A new mystery faces scientists who study stresses that trigger earthquakes both large or small: Even the rise and fall of the ocean's tides are strong enough to trigger pulses of underground tremors that periodically send seismic faults slowly slipping beneath the northwest coast, quake researchers have found.
The scientists, led by a young researcher who will soon join the U.S. Geological Survey in Menlo Park, came upon their puzzling discovery after installing highly sensitive seismic detectors on the Olympic Peninsula in Washington and across Vancouver Island in British Columbia. Readings from the detectors revealed that twice a day when it's high tide, the strength of the faint tremors increases, and twice a day at low tide, those underground tremors grow fainter.
And because it's the moon's gravity, as well as the sun's, that causes the tides to rise and fall, those heavenly bodies are, in fact, exerting their legendary influence from afar to alter seismic activity deep beneath the Earth's surface.
But no one knows how or why gravity should reach into the Earth's deep interior, conceded geophysicist Justin Rubenstein, whose work and that of his colleagues appeared in Thursday's online version of the journal Science Express.
Tremors are like tiny earthquakes, but they can never be felt on the surface. Seismologists have long known that when volcanoes are about to erupt, the Earth shudders in tremors along cracks deep beneath the volcanic mountains as the molten magma rises.
But six years ago, Berkeley seismologists performing in-depth earthquake analysis along the San Andreas Fault near quake-prone Parkfield in southern Monterey County recorded episodes of similar faint tremors far beneath the Earth's surface.
