© Kitsap SunA map from the Pacific Northwest Seismic Network shows where the earthquake swarm is centered.
How long the shaking will continue is anyone's guess.
"Sometimes these swarms last years, sometimes they're over in an hour," Pacific Northwest Seismic Network Director John Vidale said Thursday.
The chances of a significant earthquake still remain low.
"The overwhelming odds are there is nothing damaging in our near future," Vidale said.
Though disconcerting to some residents, seismologists hope the ongoing earthquake swarm, centered between East Bremerton and Bainbridge Island, will reveal new information about the orientation of fault lines running beneath central Puget Sound.
The quakes have already offered some intriguing clues. Their depth for example — some 15 miles below the earth's surface — is below the level where the Seattle Fault is believed to lie, suggesting the activity could originate from another source.
"We just don't know," Vidale said, adding more research is needed to draw firm conclusions. "We're still figuring out what details we can decipher."
A magnitude-3.3 earthquake first rattled Bremerton on May 3, accompanied by a series of smaller quakes. Earthquakes of varying magnitude were recorded in the following days, fitting the description of what scientists call an "earthquake swarm," as opposed to a single large quake followed by diminishing aftershocks.
While dozens of earthquakes have been reported on the Seismic Network's website — a popular resource for earthquake tracking — Vidale said a more in-depth review showed more than 500 events had occurred. Only a handful would have been large enough to have been felt by residents in the area.
Contrary to a common perception, the miniature quakes probably aren't relieving pressure on the fault and lessening the chance of a more powerful earthquake. Vidale said the earthquakes recorded so far simply haven't been powerful enough to dispel the buildup of energy that would generate a larger event. Instead, earthquake swarms increase the probability of a larger earthquake taking place — though that possibility remains remote.
Seismologists believe the shaking near Bremerton has occurred along a thrust fault that runs east-west below Puget Sound in an area where a plate beneath Washington is grinding north into a plate beneath British Columbia. Vidale said it's not known what triggers earthquake swarms. Movement of water deep underground is one possibility.
Whatever its cause, Bremerton's earthquake swarm showed no signs of letting up last week. More than a dozen small quakes were recorded on the Seismic Network's website between Monday and Friday.
Experts say the swarm is a good reminder to be ready for the possibility of a larger earthquake.
"These little quakes should at least serve as a reminder of where we live and that we need to be prepared," the Kitsap Department of Emergency Management reminded readers on its Facebook page. "Have your kits and plans ready and practice those plans often."
Comment: Small earthquake shakes Seattle area days after swarm near Kitsap Peninsula