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Earthquake Hazards in the Pacific Northwest Earthquake Hazard Map used in the Uniform Building Code The USGS compiles Earthquake hazard maps
using the best available information, which is often imperfect. Building standards for several levels of shaking intensity are periodically proposed by
the International Code Council (formerly the International Conference of Building Officials),
and may be adopted into legislation.
Expected Ground Motion from Future Earthquakes Shown here is a custom map
for Washington State. For the entire US, see the US Geological
Survey's
National Strong Motion pages (for Canadian
building codes refer to the Geological Survey of Canada).
The map portrays the level of ground shaking that can be reasonably
expected to occur at a site
in the next 50 years. It is based on knowledge of the
rates of earthquake occurrence and the activity of faults in
the region. The map is one of several types used to portray
earthquake hazard. The force on a building during an earthquake
is proportional to ground acceleration. Such forces are prescribed
by the UBC. During an earthquake the ground acceleration varies
with time. The acceleration values shown on the map are the
peak or maximum values expected during the earthquake.
"g" is a common value of acceleration equal to 9.8 m/sec/sec
(the acceleration due to gravity at the surface of the earth).
30% of g is the acceleration one would experience in a car that
takes 9 seconds to brake from 60 miles per hour to a complete
stop. The "2% probability
of exceedance in 50 years" refers to the fact that earthquakes
are somewhat random in occurrence. One can not predict exactly
whether an earthquake of a given size will or will not occur
in the next 50 years. The map takes the random nature of earthquakes
into account. It was constructed so that there is a 2% chance
(2 chances in 100) that the the ground acceleration values shown
on the map will be exceeded in a 50 year time period.
The degree of ground shaking (or damage) caused by an earthquake is often assigned a numerical value from Roman numeral I to XII on the Modified Mercalli (MM) Scale. The value is determined from interviews with people who felt the earthquake and from damage reports. Damage at a site depends on many factors such as the size of the earthquake, distance to the epicenter, soil conditions at a site, and type of building construction. The damage reports can be used to construct a MM intensity or "isoseismal" map. Because of ambiguities and interpretation, isoseismal maps of historic earthquakes can vary somewhat as in early studies of the 1872 earthquake, long thought to be in the North Cascades, but now believed to be near Entiat Engineers prefer a measure of ground shaking
that is more objective than MM intensity such as an instrumental
measurement of ground acceleration via a strong motion seismograph.
Since about 2000, the USGS has funded the installation of networks of strong motion accelerographs in urban areas, and created "ShakeMap", a product that combines instrumental measurements of acceleration
information about geology and soils to provide an intesity map of the affected area.
The relationship between MM intensity and acceleration,
determined by empirical observation, is included in the table below.
Modified Mercalli Intensity (abbreviated description)
Ground shaking is perceptible to humans if the acceleration exceeds 1/10 of 1% g. Structural damage in buildings not designed to be resistant usually occurs at 10% g. Accelerations caused by earthquakes have been recorded exceeding 100% g. Factors other than acceleration must also be considered in evaluating the causes of damage such as the oscillation frequency and the total duration of shaking. For example, tall buildings are most affected by low frequency ground motions while typical family residences are most affected by high frequency motions. Prepared by Prof. Anthony Qamar , Geophysics, University of Washington. Edited and amended by Ruth Ludwin 1/2008
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