Introduction to Earthquakes
Earthquakes. . . What Are Their Effects?
With nonexistent response times and the potential for catastrophic damage, earthquakes represent significant economic, social, and physical hazards. The annual loss from earthquake events in the U.S. is estimated at $1 billion, although the Northridge, California, earthquake in 1994 alone caused $20 billion in economic losses (Wang 1998). According to the Federal Emergency Management Agency (FEMA), more than 109 million people and 4.3 million businesses in the U.S. are exposed to some type of seismic hazard (FEMA 1997).
Whereas the Pacific Northwest lacks recent catastrophic seismic events, there is historical and geological evidence that suggests future events are likely and that they will be as or more intense than those experienced in the past century in California and Alaska. The Pacific Northwest is thus an area vulnerable to these events.
Defining Earthquakes
An earthquake is a sudden trembling motion caused by an abrupt release of stress on or along a fault. Earthquakes are the earth's natural means of releasing stress. The earth's crust is broken into several major plates that move. These plates are sometimes referred to as tectonic plates, and whenever they move they bump, slide, catch, or hold they put forces on themselves and each other. Faults are fractures or breaks in the earth's crust, along which one side of the break is pushed up, down, or sideways. When the force is large enough, the crust breaks. This break causes stress to be released as energy, which moves through the earth's crust in the form of waves. These waves are what we call earthquakes.
Measuring Earthquakes
Earthquakes can be measured in several ways, two of which are intensity and magnitude. Intensity measures damage to the surface and the effects on humans. The scale used to measure intensity is called the Modified Mercalli Intensity Scale. Magnitude does not depend on population and effects to ground structures, but rather on wave amplitude and distance from the epicenter. This measure uses formulas and seismograph information to determine the magnitude. The scale used to measure magnitude is the Richter scale. View a comparison of the Modified Mercalli Intensity scale to the Richter scale.
Seismic Activity in the Pacific Northwest
The Pacific Northwest is situated on the seismically active Pacific Ocean margin. Seismic activity in the Pacific Northwest is due to the convergence (or moving towards one another) of two tectonic plates, the North American Plate and the Juan de Fuca Plate. Extending from northern California to British Columbia, the Juan de Fuca Plate is being pushed underneath, or subducted by, the North American Plate at a rate of 40 to 50 millimeters per year (mm/yr). Additional seismic activity exists further offshore originating from the Pacific Plate and the Juan de Fuca Plate spreading apart from each other.
Cross section of the Juan de Fuca Plate subducting underneath the North American plate
and the Pacific and Juan de Fuca Plates spreading apart from each other. U.S. Geological Survey.
What Are the Primary Sources of Earthquakes the in Pacific Northwest?
Because of converging, or meeting, tectonic plates, earthquakes can and have originated from three primary sources in the Pacific Northwest: (Place your pointer over the earthquake types to see the location of the earthquake sources in the diagram.)
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Plates Legend 1. Pacific Plate 2. Juan de Fuca Plate 3. North American Plate |
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Earthquake Types
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To see historical occurrences of these types of earthquakes, visit historical earthquakes.
Sources: University of Washington Earthquake Hazards
Southern California Integrated GPS Network Education Module