One risk many folks worry about is earthquake damage. One need only look at aftermath photos from one of the big ones to realize, “If that happened in the area I build my retreat, no matter how strong I build, I’m doomed.”
Fortunately, it is relatively easy to assess earthquake risk. There are three resources worth taking a look at: Seismic Hazard maps from USGS, Fault Line maps from USGS, and the Recent Earthquake map by USGS. The only challenge is that they are not easy to interpret for someone not already familiar with seismic vocabulary. Fortunately, we’re here to step you through it.
Seismic Hazard Maps
These can be found here: http://pubs.usgs.gov/sim/3325/
First, the seismic hazard maps are not predicting your risk of earthquake. They are predicting odds of a ground shaking event of a certain strength within a certain time frame. So if you look at map #1 of 6, it is titled “Peak Horizontal Acceleration with 10 Percent Probability of Exceedance in 50 Years.” That’s a mouthful. What it means is that for the image zones depicted in the map, you’re looking at the amount of ground shaking that has a 10% chance of being met or surpassed within the next 50 years. That amount of ground shaking is expressed as a percent of the acceleration of gravity.
Consider that hotspot between Tennessee and Arkansas. Let’s say you were considering a retreat location there, but the site falls into that large, pale yellow ring labeled “10.” What does it mean? The 10 means ground shaking of 10%g, or about 3.2 feet per second. The fact that it is drawn on the map means there is a 10% chance that in the next 50 years that area will experience ground shaking of 3.2 feet per second or more.
The other maps have different visualizations for different degrees of shaking and different time frames and odds.
Magnitude is not Shaking
This is a key point. Everyone hears about the magnitude of a quake. But that doesn’t equate to how much damage you’ll get in the area. That varies based on local geologic conditions and such. But here’s a handy chart from USGS:
|Magnitude||Typical Maximum Modified Mercalli Intensity|
|Micro||1.0 – 3.0||I|
|Minor||3.0 – 3.9||II – III|
|Light||4.0 – 4.9||IV – V|
|Moderate||5.0 – 5.9||VI – VII|
|Strong||6.0 – 6.9||VII – IX|
|Major||7.0 and higher||VIII or higher|
And here’s a key for understanding intensity:
Abbreviated Modified Mercalli Intensity Scale
- I. Not felt except by a very few under especially favorable conditions.
- II. Felt only by a few persons at rest, especially on upper floors of buildings.
- III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.
- IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
- V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
- VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
- VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
- VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
- IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
- X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.
- XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
- XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.
So you really need a quake of 4.5 to 5.0 magnitude before you should worry about any building damage.
Fault Line Maps
The Seismic Hazard maps rely in part on the fault line maps. You can view an interactive fault line map here: http://earthquake.usgs.gov/hazards/qfaults/
Now, it is possible with rural acreage, you might be looking at a property which literally straddles a fault line. Should you be concerned? Depends on the risks you are comfortable with. Maybe that quarternary fault line hasn’t been active in over 100,000 years. Might mean that the fault has settled down and is no longer a concern, or maybe it means it’s gonna go any day now. No one can tell you for sure. But one way you might “take the pulse” is to move on to the recent earthquake map.
Recent earthquakes recorded by the USGS can be viewed here: http://earthquake.usgs.gov/earthquakes/map/
The default view options are not much use. Click the settings tab and specify that you want magnitude 4.5+ (not the default of 2.5+) because 4.5 is about the level you’ll see minor building damage. Also, a search with a long time frame on 2.5 is going to return hundreds of results and be fairly worthless to read.
Other defaults are for the last 24 hours. Change that to a couple of decades by typing in the dates in the advanced search.
Also, the default is the whole US, but you can narrow the geographic window of search even by drawing a box on the map, encompassing a region like the American Redoubt.
For example, here’s a search I ran showing all earthquakes in the last decade of 4.5+ in a box encompassing the American Redoubt.
The box captures an active area in Nevada, but that’s easy enough to ignore and I can just focus on quakes in the states of interest.
Putting It All Together
The USGS (and anyone else with a sense of our modern, convoluted, legal liability system) will swear up and down that they can’t predict if your property will experience a quake or not.
What you can do is look at the Seismic Risk maps and decide what level of risk you are comfortable with. Less risk is less risk, but different folks have different tolerance for risk.
Next, look at the Fault line maps. Living in a particular Seismic Risk zone might not distinguish living right on a fault line versus living a hundred miles from it. How close are you to the fault lines on which the Seismic Risk maps are based? But is the nearby fault line the one you should worry about?
For that, search recent quakes and see where the action has been lately. It might be a good idea to stay away from active areas like Yellowstone, or North Nevada.
These resources don’t tell you whether you’ll experience a quake at your retreat. But they can give you a gut feel for whether or not you should waste energy worrying about it.