An earthquake is the shaking of the ground caused by an abrupt shift of rock along a fracture in the earth, called a fault. There are three categories of quakes, and each type may affect Wasco County. One way earthquakes are categorized is by type. The first is a shallow or crustal quake. These occur at a depth of 5 to 10 miles beneath the earth’s surface. These quakes are associated with fault movement within a surface plate. The second type of earthquake is an intraplate, or “deep” earthquake. Intraplate quakes occur when an earthquake on a geologic plate affects another plate. In Pacific Northwest geology, intraplate quakes happen when the Juan de Fuca plate breaks up underneath the continental plate, approximately 30 miles beneath the earth’s surface. The third type of quake is a subduction zone earthquake. These occur when two converging plates become stuck along with their interface. Continued movements between the plates will build up energy across the locked surface until the plates abruptly slip along the interface when the strain is released.

Another way to categorize earthquakes is a “Convergent Boundary,” Divergent Boundary” or “Transform Fault.” “Convergent Boundary,” where one plate is forced over another plate during movement creating a thrust fault; “Divergent Boundary,” where plates are forced apart, usually forming a Rift Zone; and “Transform Fault,” where plates slip by each other (also referred to as Strike-Slip)1. The scope of the damage is a function of earthquake magnitude and level of community preparedness. Damage could range from minimal to moderate loss of life and destruction of property. The entire County population, property, commerce, infrastructure, and services may be vulnerable to an earthquake.

There is no recent history of major earthquakes in Wasco County, though County residents do occasionally feel some minor earthquakes. However, geology clearly shows that the County has been impacted by significant events in the last 500 years.

Earthquakes in Wasco County are most likely to originate from two sources: 1) the Cascadia Subduction Zone and 2) faults near the eastern end of the Columbia River Gorge.

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