The following article is from our associate Steve Jones at Alabamaquake.com
“A heat wave in San Francisco is rare, but when one occurs, it usually lasts three days… Tuesday, April 17, 1906, was the first of three such days, and by evening it was still breathlessly hot… I heard tales that horses in their stalls were restless much of that night. This, linked with the unseasonable weather – “earthquake weather” – some were to call it, would be remembered by the superstitious as portents of impending disaster… (on Wednesday morning, April 18th) I was awakened by a violent shaking of the house accompanied by a roar and the sound of cracking wood… I could hear the crash of falling bricks outside and heard my father say, “This is certainly a real earthquake.” (from MEMORIES OF THE SAN FRANCISCO EARTHQUAKE AND FIRE OF 1906, by Howard T. Livingston)
Folklore applies the term “earthquake weather” to a sultry, ominously uneasy period said to precede large earthquakes. Scientists have generally dismissed suggestions that weather could have anything to do with the tremendous underground forces associated with an earthquake. Yet, many people who have ample experience with seismic events insist that quakes and weather are somehow connected.
“Earthquake weather” is signaled by close, oppressive conditions: hot and humid weather, with thin, wispy clouds appearing overhead and no predicted rain. Stories of “earthquake clouds” describe long, straight cloud forms that appear for days over a fault before it finally shakes. One theory is that a squeezing motion of the earth prior to a quake generates electric currents that decompose trapped ground water and release hydrogen and oxygen ions into the air, with these ions then producing the clouds and the unusual calm characteristic of “earthquake weather.”
In the 4th century B.C., the philosopher Aristotle proposed that earthquakes were caused by winds trapped in caves. Small quakes were thought to have been caused by air pushing on the cavern roofs, and large ones by the air breaking through the earth’s surface. This idea said that because a large amount of air had been trapped for some time underground, the “earthquake weather” would be hot and calm before a quake. And ever since, many people have pointed to various signs of the weather as being linked to earthquakes.
Some have drawn conclusions that more quakes tend to occur in the winter than the summer, usually when there is a surge of unusually warm weather. Malcolm Johnston, a research geologist at the USGS Menlo Park California Science Center, said many earthquakes occur at depths of six miles or greater – far removed from the surface weather conditions. “People have argued that you’re heating the earth…but we’re talking no more than a couple of meters into the crust,” Johnston said. “Further down than that, you wouldn’t even know it’s a hot day outside.”
Another theory says that it’s not the temperature changes causing earthquakes, it’s that the high-pressure weather system creating the sultry, oppressive heat is pushing on the ground, disturbing the underlying faults. Johnston said that’s unlikely. Such a pressure change underground would be minuscule: on the order of one part in 100 million, he said. And he said that the increased downward pressure on faults would make it harder for an earthquake to occur, not easier.
The bottom line: there is no such thing as “earthquake weather”. Statistically, there is an approximate equal distribution of earthquakes in hot weather, cold weather, rainy weather, etc. While some may hold to the common misconception that earthquakes are more likely to occur during hot and dry weather (this idea dating to the ancient Greeks), earthquakes of course take place miles underground, and so are not influenced by surface weather conditions.
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