Every so often, a fault line shifts and an earthquake shakes some part of America, and it prompts national headlines.
Yet, you may wonder, why are some regions in America so much more prone to earthquakes than others?
Why do California and Alaska constantly see Earthquakes while Nebraska never does?
The frequency of earthquakes all comes back to fault lines in America.
Here’s what you need to know about fault lines and how they can affect the planet we live on.
1. What are fault lines?
A fault is a fracture or a break between two blocks of rock.
Fault lines allow the blocks to move relative to each other.
This movement can occur quickly (in the form of an earthquake) or slowly (in the form of a creep).
Fault lines can range in length from a few millimeters to thousands of kilometers.
Most faults produce repeated displacements over geologic time.
When an earthquake occurs, the rock on one side of the fault line will slip suddenly with respect to the other.
The fault surface can be horizontal or vertical or some arbitrary angle in between.
We’ll discuss the different angles of the faults below along with the different types of fault lines.
2. What are the different types of fault lines?
There are several different types of fault lines.
Here are the primary ones you should know.
Normal fault: A dip-slip fault (or a fault where the blocks move vertically) is where the block above the fault has moved downward relative to the block below.
The type of faulting occurs in response to tension and is often observed in the Western United States Basin and Range province as well as along oceanic ridge systems.
Reverse fault: A dip-slip fault in which the upper block (along the fault plane) moves up and over the lower block.
This type of fault is common in areas of compression such as regions where one plate is being subducted under another.
When the dip angle is shallow, a reverse fault is often described as a thrust fault.
An example of this is in Japan.
Strike-slip fault: A fault on which the two blocks slide past another.
The San Andreas Fault is an example of a right-lateral fault.
Left-lateral strike-slip fault: This is when the displacement of the far block is to the left when viewed from either side
Right-lateral strike-slip fault: This is when the displacement of the far block is to the right when viewed from either side
3. How can you classify the direction of slip along fault lines?
Another way to classify fault lines is the direction of the slip.
Scientists use the angle of the fault with respect to the surface — also known as the dip — and the direction of the slip along the fault to classify faults.
Dip-slip: Faults that move mostly vertically; these can be described as either normal or reverse (thrust), depending on their motion
Strike-slip: Faults that move horizontally; these can be classified as either right-lateral or left-lateral
Both dip-slip and strike-slip faults are known as oblique-slip faults.
4. What is the relationship between faults and earthquakes?
Earthquakes occur on faults.
Strike-slip earthquakes occur on strike-slip faults.
Normal earthquakes occur on normal faults.
Thrust earthquakes occur on thrust or reverse faults.
When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other.
A fault’s surface can be vertical, horizontal, or another angle to the Earth’s surface.
The slip direction can also be at any angle.
5. How do I find the nearest fault to a property or specific location?
You can click on the “Basemaps and Overlays” icon in the upper right corner of the map.
Next, check the box for “U.S. Faults.”
Then, mouse over each fault to get a pop-up window with the name of the fault.
You can also use the Quaternary Fault and Fold Database below to view faults within the U.S.
6. Where can I find a fault map of America?
Visit the Quaternary Fault and Fold Database to view an online map of the U.S. Quaternary Faults.
These are faults that have been active in the last 1.6 million years.
The interactive map allows you to view the faults online and use a separate database search function.
7. What are the most dangerous fault zones in America?
The United States has numerous seismically active fault zones that can cause quite a bit of damage.
Here are the most dangerous.
Hayward Fault Zone
This Hayward Fault is 74 miles long and runs the east side of the San Francisco Bay.
It has a capable magnitude of 7-8 and a strike-slip fault type.
The last recorded event was 1868 in Hayward City.
There is a 31 percent chance of a magnitude 6.7 earthquake or greater occurring along the Rogers Creek-Hayward Fault in the next 30 years and a 4 percent chance of it being an 8 or greater.
Overall, this highly unstable fault in California has been threatening the San Francisco Bay Area for generations.
Seismologists consider this to be a “ticking time bomb.”
Cascadia Fault Zone
The Cascadia Fault is 620 miles long off the coast of Washington and British Columbia.
Its capable magnitude is a megathrust quake exceeding 9.0.
The fault type is subducting.
The last large event was in 1700, but there is a 37 percent chance of an 8.0+ in the next 50 years.
Newport Inglewood Rose Canyon Fault Zone
This fault is 47 miles long at the heart of Los Angeles (running north-south through the center of West LA and into South LA).
It has a potential magnitude of 6.0 to 7.4 and is a right-lateral strike-slip.
The last known event was in 1933 in Long Beach and killed 120 people.
