State Temblors Not Linked to Mexico Quake

The geological forces along the Pacific Ocean’s floor that were responsible for Thursday’s massive earthquake in Mexico have no relationship to those that cause temblors in California, earthquake specialists said Friday.

The occurrence of the damaging Mexican quake will have no effect on increasing or decreasing the potential for quakes in California, the scientists said. And the seismic wave from the quake that temporarily raised the entire state of California about one inch Thursday was a normal event and no cause for worry, they added. The quake registered 7.8 on the Richter scale, a measure of the total release of energy.

Geophysicists at the National Earthquake Information Center in Golden, Colo., on Friday placed the quake’s epicenter halfway between the cities of Manzanillo and Acapulco along Mexico’s Pacific coast, at 18.1 degrees north latitude and 102.3 degrees west longitude in the state of Michoacan. The exact location could be altered depending on data retrieved from Mexico in the next week or two, earthquake center geophysicist Willis Jacobs said, but it is not expected to change significantly.

The quake occurred along the Middle America underwater trench, where the sea floor plate is thrusting under a continental plate at a rate of six centimeters per year, Christopher H. Scholz of Columbia University’s Lamont-Doherty Geological Observatory, said Friday. The temblor was expected, though no prediction was possible because of a lack of data, Scholz said. The observatory’s work several years ago found that no quake had taken place in the area for as long as 200 years despite its active geologic nature.

Scholz said a 1973 quake measuring 7.5 occurred on the northern end of Thursday’s quake location, and that a 1979 quake measuring 7.6 took place on the southern end. That left a so-called seismic gap, he said, that Thursday’s quake filled. Such potential seismic gaps also exist in Alaska.

However, Scholz said the underthrusting that causes such quakes ends south of the mouth of the Gulf of California. California’s seismic activity results from two continental plates rubbing against each other horizontally, rather than one thrusting under another, he said. There is no connection between the two areas, Scholz said.

Shock waves from Thursday’s quake caused California to vibrate up and down about an inch, UC Berkeley seismologists said Friday. Such waves are normal from large quakes around the world, however, and are the means by which remote monitoring stations determine approximate energy releases. The waves have no relationship to local quake activity, Berkeley’s Robert Uhrhemmer said.