Thousands of people throughout the Puget Sound region felt a 4.8-magnitude earthquake Tuesday night centered between San Juan Island and Vancouver Island.
No major damage was reported, but seismologists are wondering about the role played by slow-slip tremors.
Slow slip is when the earth moves over several weeks, not in seconds as in an earthquake.
That happened right before the major 2011 earthquake off Japan, which caused a deadly tsunami and the Fukushima nuclear disaster.
John Vidale at the Pacific Northwest Seismic Network says there has been slow slip under Vancouver Island over the past week. He thinks it could have prompted the 4.8 quake.
"We wonder about that, since the slow slip does load the part of the subduction zone that we expect to deliver a magnitude 9 earthquake some day,” Vidale said. “But it's complicated, so it's something we're studying pretty carefully."
One tool that scientists are using is a system of sensors that can detect an earthquake right before it happens.
Vidale says the system worked Tuesday night.
"The system was running, and it issued a warning,” he said, “and in this particular case we could recognize the earthquake within a few seconds of when the first waves hit the surface."
He says eventually his team will use those extra 10 seconds or so to warn the public.
For now, the early warning system in Washington state isn't fully funded.