PARKFIELD, Calif., April 13 (UPI) -- Earth tremors usually, but not always, are related to a volcano and such vibrations have been recorded in the San Andreas Fault near Parkfield, Calif.
Scientists believe the fault tremors may be related to activity at a subduction zone -- a place where one of Earth's constantly moving tectonic plates slips beneath another.
To determine whether the San Andreas Fault is moving with the tremors, scientists are installing instruments to measure the tremors' activity.
"Unlike the sharp jolt of an earthquake, tremors within Earth's crust emerge slowly, rumbling for longer periods of time," said Kaye Shedlock, a program director at the National Science Foundation. "Although not in this case, tremors are usually produced by magma moving in cracks or other conduits beneath a volcano."
Scientists said the rumblings are the first recordings of non-volcanic tremors in a deep borehole, providing geologists with data to better understand such mysterious underground movements.
