The quake-prone geologic fault under South Whidbey extends farther east than previously suspected, and may affect the severity of earthquakes in the Cascade foothills, scientist said.

What about the severity of earthquakes on Whidbey?

“It doesn’t affect South Whidbey very much, if at all,” said Tim Walsh, a chief hazard geologist with the state Department of Natural Resources who has been studying earthquake faults in the Puget Sound region for more than 25 years.

“Then again, it might,” he added.

DNR geologists recently released new maps showing that the southern Whidbey fault runs much farther south and east than once thought, and connects to the Rattlesnake Mountain fault zone in east King County.

Although not as well known as the Seattle fault, the southern Whidbey Island fault, which begins near Port Townsend, comes ashore near Lagoon Point about five miles northwest of Freeland and continues south and east, is considered capable of generating equally large earthquakes.

The fault represents a potential seismic hazard to residents of the Puget lowland, scientists say. Cities at greatest risk are Everett, Seattle, Port Townsend, and even Victoria, British Columbia.

Researchers have determined that the Rattlesnake Mountain fault zone, which was originally mapped by Walsh in 1984, extends from near North Bend and through the Snoqualmie River Valley in the vicinity of Fall City and Carnation.

The new mapping shows that it is the continuation of the South Whidbey fault, which scientists say is about 100 miles long, and is probably several miles wide.

Walsh said further research may verify his theory that the combined faults extend all the way to Cle Elum.

Walsh said that while the potential severity of quakes is related to the length and breadth of a fault, a quake rarely occurs along an entire fault line.

“The San Andreas Fault in California is hundreds of miles long, but it’s never had a quake along the entire thing,” Walsh said.

He said scientists have known about the southern Whidbey fault for decades.

Walsh said that Whidbey Island through the years has been subjected to a number of small quakes, roughly 2 or 3 on the Richter scale.

A small quake was registered in the Coupeville area just this summer.

But scientists theorize that the area has suffered only four quakes above 6 or 7 on the Richter scale in the past 13,000 years, the last one about 2,300 years ago.

Are we due for a big one?

“That’s a tricky question,” Walsh said. “If you flip a coin eight times and it come up heads, you may say you’re due for a tails. But the probability is still the same, 50-50.”

He said the rule of thumb among earthquake researchers is: “The bigger the event, the rarer it is.”

Walsh said that because there remains a danger of earthquakes on South Whidbey, the issue is taken into consideration when it comes to building codes.

He said earthquake insurance may be beneficial, but often requires such high deductibles that you end up insuring only against total loss.

Walsh’s best advice?

“Make sure your house is bolted to the foundation,” he said.