More than 100 earthquakes have rocked the off-shore waters of the Pacific Northwest in recent days, causing some concern among those along the seismically active Washington and Oregon coast that something larger was looming. While the weekend started on a shaky note, it appears the new work week is starting with a pause, with no earthquakes detected in this region yet today.
A total of 105 earthquakes struck off the coast of Oregon in the last 7 days, including a 4.4 which struck yesterday morning. Most of the earthquakes in this swarm have ranged in intensity from 3.2 to 5.8 magnitudes. These earthquakes are part of a swarm that began December 7. The first earthquake in the series was a 4.2 magnitude shaker which struck at 5:20 am local time. In the first 24 hours of the event, there were more than 55 earthquakes with a magnitude of 3.4 or greater. The greatest in the entire swarm was a 5.8 thus far.
There has been no tsunami threat from the Pacific Northwest swarm. USGS says, “Because these quakes are generally small to moderate in magnitude and have a strike-slip focal mechanism (lateral or horizontal motion, not vertical), they are unlikely generate tsunamis. However, if they become larger, they might cause some increased local waves.” In addition to USGS, the National Weather Service Tsunami Warning Center has also issued their own bulletins on the stronger quakes in the swarms, adding there was no imminent threat of tsunami.
According to USGS, the swarm is located roughly 200 miles west of the Cascadia Subduction Zone and is ongoing along the Blanco Fracture Zone, which is a strike-slip fault system on the edge of the Juan de Fuca plate.
This isn’t the first earthquake swarm to strike this region in recent times. In 2003, a swarm struck within 31 miles of today’s swarm. The 2003 swarm included a magnitude 6.3 earthquake along with 4 other earthquakes that had magnitudes of 5.1 – 6.7. The 2003 swarm only lasted 24 hours; scientists aren’t sure when the ongoing swarm will end.
According to USGS, an earthquake swarm is a sequence of small to moderate earthquakes within a relatively small area that don’t fit the pattern of a mainshock-aftershock sequence. Swarms are usually short-lived, but can continue for days, weeks, or sometimes months, and often recur at the same locations.
The last earthquake from this swarm occured yesterday; if no earthquake occurs today and tomorrow, the swarm will be considered over. One earthquake struck yesterday, none the day before, and 18 the day before that day.
Scientists will continue to monitor the area for any change in seismic behavior; the Tsunami Warning Center will also monitor ongoing earthquake activity and will issue a tsunami advisory if needed.