Scientists have found evidence that ocean tides and the moon's gravitational force may combine to trigger earthquakes in some regions of the world.
The research shows a strong statistical correlation between lunar influence and large earthquakes in southern California. This effect, the scientists say, will next exert its strongest influence in November, 1987, creating a "high potential" for a major earthquake along the San Andreas and related faults in California.
"We're not predicting the date of the next earthquake," cautioned Dr. Steven Kilston, an astrophysicist with the Hughes Aircraft Co. "But our study shows there are periods when major earthquakes may be more likely to occur in the region."
The scientists emphasized that earthquakes result primarily from the release of accumulated geologic stress along faults in the Earth's surface, not from lunar and tidal forces. But Kilston said in a telephone interview that these forces "apparently combine to trigger the release of stresses that cause quakes."
Kilston and co-author Dr. Leon Knopoff, an internationally respected geophysicist and senior faculty member at UCLA, published their findings in the current issue of the British science journal Nature.
The study found statistically significant associations between the occurrence of large earthquakes in a narrow geographical region of southern California and the 12-hour, two-week and 18.6-year phases of the moon's gravitational effects on the Earth. These associations did not appear with smaller tremors or aftershocks, the researchers found.
"A literal extrapolation of our observations," the study said, "implies that during a window of a few years' width astride this date November, 1987 , at times near full or new moon, and near sunrise or sunset, one might be more likely than otherwise to observe one or more large earthquakes in southern California."
Dr. David Hill, a geophysicist with the U.S. Geological Survey (USGS) in Menlo Park, Calif., called the study "fascinating."
"Scientists have tried to establish these correlations before, with mixed results," Hill said. "If this study holds up, it looks as if it is going to be very useful for earthquake prediction."
Most other studies that sought to show tidal and lunar gravitational effects have been unsuccessful. Both Knopoff and Thomas Heaton, a senior USGS researcher in Pasadena, Calif., earlier published research that discounted any significant link between phases of the moon and earthquake probability.
Previous studies looked at large numbers of earthquakes throughout the world, but the study published last week was restricted to major earthquakes in a narrow geographical region of California where the San Andreas fault is oriented approximately northwest-southeast. The region extends about 250 miles north from the Mexican border between 33 and 36 degrees north latitude.
Kilston said faults oriented in different directions would not be subject to the effect. Previous studies that took into account all earthquakes regardless of fault orientation would not be expected to show a significant correlation, he said.
Kilston said he hopes to analyze earthquakes and lunar phases in other areas of the world, such as Japan and China, where fault orientations may be similar to those studied in California.
Prof. Lynn R. Sykes of Columbia University said in an interview that the geologic stress caused by the moon's gravitational effects "may be a kind of last straw." He agreed that such external forces could be a factor in the origin of earthquakes and may affect their timing.
"There've been a lot of quack estimates in the area of lunar correlation," Sykes added. "But Knopoff is widely recognized as one of the world's foremost experts on the statistical aspects of earthquakes. I would give the report great credence."
The research was funded in part by grants from the National Science Foundation's Earthquake Hazards Mitigation Section.