Offshore Earthquakes

On October 22, 2018, three fairly large earthquakes (two M6.5 and one M6.8) struck offshore southwest of Port Hardy, Canada. They occurred at 9:39 p.m., 10:16 p.m., and 10:22 p.m. PST, respectively. A notification was sent to my phone after each one.

At that time, seeing one M6 earthquake didn’t phase me much. Seeing three this large back-to-back had my heart racing. I didn’t sleep well that night! This page was born out of that night’s anxiety.

The December 2021 earthquake swarm, shown on the left here, included a whopping 18 earthquakes with magnitudes ranging between 5.0 to 5.8.

Like most, I tried to figure out if I should be concerned. The swarm became the best teaching moment I’ve experienced regarding the risk level of “smaller” offshore earthquakes.

The Pacific Northwest Seismic Network (PNSN) posted a great write-up about the activity in its December 2021 Seismo Blog. Further discussion can be found on Facebook’s Pacific Northwest Earthquake Discussion Group. I highly recommend joining this group, as its admins are some of the nation’s top field experts. The most helpful comment I read (there were many that I found helpful) came from UW Professor and Director of PNSN Harold Tobin.

“It’s all about the locations and magnitudes. The Blanco fault lies entirely offshore and at a remove from the Cascadia subduction fault. Even an M7 out there wouldn’t trouble me much (it’d be interesting seismologically but not really a hazard). If we had a bunch of 5+ actually located right on the sub-zone fault, well, that would be cause for genuine concern.” You can read through the post here.

Because of the type of fault it occurred on and its distance from the Cascadia Subduction Zone, the swarm did not pose a threat to the Pacific Northwest. Phew!

Hopefully, reading through the resources hyperlinked above will help you worry less about these offshore earthquakes.

Image on the right from the USGS ArcGIS Maps.

Could Smaller Earthquakes Relieve Stress?

While some people worry these offshore earthquakes will increase the risk, others wonder if they could relieve the built-up stress. According to the Government of Canada, smaller earthquakes are unlikely to lower the risk level. Here is a print screen from their earthquake Q&A page.

A Little More History

Offshore earthquakes in the Pacific Northwest are common. Between January 1, 1980, and December 31, 2021, 48 earthquakes with a magnitude 6 or above struck offshore. Five of those earthquakes fell in the M7 range near the Mendocino triple junction where the Gorda plate, the North American Plate, and the Pacific Plate all meet (very active region!), so even large earthquakes near the southern end of the CSZ don’t mean a larger earthquake is imminent.

USGS Query

Offshore earthquakes Between 1/1/1990 & 12/31/2021

The region has experienced many of these “smaller quakes” (i.e., not M8 or 9.0 megaquakes), ranging from 23 quakes in 1993 to a whopping 156 in 2008! The year 2021 came in a close second, with 87 of the 144 earthquakes occurring between December 7th and 10th.

USGS Query

M4+ earthquakes/month from 1980-2020

Divided into 4 “decade” charts for easier viewing. Data derived from running USGS queries

Anything smaller than a magnitude 4 earthquake is so commonplace that I have turned off notifications for them in my earthquake alert phone applications. That said, curiosity had me running the data for magnitude 3 range earthquakes. Here are the yearly (top chart) and monthly counts (second chart) from 1990 through 2020. In them, we see that the past twelve years, while busy, look quieter than the previous nineteen.

USGS Query

I’ve added this page because I think it’s important to know when to be worried. Yes, history shows us we may not be far off from a major earthquake, but seeing articles in the paper about these smaller offshore earthquakes which are not occurring on the Cascadia Subduction Zone shouldn’t be cause for alarm. Use the headlines as reminders that we live in Earthquake Country. Being 2-weeks-ready is just smart.