Boundary County

All Hazards Mitigation Plan

Section 8

Hazard Profiles - Earthquakes

Definition and Descriptions Earthquake 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.


Breaks 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.


Foreshocks,” 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.


Potential Damage   Earthquakes are measured in two ways:


·        Magnitude (measures energy released)

·        Modified Mercalli Intensity Scale (measures physical effects)


An 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 populated area.


The 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 structures.


Earthquakes 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.


The damage associated with each earthquake is subject to several variables:

·                    Seismic Activity

·                    Geology and Soils

·                    Development

·                    Time of Day

·                    Shaking

·                    Ground displacement


Seismic Activity  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 earthquakes.


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.


Development  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.


Shaking 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.


Ground displacement 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.