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PARKFIELD, Calif., July 9 (UPI) -- U.S. scientists say they observed changes in seismic wave speeds before two small California earthquakes -- a finding that might lead to quake forecasts. Study co-author Paul Silver of the Carnegie Institution said the discovery was made at the San Andreas Fault Observatory at Depth, located halfway between Los Angeles and San Francisco. The observatory consists of two holes drilled into the fault zone. The researchers told the BBC they generate seismic waves deep in one hole and then time their arrival at a seismometer in the other hole. The speed of the waves varies due to stress-induced cracks opening and closing in the rocks.