A 4.3 magnitude earthquake in Jefferson County at 7:21 p.m. on Sunday, Oct. 8, seems not to have left any damage, and was confirmed to pose no tsunami danger by the Jefferson County Department of Emergency Management.

Harold Tobin serves as the state seismologist for Washington, as well as the director of the Pacific Northwest Seismic Network, and he confirmed that the epicenter of the earthquake was “where Marrowstone Island and Indian Island meet,” less than two miles southwest of Marrowstone, but at a depth of 35 miles.

Although Tobin conceded that the “average earthquake” carries with it as high as a one-in-five chance of being followed by a larger earthquake, he hastened to add that such statistical probabilities are drawn from globally-based data, rather than anything pertaining specifically to this event, or even the local region where it occurred.

Thus, while aftershocks are not uncommon for earthquakes, Tobin noted that no such aftershocks have been detected, nor has he received any reports of significant, or even minor, structural damage stemming from the earthquake.

Tobin laughed as he recalled that all three local broadcast television news networks contacted the U.S. Geological Survey within half an hour of the earthquake.

“With a 4.3 magnitude earthquake, it’s widely felt, but the shaking is relatively weak,” Tobin said. “This is because even those who are relatively close to the epicenter are still an additional 35 miles removed from it, due to its depth. As it gets closer to the surface of the earth, say within a distance of 10 miles instead, the earthquake would be felt more strongly.”

Tobin compared it to the ripples in a pond from dropping a pebble in the water, and how those ripples grow smaller the further they reach.

Tobin also pointed out that the earthquake magnitude scale measures factors of 10, so a 5.3 earthquake would be 10 times stronger than the one on Oct. 8, while a 6.3 earthquake would be 100 times stronger.

“At 5.3, you might see some cracks in the walls,” Tobin said. “At 6.3, it would be strong enough to affect structures. The 2001 Nisqually earthquake was a 6.8, and that did serious damage.”

At the same time, Tobin warned against listening to any online prognosticators who might attempt to characterize this earthquake as part of an increasing pattern of such events.

“There’s no evidence that’s true,” Tobin said. “Earthquakes are always happening randomly somewhere around the world. It was as recently as 2019 that Monroe experienced a 4.6 earthquake. Every few years, we get something that’s around a 4 on the magnitude scale.”

And for those who had early warning “shake alerts” installed as apps on their mobile phones, Tobin reassured them that neither the apps nor their phones were malfunctioning.

“The earthquake was detected, but it didn’t meet the minimum threshold set by the U.S. Geological Survey to issue a warning.

“If it had, all our cellphones would have gone off, like they did when that emergency alert test was sent out the week before,” Tobin said.

For more information on the earthquake, log onto the U.S. Geological Survey website at earthquake.usgs.gov/earthquakes/eventpage/uw61965081/executive online.

Jefferson County residents are encouraged to fill out the "Did You Feel It?" survey on that webpage, to help researchers better understand the impacts of the event.

Since county residents live in a region with a relatively high probability of earthquakes, the Jefferson County Department of Emergency Management’s website provides more information on how to prepare at co.jefferson.wa.us/1067/Preparedness-Planning online.