Using a FEMA-developed methodology to help communities and planners estimate potential earthquake losses, researchers found a simulated shallow, 6.8-magnitude earthquake southwest of Ellensburg would likely injure about 60 — some critically or even fatally, heavily damage or destroy about 600 buildings and do about $215 million in damage over several counties.
A report on the simulation, a strong quake along the northern end of Manastash Ridge northwest to a fault line beneath Cle Elum Ridge, was part of the material shared at Friday’sconference on seismic hazards at Central Washington University.
The 2001 Nisqually earthquake in Washington occurred deeper in the earth than the simulated quake, and did less relative damage. Shallow quakes tend to do more damage than deeper ones of comparable strength, primarily because seismic waves lose energy as they travel through more of the ground.
An earthquake that size in 1995 in Kobe, Japan, killed more than 6,000 people and did tens of billions of dollars of damage.
The simulation assumes a slip on a 19-mile set of thrust fault lines, one of which turns and follows Manastash Creek west. Another turns to become another kind of fault, a right-lateral strike-slip fault, just south of Cle Elum. It would ultimately affect 10 counties, with Yakima and Kittitas County taking the brunt of its force.
A fault, broadly, is a break in the earth’s crust. A thrust fault occurs where one layer of rocks is pushed up and over another higher layer, often placing older rocks above younger.
In strike-slip faults, the offset motion of the meeting plates is mostly horizontal, with little vertical movement.
Many small aftershocks would likely follow a quake like the one in the simulation, a few large enough to cause additional damage. The modeled quake would probably lead to soil liquefaction, causing the sediments to lose their strength and behave like fluids and geyser-like eruptions of sediment and water to appear. The quake could also cause landslides.
When the shaking stops, most debris removal would be in Kittitas, Yakima and King counties — about 65,000 tons worth — and the losses in income would total about $47 million.
The Washington State Military Department Emergency Management Division, the state Department of Natural Resources, the U.S. Geological Survey and the Federal Emergency Management Association produced the report.