Scientists from Academia Sinica’s Institute of Earth Sciences (IES) and the Department of Terrestrial Magnetism of the Carnegie Institution of Washington have made the surprising discovery that slow earthquakes are triggered by typhoons—in eastern Taiwan, at least. The researchers published their findings in Nature (Volume 459 Number 7248) on June 11.

Slow earthquakes are non-violent fault slippage events that take hours or days instead of a few brutal seconds to minutes to release their potent energy.

Taiwan has frequent typhoons in the second half of each year, but is typhoon free during the first four months. During a four-year study period from August 2003 to August 2007, researchers identified 20 slow earthquakes that each lasted from several hours to more than a day. Eleven of the 20 slow earthquakes coincided with typhoons and were also stronger and characterized by more complex waveforms than the other slow events. The scientists discovered that the reduced atmospheric pressure caused by the typhoons unclamps the fault, triggering a slow quake.

"Typhoons reduce the atmospheric pressure on land, but do not affect conditions at the ocean bottom, because water moves into the area and equalizes the pressure," explained one of the coauthors of the Nature article, Dr. Selwyn Sacks, of the Department of Terrestrial Magnetism at Carnegie. "The reduction in pressure above one side of an obliquely dipping fault tends to unclamp it."

"These data are unequivocal in identifying typhoons as triggers of these slow quakes. The probability that they coincide by chance is vanishingly small," remarked another coauthor Dr. Alan Linde of Carnegie.

The researchers speculate that the reason devastating earthquakes rarely occur in eastern Taiwan is because the slow quakes act as valves, frequently releasing the stress along small sections of the fault, eliminating the situation where a long segment sustains continuous high stresses until it ruptures in a single great earthquake.

The current research is borne out of a cross-Pacific collaboration to monitor the crustal deformation in eastern Taiwan using borehole strainmeters. In 2003, earth scientists in California started the Plate Boundary Observatory (PBO), a geodetic observatory designed to study the three-dimensional strain field resulting from deformation across the active boundary zone between the Pacific and North American tectonic plates in the western United States. A similar observatory, the Plate Boundary Observatory in Taiwan (PBO-T), was setup to study the strain field in Taiwan over the plate boundary between the Eurasian and the Philippine Sea Plates.

The core instrumentation for PBO-T is a carefully designed and integrated network of borehole strainmeters (BSM) and Global Positioning System (GPS) receivers. The GPS network provides information about the total accumulated strain over distances of several kilometers.

"Borehole strainmeters can measure the strain changes of the rock on a site. Non-seismic released strain can also be observed by borehole strain data," explained the former director of the IES, Dr. Typhoon Lee, the aptly named Academician who initiated the cooperation.

"In this study, we monitored deformation in eastern Taiwan using three highly sensitive borehole strainmeters located about 5 to 15 km apart, installed 200-270 meters deep. These devices detect otherwise imperceptible movements and distortions of rock," explained the first author Dr. Chiching Liu.

The group now intends to expand their instrumentation and monitoring to different tectonic regimes of the same plate boundary in eastern Taiwan.

The publication entitled " Slow Earthquakes Triggered by Typhoons" Nature Volume 459 Number, 7248 (2009) can be found at the Nature website: http://www.nature.com/nature/journal/v459/n7248/full/nature08042.html The complete list of authors is: Chiching Liu, Alan T. Linde and I. Selwyn Sacks.