On the night of July 15, 1936, the 5,500 or so residents living in the Tri-Cities probably didn’t expect to get much sleep when they went to bed.

But the reason they were up wasn’t because of the heatwave that blanketed the Tri-Cities and the nation with sweltering 110-degree temperatures.

According to historical accounts, “the 1936 heatwave was one of the deadliest in North American history, prompting people across the country to sleep on roofs, in parks, or on porches to survive.”

Only 1% of the nation’s homes in the mid-1930s had air conditioning and that was reserved for the affluent. More than half the farms dotting the Horse Heaven Hills, and homes with rural connections to Pasco, Kennewick and Richland functioned without electricity. 

At 11:08 p.m. that night, however, heat became a secondary concern because what kept them awake was not the heat.  

It was a rumbling, rolling, glass-shattering, wall-cracking, chimney-crashing, disorienting disruption to their lives when a 6.1 earthquake struck. The epicenter was about 50 miles away near Walla Walla and Milton-Freewater, Oregon. 

It was the most powerful ever recorded here.

State Line quake

Called the State Line Earthquake of 1936, it was felt in Spokane, in Idaho near the Canadian border, and in Arlington, Oregon. 

Seismographs in San Diego, California, recorded it.

“The 1936 State Line Earthquake…is still considered the most powerful and destructive earthquake in the immediate Northeast Oregon/Southeast Washington region in recorded, instrumental history,” according to historical accounts.

A 6.1 earthquake will unleash the explosive force equivalent to more than 1 million tons of TNT. On the Modified Mercalli Scale, it had a maximum intensity of VII, meaning “very strong.” The scale runs from I to XII and is an intensity scale based on observable effects on people, property and the environment.

“While larger earthquakes have occurred in the Pacific Northwest (such as the 1949 and 2001 Puget Sound quakes), the 1936 event holds a unique, top-tier spot in the specific Mid-Columbia region’s records,” according to accounts of such activity.

A listing of damage in the Tri-Cities from 90 years ago is difficult to obtain, but the force of a VII intensity quake centered 6 miles northwest of Milton-Freewater and 6 miles southwest of Walla Walla likely would not have heavily damaged well-built structures. However, accounts of its strength suggest it could have brought down poorly constructed buildings.

The 1936 quake was a crustal earthquake, which are closer to the surface. A buffer of 50 miles will reduce their intensity, but with a shallower eruption, seismic waves carrying destructive shaking can travel longer distances.

Homeowners in Prosser more than 80 miles away reported damage to their homes.

The most severe damage was near the epicenter itself. Windows broke and chimneys collapsed, even as far away as Waitsburg.  

At least one home near Milton-Freewater was nearly destroyed, and others were severely damaged. 

Large cracks in Athena, Oregon, homes forced residents to evacuate. In one cemetery near Walla Walla, 70% of the headstones were rotated clockwise. 

A high school and a grade school which had been physically connected were separated by three inches. 

Two freight cars at a station in Dayton were lifted off the tracks. In Umatilla, pendulum clocks stopped. Concrete pavement cracked.

A crack near Milton-Freewater measured1,500 feet long. Another was 300 feet long and 6 feet wide. 

Water flowed from cracks in the ground near Umapine, Oregon. Water levels in wells increased, and a dry creek near Milton-Freewater began flowing with water again. 

The ground dropped in places.

For the next six months, some 50 aftershocks were felt, with several dozen occurring during the night after the quake, including three in Hermiston. 

It could happen here

The Tri-Cities is not prone to earthquakes, but the possibility exists for a crustal earthquake, less powerful, but still potentially devastating, than megathrust earthquakes producing the most powerful earthquakes ever known with magnitudes of 9.0 or more. A 9.0 magnitude is 1,000 times more powerful than a 7.0.

Two faults intersect at the Tri-Cites, both capable of 7.0 magnitude earthquakes. The Wallula Fault Zone runs along the northern base of the Horse Heaven Hills for 75 miles. The Yakima Fold Belt stretches from the Hanford Nuclear Reservation to Ellensburg, comprising a region of 5,400 square miles.

Because they intersect at the Tri-Cities, they create a unique hidden threat to the area. An underground sedimentary basin 2 to 2 ½ miles beneath the Tri-Cities also adds to the threat. It is known as the Pasco Gravel Low/Sub-Basin and it is 5 miles deep. 

An earthquake in either fault would cause the sediment to act like a bowl of jelly, causing seismic waves to be prolonged and stronger on the surface, affecting Tri-City structures longer and with expected greater damage.

While a megathrust quake can’t happen here, if a large one struck western Washington, it could be felt here and its effect could set off the bowl-of-jelly effect below the Tri-Cities.

Gale Metcalf of Kennewick is a lifelong Tri-Citian, retired Tri-City Herald employee and volunteer for the East Benton County Historical Museum. He writes the monthly history column.