Heavy Rainfall Can Cause Huge Earthquakes

And another large earthquake, a magnitude 6.4 temblor that rocked Taiwan in 2009, occurred only seven months after the area had been hit by Typhoon Morakot, which dropped 9.5 feet (2.9 meters) of rain in five days. Hurricanes are called typhoons in parts of Asia.
To put that in perspective, "that's about five times the average [annual] rainfall of San Francisco ... in five days," Wdowinski said last week at a meeting of the American Geophysical Union in San Francisco.
Quakes Triggered by Rain-Induced Erosion
To test the rainfall-earthquake link, Wdowinski dug through the past 50 years of earthquake and weather records for Taiwan, an island that experiences a lot of severe rainstorms and earthquakes.
He found that a magnitude 7.6 earthquake had struck in 1999, only three years after Typhoon Herb soaked Taiwan with 6.6 feet (2 meters) of rain.
Overall, his analysis revealed that Taiwan's large earthquakes—deemed as magnitude 6 and higher—were five times more likely to occur within four years after such storms than if the storms had had no effect.
The weight of the water itself does not trigger the earthquake—rather, it's the ensuing erosion from landslides, which subsequent storms steadily wash into the sea.
"There's less stress [on the underlying rocks], and it's easier for the fault to move," he said. "These are small changes, but are apparently enough to trigger the earthquake."