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10/01/1998 07:00:00

Caltech Question of the Month: Is there any such thing as "earthquake weather"?

Submitted by Maureen Castro, Oakland, California, and answered by Kate Hutton, Seismologist, Caltech.

There is a popular notion that earthquakes happen more often in certain kinds of weather. Unfortunately, the description of the preferred weather varies geographically and with the person providing the description.

Traditionally, earthquake weather was still and sultry. According to Aristotle, earthquakes were caused by winds trapped underground. Less wind above the surface must mean more below, and hence earthquakes were more likely. In California, however, which frequently has hot weather, earthquakes are often associated with the Santa Ana condition. In particular, the Northridge quake, "the earthquake" that is currently on people's minds, happened on an unseasonably warm day. Case proven, right?

In actuality, earthquakes are caused by the accumulation of strain in the crust due to the motion of tectonic plates. Most hypocenters (the place where the quake starts, directly below the epicenter) occur at least five miles below the surface, for large quakes. What is going on in the atmosphere simply does not affect conditions at that depth. Weather doesn't even affect the temperature in a well-designed wine cellar!

Besides, we have to remember that Alaska has several times the number of earthquakes that we do here. In Alaska, earthquake weather is cold and snowy!

As an aside, weather could very easily influence the scale of the disaster caused by an earthquake. Consider the effect of Santa Ana winds on fires and the effect of rain on the ubiquitous California mudslide in your worst-case planning scenarios.