Fault study:
Instruments are lowered into ground to learn what tiny tremors mean.
Mysterious seismic tremors deep inside the Earth have puzzled earthquake scientists for more than a decade, so on Thursday scientists lowered a package of sensitive instruments into the San Andreas Fault to learn what those tiny quakes mean.
The most recent surge of underground tremors followed the destructive Napa earthquake in August. For 100 days in a row, wave after wave of them triggered seismometers 250 miles away near the tiny southern Monterey County towns of Parkfield and Cholame, said Robert Nadeau, a geophysicist at the UC Berkeley Seismological Laboratory.
At Cholame on Thursday, seismologist Peggy Hellweg, the operations manager at the seismological lab, and her colleagues lowered a compact array of three seismometers, known as the Tremorscope, into the ground through a concrete- encased borehole drilled 750 feet down.
One instrument was a highfrequency geophone that registers the faint, high- pitched noises of the tremors as the rocks underground grind along tiny hairline fault strands.
Another was a broadband package that, as Hellweg described it, “can measure every frequency from the deep burping of whales in the ocean to the high- pitched whine of a chainsaw.”
The third, she said, is a strong motion instrument that measures the ground- shaking of an earthquake from signals so powerful they can eclipse the signals of the other instruments.
The borehole with its instruments is located on a sandy knoll in the Cass Vineyard and Winery, one of the many new vineyards that have made the quake- prone region around nearby Paso Robles ( San Luis Obispo County) a burgeoning wine center.
The borehole is the first of four that will hold seismometers to gather data on the tremors beneath the region.
“Those tremors seem to be very sensitive to changes in stress in the deep Earth, so when there’s slip along the roots of a fault and a big earthquake is coming, the tremors just go crazy,” Nadeau said.
Nadeau has been pondering the nature of the tremors since they were first detected nearly 15 years ago near Parkfield, which touts itself as the “Earthquake Capital of the World.” Those tremors continued for 110 days.
That crossroads village is famed because at least seven large quakes with magnitudes of about 6 have struck there at an average of once every 22 years since 1857, when the greatest earthquake ever to hit California first ruptured the ground there.
Known as the Fort Tejon quake, it had an estimated magnitude of 7.9 — about the same as the more infamous San Francisco quake of 1906, but because virtually no one lived around there back then, it affected very few people.
The most recent quake occurred in Parkfield in 2004 with a magnitude of 6, and by then the area was filled with instruments on the surface and underground in an ongoing quake- detection experiment by the U. S. Geological Survey.
Three years earlier, however, one sensitive seismometer in a deep borehole in Cholame had recorded the first wave of the unexplained tremors. They are the ones that lasted 110 days and caught Nadeau’s attention, he recalled.
“We have seen this kind of tremor activity inside volcanoes when magma is rising inside them before eruptions,” Nadeau said.
“But in the tremors we’ve recorded, there’s no volcanism, and none of the seismic waves that are typical of earthquakes.”
Another puzzle is the fact that some surges of the tremors tend to precede significant earthquakes by a few years, as they did at Parkfield before the 2004 quake, while others follow similar- size quakes, as they did after the recent Napa quake, Nadeau said.
The seismic detectors, now functioning 750 feet down in a region where some kind of ground- shaking is virtually constant, are focused on solving that seismic mystery.
“When there’s slip along the roots of a fault and a big earthquake is coming, the tremors just go crazy.”
Geophysicist Robert Nadeau