An earthquake is the sudden release of stored energy. Most earthquakes occur along a fracture within the earth, called a fault. The shaking caused by this sudden shift is often very small, but occasionally large earthquakes produce very strong ground shaking. It is this strong shaking and its consequences – ground failure, landslides, liquefaction – that damages buildings and structures and upsets the regional economy.
Washington, especially the Puget Sound basin, has a history of frequent earthquakes. More than 1,000 earthquakes occur in the state each year. A dozen or more are strong enough that people feel ground shaking. Occasionally, earthquakes cause damage. The state experienced at least 20 damaging events in the last 125 years.
The Cascadia subduction zone, the fault boundary between the North America plate and the Juan de Fuca plate, lies offshore from northern California to southern British Columbia. The two plates are converging at a rate of about 2 inches per year. In addition, the northward-moving Pacific plate is pushing the Juan de Fuca plate north, causing complex seismic strain to accumulate. The abrupt release of this slowly accumulated strain causes earthquakes.
The Pacific Coast, Strait of Juan de Fuca, Puget Sound, and large lakes are at risk from tsunamis, which are trains of waves that threaten people and property along shorelines. Sudden raising or lowering of the earth’s crust during earthquakes generally causes a tsunami, although landslides and underwater volcanic eruptions also can generate them. Movements of the sea floor or lake bed, or rock fall into an enclosed body of water, displace the water column, setting off a series of waves that radiate outward like pond ripples.
- Example: Projected Tsunami Travel Times based on January 23, 2018, 8.0 earthquake near Kodiak AK and the subsequent tsunami warning that was activated, but cancelled when Jefferson County Department of Emergency Management and the Emergency Operation Center determined there was no longer threat.
- Port Ludlow Tsunami Inundation Map
On a quiet Saturday in March 22, 2014, the small, tight-knit community of Oso, Wash. was shaken with one of the deadliest landslides in our nation’s history. The slide killed 43 people, destroyed dozens of homes and cabins, covered a mile stretch of State Route 530 and shook the rural community and its surrounding neighbors.
A landslide is the movement of rock, soil and debris down a hillside or slope. Landslides take lives, destroy homes, businesses, and public buildings, interrupt transportation, undermine bridges, derail train cars, cover marine habitat and damage utilities.
Ground failures that result in landslides occur when gravity overcomes the strength of a slope. While gravity is the primary reason for a landslide, there can be other contributing factors, including:
- Saturation, by snowmelt or heavy rains, that weaken rock or soils on slopes.
- Erosion by rivers, glaciers, or ocean waves that create over-steepened slopes.
- Topography of a slope – its shape, size, degree of slope and drainage.
- Stress from earthquakes magnitude 4.0 and greater can cause weak slopes to fail.
- Excess weight, from accumulation of rain or snow, from stockpiling of rock or ore, from waste piles, or from manmade structures, may stress weak slopes to failure.
- Human action, such as construction, logging or road building that disturbs soils and slopes.
Forest or Wildland Fires
Wildland fires are fires caused by nature or humans that result in the uncontrolled destruction of forests, brush, field crops, grasslands and real and personal property.
The wildland fire season in Washington usually begins in early July and typically culminates in late September when regular rain returns to the Northwest. However, wildland fires have occurred in every month of the year.
The Washington Department of Natural Resources protects 2.5 million acres of state-owned land and 10 million acres of land in private ownership through legislative directive.
The department fights about 900 wildland fires per year across the state, about 70 percent are in Eastern Washington. Most are small, usually extinguished while they are less than one acre in size. Wildland fires can spread to more than 100,000 acres, depending on a number of factors, and may require thousands of firefighters and several months to extinguish. Federal, state, county, city and private agencies and private timber companies provide fire protection and firefighting services on forestlands in Washington.
A severe storm can be anything involving strong winds and large hail, thunderstorms, tornados, rain, snow, or other mixed precipitation. Typically, transportation issues and loss of utilities come about because of a severe storm.
The following are severe storm elements (using National Weather Service definitions):
- High winds – Storms with sustained winds of 40 mph or gusts of 58 mph or greater, not caused by thunderstorms, expected to last for an hour or more.
- Severe Thunderstorm – Storms that produce winds of 58 mph or greater or three-quarters of an inch or larger hail.
- Winter storm – A storm with significant snowfall, ice, and/or freezing rain; the quantity of precipitation varies by elevation. Heavy snowfall is 4 inches or more in a 12-hour period, or 6 or more inches in a 24-hour period in non-mountainous areas; and 12 inches or more in a 12-hour period or 18 inches or more in a 24-hour period in mountainous areas.
- Coastal flooding – Flooding in coastal areas caused by storm surge, astronomical high tides, or a combination of them.
Floods cause loss of life and damage to structures, crops, land, flood control structures, roads, and utilities. Floods also cause erosion and landslides, and can transport debris and toxic products that cause secondary damage. Flood damage in Washington State exceeds damage by all other natural hazards.
Since 1970, every county in Washington state has received a Presidential Disaster Declaration for flooding. While not every flood creates enough damage to merit such a declaration, most are severe enough to warrant intervention by local, state or federal authorities.
Although floods can happen at any time during the year, there are typical seasonal patterns for flooding in Washington state, based on the variety of natural processes that cause floods:
- Heavy rainfall on wet or frozen ground, before a snow pack has accumulated, typically cause fall and early winter floods.
- Rainfall combined with melting of the low-elevation snow pack typically cause winter and early spring floods.
- Thunderstorms typically can cause a flash flood during the summer in Eastern Washington; on rare occasions, thunderstorms embedded in winter-like rainstorms cause flash floods in Western Washington.