At 4:35 a.m. Pacific time on June 29, the sea floor off Oregon gave a hard shrug. The U.S. Geological Survey put the quake at magnitude 5.5, centred about 135.5 miles west of Bandon, at a depth of roughly 6.2 miles.
Almost nobody on shore noticed. By the time the USGS tallied reports, seven residents had said they felt it. NBC16 noted that no one along the southern Oregon coast had reported shaking at all, and the Register-Guard wrote that coastal residents likely did not feel it.
No tsunami followed. An early National Weather Service message carried a slightly lower figure of 5.4 and said a tsunami was not expected.
A cluster, not a single jolt
The big quake had company. A magnitude 3.9 shock followed at about 5:47 a.m., and a 3.4 at 6:38 a.m., both in the same stretch of ocean floor, according to the Register-Guard.
All of it happened along the Blanco Fracture Zone, an active fault system of approximately 200 miles on the Juan de Fuca Plate, off Oregon and Washington. Here the Pacific and Juan de Fuca plates slide past one another rather than colliding. Hundreds of earthquakes strike the zone each year, and many are too small to register as anything but a squiggle on a seismometer.
What scientists told local reporters
Worry about the Cascadia Subduction Zone comes quickly to mind for people on this coast. Brian MacMillan, chief meteorologist at FOX 13 Seattle, addressed it directly: “People always get concerned about this, because of the Casscadia Subduction Zone, but this is actually not along that fault.” Near Brookings, he added, quakes of about this size are typical.
Leland O’Driscoll, director of the University of Oregon’s Hazards Lab, took the same view. “It is normal to see this,” he told The Register-Guard, saying activity like this shows up on the Blanco Fracture Zone once or twice a year. “People should not be worried. It’s far enough from land that it should not affect the Cascadia Subduction Zone.”
Distance also decided whether phones would buzz. O’Driscoll explained that a quake above magnitude 5.0 closer to shore would trigger ShakeAlert, the USGS system that warns cellphones before damaging shaking arrives. This one was too far out.
Aftershock odds
For families who live near the water and want numbers, the USGS aftershock forecast gave them. Over the following week, the chance of at least one aftershock of magnitude 5.0 or greater stood at 3%. For magnitude 4.0 or greater it was 18%, and for magnitude 3.0 or greater, 59%.
The USGS asks anyone who did feel the quake to report it on its website. The agency describes this as citizen science.
Sources
- Magnitude 5.5 earthquake strikes off Oregon coast | FOX 13 Seattle
- Multiple earthquakes rumble off Oregon coast in Blanco Fracture Zone – Eugene Register-Guard (via AOL)
- 5.5 magnitude earthquake rumbles off Oregon coast; no tsunami expected – KPTV
- National Weather Service says no tsunami threat after 5.5 quake off Oregon coast – NBC16
