The 1936 State Line earthquake (also referred to as the 1936 Milton-Freewater earthquake) struck at 23:08

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on July 15, 1936. The earthquake had an estimated magnitude of 5.8 and a maximum Mercalli intensity of VII (''Very strong''). The epicenter was near the

Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...

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state line approximately northwest of

Milton-Freewater, Oregon Milton-Freewater is a city in Umatilla County, Oregon, United States. The city received its current name in 1951 when the neighboring rival cities of Milton and Freewater voted to merge. The population was 7,151 at the 2020 census. It is part of ...

and southwest of Walla Walla, Washington and was felt throughout the

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, including as far away as

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near the Canadian border and by seismographs as far away as

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Geology

Larger earthquakes like the 1936 State Line earthquake are not uncommon along the Olympic–Wallowa Lineament, a series of faults stretching from Port Angeles, Washington to the Wallowa Mountains in northeast Oregon. Another earthquake estimated near a magnitude 6 struck the Walla Walla valley in 1882. The earthquake occurred in the Touchet Ridge, a spur leading west off the Blue Mountains that is known as the Horse Heaven Hills west of the

Wallula Gap Wallula Gap () is a large water gap of the Columbia River in the northwest United States in southeast Washington. It cuts through the Horse Heaven Hills basalt anticlines in the Columbia River Basin, just south of the confluence of the Walla ...

. There is both the old fault along the ridge as well as a newer fault leading from the ridge into the valley. To the north of the earthquake is the Hite Fault, which lies approximately parallel to the west slopes of the Blue Mountains in

Walla Walla County Walla Walla County is a county located in the U.S. state of Washington. As of the 2020 census, its population was 62,584. The county seat and largest city is Walla Walla. The county was formed on April 25, 1854 and is named after the Walla Wall ...

. The Hite Fault is thought to be the boundary between the more stable North American craton to the east and accreted material to the west. While there is no evidence of a

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earthquake event along the Hite Fault, the

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and several Washington state agencies have run scenarios regarding a future earthquake on the fault. There have been 66 earthquakes felt in the area since 1936. Most of these have had a magnitude below 4, though one event in November 1991 was 4.3.

Earthquake

The ground near Milton-Freewater exhibited cracks over an area that was long by up to , one of which being long and wide in places. Near Umapine there were cracks in the ground up to wide that had water flowing out of them, demonstrating

soil liquefaction Soil liquefaction occurs when a cohesionless saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during an earthquake or other sudden change in stress condition, in ...

. Some cracks were deep. Residents in Umapine reported being woken during the night. Wells in Milton-Freewater saw an increase in water level and a creek near a town, which had previously run dry, began flowing immediately after the earthquake. Observers in Walla Walla noted rumbling noises immediately preceding the first shocks. At one point the ground dropped by . About 70% of headstones at a nearby cemetery were found to have been rotated clockwise. 1936 State Line earthquake isoseismal map

1936 State Line earthquake isoseismal map

Two small foreshocks were recorded in the three hours before the mainshock.

Damage

Severe damage was widespread throughout Milton-Freewater and Walla Walla. In

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, people were forced to evacuate their homes due to large cracks found within their walls. Rocks were reported to have wandered into some intersections and two freight cars were shaken off the tracks at Blue Mountain Station in

Dayton Dayton () is the sixth-largest city in the U.S. state of Ohio and the county seat of Montgomery County. A small part of the city extends into Greene County. The 2020 U.S. census estimate put the city population at 137,644, while Greater Da ...

. The shaking was reported as being strongest near State Line. Very strong shaking occurred in Milton-Freewater, breaking windows and collapsing chimneys that had been built longer than ten years previous to the earthquake. Up to $3,000 ($53,000 in 2018) in damage was done to canned goods as well as $8,500 ($151,000 in 2018) in damage to school buildings. A new home west of town was nearly destroyed, and extensive damage was dealt with two cement homes that had been built around 1916 about west of town. In Umapine a handful of homes were badly damaged. The grade school and high school, which were joined, were found to be separated by . Chimneys on roofs collapsed in numerous locations, as far away as Waitsburg. Pendulum clocks stopped in Umatilla and plaster was found to have been cracked in

Prosser Prosser may refer to: __NOTOC__ Places ;United States * Prosser, California, a former settlement * Prosser Creek, California * Prosser, Nebraska, a village * Prosser, Washington, a city ;Australia * Electoral division of Prosser, Tasmania * Prosser ...

Aftershocks

Over 50 aftershocks were recorded intermittently until mid-November 1936. Several dozen were felt the night of the event in the immediate area, with only three of them reaching to Hermiston. These earthquakes occurred in both Washington and Oregon, owing to the close proximity of the mainshock to the state line. Aftershocks measuring IV on the Mercalli scale were recorded in August, with the ones that struck in November being measured as III.

References

External links

* {{Earthquakes in the United States 1936 natural disasters in the United States 1936 earthquakes Earthquakes in Oregon 1936 in Oregon