We live on the thin crust of a layered Earth. The crust or
surface of our planet is broken into large, irregularly shaped
pieces called plates. The plates tend to pull apart or push together
slowly, but with great force. Stresses build along edges of the
plates until part of the crust suddenly gives way in a violent
movement. This shaking of the crust is called an earthquake.
The crust breaks along uneven lines called faults. Geologists
locate these faults and determine which are active and inactive.
This helps identify where the greatest earthquake potential exists.
Most faults mapped by geologists, however, are inactive and have no
earthquake potential.
When the crust moves abruptly, the sudden release of stored force
in the crust sends waves of energy radiating outward from the fault.
Internal waves quickly form surface waves, and these surface waves
cause the ground to shake. Buildings may sway, tilt, or collapse as
the surface waves pass.
The constant interaction of crustal plates in western North America
still creates severe earthquakes. Idaho is situated where the Basin
and Range and Rocky Mountain geomorphic provinces meet. Most of
Idaho has undergone the effects of tremendous crustal stretching.
Central Idaho's high mountain ranges are striking evidence of these
powerful earth movements over millions of years. The Borah Peak
earthquake of 1983 was another event in the stretching that forms
long deep valleys and tall, linear mountain ranges. Earthquakes from
the crustal movements in the adjoining states of Montana, Utah, and
Nevada also cause severe ground shaking in Idaho.
Earthquakes are measured in two ways. One determines the power;
the other describes the physical effects. Magnitude is calculated by
seismologists from the relative size of seismograph tracings. This
measurement has been named the Richter scale, a numerical gauge of
earthquake energy ranging from 1.0 (very weak) to 9.0 (very strong).
The Richter scale is most useful to scientists who compare the power
in earthquakes. Magnitude is less useful to disaster planners and
citizens, because power does not describe and classify the damage an
earthquake can cause. The damage we see from earthquake shaking is
due to several factors like distance from the epicenter and local
rock types. Intensity defines a more useful measure of earthquake
shaking for any one location. It is represented by the modified
Mercalli scale. On the Mercalli scale, a value of I is the least
intense motion and XII is the greatest ground shaking. Unlike
magnitude, intensity can vary from place to place. In addition,
intensity is not measured by machines. It is evaluated and
categorized from people's reactions to events and the visible damage
to man-made structures. Intensity is more useful to planners and
communities because it can reasonably predict the effects of violent
shaking for a local area.
Idaho has experienced the two largest earthquakes in the contiguous
United States in the last forty-plus years the 1959 Hebgen Lake
earthquake (7.5M) and the 1983 Borah Peak earthquake (7.3M). The
Borah Peak event shattered windows, cracked walls, and killed two
people. Five schools in this sparsely populated area sustained
nearly $10 million in damages.
