become a familiar measure of an earthquake’s size.
The revisions were based on “surface wave magnitudes,” which is different from Richter scale readings but nevertheless yield magnitude measurements, he said.
The same procedures, in part, prompted seismologists to downgrade the size of the Oct. 1 Whittier temblor from 6.1 to 5.9.
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