FEMA defines an earthquake as a sudden slipping or movement of a portion of the earth’s crust, accompanied and followed by a series of vibrations. This is a fairly innocuous description of what can be a traumatic event, to which anyone who has experienced an earthquake can surely testify.
The magnificent Teton range, although peaceful now, was formed through thousands of major earthquakes in the 7 to 7.5 magnitude range over 13 million years. Each of these earthquakes are estimated to have caused 3 to 6 feet of vertical ground movement (Windows Into the Earth; Smith and Siegel, 2000). There have not been any quakes of this magnitude along the Teton fault in recorded history, but it is still considered an active fault. In fact, there are many faults in Teton County. View a map of those fault lines (JPG).
The main fault, the Teton Fault along the base of the Tetons, can produce a significant earthquake in the range of 7.0 to 7.5 according to Dr. Robert Smith of the University of Utah. The Teton fault is a normal fault, which means that the mountains rise and the valley drops during episodes of movement.
Chances of an Earthquake Happening
You can view a peak acceleration map (PDF) for Teton County generated by the USGS National Seismic Hazard Mapping Project of 2002. It does not show the chances of an earthquake occurring. What it does show is the peak acceleration of a theoretical particle on the ground during an earthquake. The entire map shows that in the next 50 years there is a 10% probability that the peak acceleration shown by the contour lines will be exceeded at any given point on the map. The unit of measurement for the contour lines represents the percentage of acceleration of gravity, which is 9.8 meters per square inch or 32 feet per square inch.
So for instance, the orange spot on the Yellowstone/Teton boundary says "30", which means that in this area, within the next 50 years, there is a 10% chance that an earthquake will cause an acceleration on the ground greater than 30% the rate of gravity. Remember, acceleration doesn't necessarily tell us how fast the ground is moving (velocity), but how much change there is in the velocity of the ground over a period of time. It is like accelerating in your car from 0 mph to 55 mph: it doesn't happen instantly.
You need to build up to it, so at first you are moving at 5 mph, then a minute later 25 mph, then a minute later 45 mph, etc. If this were the case, you would be accelerating at 20 miles per hour per minute.
What This Information Means
How does this information help us? Well, if you are building a structure, you would want to know what the highest probable acceleration of your foundation might be during an earthquake so that you can take the proper precautions in construction. Planning and building departments also use this data to create building codes for safer structures in earthquake prone areas such as Teton County. Earthquakes and geology are fascinating topics that cannot be completely covered here. If you would like to learn more, check out the USGS's "Learn" website.
There aren't any activities that you as a citizen can do to increase your risk of experiencing an earthquake. The main risk factor is in where you choose to reside:
- Living near an active fault: If you live in Teton County, you live near an active fault. A look at the map at the top of the page shows the extensive fault lines throughout Teton County. The Teton Fault by itself is large enough that it can affect the entire county and beyond.
- The level of activity of the fault: In theory, an active fault that has more frequent smaller earthquakes is less likely to have a major earthquake. The reasoning is that the pressure in the fault is being released in small doses more frequently, as opposed to being built up and finally releasing in a large earthquake. The Teton Fault is still considered active, and has not had a major earthquake in recorded human history, so it would fall into this category.
- Underground nuclear testing: There have been cases of underground nuclear testing causing fault ruptures in areas of Nevada. Luckily, we don't have any nuclear testing in Teton County.
- Extensive Mining: Very extensive mining in some cases has been shown to cause earthquakes. Although artificial, they can still be destructive. Mining that has occurred in Teton County is unlikely to cause any significant earthquakes in and of themselves