Hazard Profiles - Earthquakes
Definition and Descriptions
energy travels in waves radiating outward from the point of release.
When these waves travel along the surface, the ground shakes and rolls,
fractures form, and water waves may be generated.
Earthquakes generally last a matter of seconds but the waves may travel
for long distances and cause damage well after the initial shaking at the point
of origin has subsided.
in the crust associated with seismic activity are known as “faults”
and are classified as either active or inactive.
Faults may be expressed on the surface by sharp cliffs or scalps or may
be buried below surface deposits.
minor releases of pressure or slippage, may occur months or minutes before the
actual onset of the earthquake. “Aftershocks,” which range from minor to major, may occur for
months after the main earthquake. In
some cases, strong aftershocks may cause significant additional damage,
especially if the initial earthquake impacted management and response functions
or weakened structures.
Earthquakes are measured in two ways:
Magnitude (measures energy released)
Modified Mercalli Intensity Scale (measures
earthquake of Magnitude 2.5 or less is usually not felt.
Dishes rattling and china shaking occur at Magnitude 3.0. Magnitudes
greater than 6.5 are devastating events when the earthquake strikes in or near a
Modified Mercalli Intensity Scale is a subjective description of the physical
effects of the shaking based on observation at the event site.
The damage from earthquake shaking is a function of several factors like
distance from the epicenter and local geology and soils.
On the Modified Mercalli Intensity 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 and is evaluated
from the people’s reactions to events and the visible damage to man-made
of intensity III may be felt, IV are generally felt, and V are definitely felt.
Damage begins at intensity V and starts to become significant at VII for
poorly constructed structures. Intensity
VII is used as a threshold for “significant” event.
Damage is widespread at intensity X and “total” at XII.
damage associated with each earthquake is subject to several variables:
Geology and Soils
Time of Day
The properties of earthquakes vary greatly from event to event.
Some seismic activity is localized, while other activity is widespread.
Earthquakes can be very brief, or last for a minute or longer.
The depth of release and type of seismic waves generated also play roles
in the nature and location of damage; shallow quakes hit the area close to the
epicenter harder, but tend to be felt across a smaller region than deep
Geology and Soils
The surface geology and soils of an area influence the propagation
(conduction) of seismic waves and how strongly the energy is felt.
Generally, stable areas experience less destructive shaking than unstable
areas. The location of a community
or even individual buildings plays a strong role in the nature and extent of
damage from an event.
A small earthquake in the center of a major city can have far greater
consequences than a major event in a thinly populated place.
Time of Day
The time of day of an event controls the distribution of the population
of an affected area. On work days, the majority of the community will transition
between work or school, home, and the commute between the two.
The relative seismic vulnerability of each location can strongly
influence the loss of life and injury resulting from an event.
While damage can occur by movement at the fault, most damage from earthquake
events is the result of shaking. Shaking
also produces a number of phenomena that can generate additional damage.
includes landslides and avalanches. Liquefaction
and subsidence can occur during an earthquake, i.e. soils liquefy and/or subside
when impacted by the seismic waves. Seiches (an oscillating wave created by the rocking of an
enclosed body of water) can also occur.
The USGS has designated a group of several faults as the North Idaho Fault System. There are no documented faults in Boundary County, however, the “Purcell Trench” runs down the center of the Kootenai Valley floor, as seen on Map 8-1 below. Map 8-2 shows the locations of faults in Idaho and western Montana, some of which could impact Boundary County in the event of a large earthquake.