Study finds source of tremors

WALNUT CREEK — Rumbles deep underground are caused by water being controlled by the sun and moon, UC Berkeley seismologists have found in a new study that could lead to a better understanding of earthquakes.

The study of a portion of the San Andreas fault revealed that underground fluids move like the tides, the scientists wrote in an article published Wednesday in the journal Nature. Geologists had long wondered what caused the frequent rumbling 15 miles below the surface, said co-author Roland Burgmann, a Berkeley professor of earth and planetary science.

“People had looked for those kinds of relationships for decades,” said Burgmann, who wrote the paper with seismologist Robert Nadeau and doctoral student Amanda Thomas. “Now, with these tremors, there’s a very strong relationship.”

Highly pressurized water essentially lubricates certain faults, including the San Andreas in California, far below the portion of the fault that causes measurable earthquakes, scientists found. Geologists previously were not sure whether water existed at that depth, Nadeau said.

The relationship between the deep tremors and earthquakes remains unclear, he said. Scientists have measured a burst of the tremors before at least one quake, Nadeau said, although it is not clear whether such measurements can be used to predict earthquakes.

The discovery will help scientists better understand faults, Nadeau said.

“People didn’t know how to look for (tremors) before,” he said, calling the discovery “a Rosetta stone” that will help translate the rumbling into more useful data.Among the next questions: Are the tremors increasing stress on a “locked” portion of the San Andreas north of San Juan Bautista?

Seismologists believe that stretch of the fault may be overdue for a large earthquake.

The group measured the tremors near the seismically active town of Parkfield, where seismologists have spent years tracking the movement of the San Andreas. The deep, low-frequency shaking does not register a magnitude on seismographs, and the signals showing up on the measuring devices long were thought to be background noise — such as ocean waves or traffic.

Though scientists noted that a major 2002 Alaska earthquake set off deep tremors on other parts of the planet, it was not previously known that the much more minute forces of the sun and moon could have a similar effect, said Kenneth Creager, a professor of earth sciences at the University of Washington.

“Seeing that (underground water) is sensitive to even smaller stresses is significant,” he said, adding that he’s curious to find out how the phenomenon is related to earthquakes. “They could trigger (temblors), or they could help us predict an earthquake.”