Wake-up Call: 6.5 Quake a Reminder to Prepare


This morning's earthquake recorded at HSU. - FACEBOOK
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  • This morning's earthquake recorded at HSU.
Today’s magnitude-6.5 quake centered 100 miles off the coast of Ferndale has already produced a trio of aftershocks and more are likely to follow in the weeks to come, earthquake experts note.

The 6:50 a.m. quake was caused by an east-west strike slip on the Mendocino fault, which runs along the boundary of the Pacific and Gorda plates, according to Humboldt State University geology professor Lori Dengler.

It’s the North Coast’s hotbed of seismic activity, she said in an email to the Journal, and produced a magnitude-7 earthquake in September of 1994 that was located about 26 miles east of today’s epicenter.

Because this fault is moving in a horizontal direction, these quakes don’t pose a tsunami threat and are unlikely to trigger submarine landslides, which can also cause tsunamis.

Originally listed as a magnitude-6.8 by the USGS, today’s quake was later revised to a 6.5. The aftershocks have so far included a magnitude-2.4 at 8:24 a.m. on shore near Petrolia, a 5.2 at 8:32 a.m. located 107 miles west of Cape Mendocino and a 4.7 situated 36 miles west of Petrolia at 8:33 a.m.

“There is always a small chance that today’s quake could trigger activity on the Mendocino fault closer to the coast,” Dengler said, adding that that segment ruptured in 1994 so most of the accumulated strain may have been released. “There is also a small chance it could trigger fault slip in adjacent areas of the Gorda plate producing a quake similar to the February 1995 magnitude 6.6.”

Dengler notes that Gorda quakes located far offshore will be felt but are unlikely to cause damage.
Hundreds of people from Oregon, Humboldt, Del Norte and Mendocino reported feeling the quake, with most describing weak to light shaking.

This morning’s quake is yet another reminder that the North Coast is an earthquake prone area (recent activity includes a 4.6 on Tuesday near Crescent City and a 4.3 on Monday near Rio Dell) and the best defense is preparation. For more information, visit the Living on Shaky Ground website.

Free copies of the earthquake preparedness magazine “Living on Shaky Ground: How to Prepare for Earthquakes and Tsunamis in Northern California” can also be requested by leaving a message at 826-6019.

For up-to-date tsunami information, Dengler said to forget apps and sign up for text messages from the most direct source: the National Tsunami Warning Centers. In the U.S., send a text message to 40404 with 'follow NWS_NTWC' for NTWC messages, and 'follow NWS_PTWC' for PTWC messages. To stop receiving NTWC text messages, you can text 'stop NWS_NTWC' to 40404.

Dengler also notes that the Cascadia subduction zone “poses our largest magnitude earthquake threat.”

“Today’s quake is not likely to have made a significant difference to the long-term strain accumulation that will eventually be released by an earthquake in the magnitude 8 to 9 range,” she said. “That being said, that earthquake will come – maybe this afternoon and maybe 200 years from now. The only thing that is certain is that we are one day closer today than we were yesterday.”

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