The next event is unknown.
Most older buildings have been retrofitted for seismic activity, and newer buildings are as safe as they can be.
These buildings are being constructed to prevent loss of life.
San Andreas Fault Zone – Salton Sea/San Gorgonio
You’ve likely heard of the San Andreas Fault Zone that’s 100 to 300 miles from Salton toward Cajon Pass and runs from the Salton Sea from the south up to the north of LA (although, the entire fault stretches up to San Francisco).
It has an 8+ capable magnitude and is a strike-slip fault type.
The last event is not recorded in human history, but there is a geologic body of evidence.
There is no prediction for its next event.
Recent data has shown that the entire fault line could potentially rupture along its entire 800-mile length beginning with a quake at either end.
It’s not probable, but it is possible.
Scientists estimate that a quake (if it were to happen) could be as high as 8.3 on the Richter scale.
New Madrid Fault Zone
The New Madrid Fault Zone is 150 miles long and extends through five states, including Illinois, Missouri, Arkansas, and Tennessee.
As an intraplate fault, it has a 6.5-7.5 capable magnitude.
Its last large event was in 1812, and it has a 7 to 10 percent chance of a 7.5 within the next 50 years.
It has a 25 to 40 percent chance of a magnitude 6.0 earthquake within the same time frame.
Ramapo Fault Zone
The Ramapo Fault is 185 miles located between the northern Appalachian Mountains and Piedmont areas to the east in New York, New Jersey, and Pennsylvania.
It has a 6.0 to 7.0 capable magnitude.
While there is no projected timeframe for the next event, it last had a large event in 1884.
If there were to be a large event in the future (even at 5.0), then it could cause a catastrophic loss of life due to the density of the population and lack of seismic preparedness to the structures.
The last event was shaken by an earthquake that ruptured near the Ramapo Fault System.
Wasatch Fault Zone
The Wasatch Fault is 240 miles east of Salt Lake along the Wasatch Range.
It has a 7.5 magnitude capability.
It is one of the world’s longest normal faults.
As the last event was in 1847, it is well overdue for an earthquake.
Denali Fault Zone
The Denali Fault zone is 200+ miles in Alaska’s Denali National Park.
It is a strike-slip fault with a 7.9+ capable magnitude.
The last event at this fault was in 2002.
Due to the recent slippage and possible release of stress on the fault, it is difficult to say when it might rupture again.
However, the fault is very capable of delivering a big bang.
Hawaii’s Big Island
Hawaii’s island chain was formed by volcanism, and the Big Island continues to be shaped by these forces.
Volcanoes and earthquakes go hand in hand.
However, the fault movement on the island is not caused as much by the slipping in the faults as it is the volcanism.
The volcanism prompts the faults to slip as the island expands.
8. Why are there so many earthquakes and faults in America?
The region that is now the United States has been tectonically active since the supercontinent Pangea broke up roughly 200 million years ago.
This is partly because it is close to the western boundary of the North American plate.
When the San Andreas Fault system formed 25-30 million years ago, the Pacific and North American plates created many faults in California, which accommodate lateral motion between the plates.
North and east of California, the Basin and Range province between the Wasatch Mountains in Utah and the Sierra Nevada Mountains in eastern California are also actively spreading and stretching westward.
In the American southwest, Mexico and west Texas also have a similar spreading that has opened up a north-south rift that starts in central Colorado and extends into Northern Mexico.
The geologic conditions and plate tectonic setting in the American West have resulted in the region being underlain by a relatively thin crust and having high heat flow.
This can favor relatively high deformation rates and active faulting.
On the other hand, the Central and Eastern U.S. has a thicker, colder, older, and more stable crust.
It’s thousands of miles from active plate boundaries.
The rates of deformation are low in this region.
Still, this region has seen some major earthquakes in history.
These include New Madrid, Missouri, Charleston, South Carolina, and Cape Ann, Massachusetts.
9. What are the five most dangerous American earthquake hot spots outside of California?
When you think of earthquakes in America, California may automatically come to mind.
However, this state isn’t the only one with a serious earthquake hazard.
In other parts of the country, there are lesser-known fault zones that pose a threat.
Here are some other hazardous seismic hot spots that you may want to consider.
The Pacific Northwest
Did you know that the biggest earthquakes in the country are not in California?
North of the San Andreas Fault, the ocean crust is being forced beneath the North American continent.
This presents a much greater hazard in terms of earthquake magnitude.
The area is known as the Cascadia Subduction Zone.
It’s a 680-mile stretch of colliding landmass 50 miles offshore of Oregon, Washington state, and southern British Columbia.
Near New Madrid, Missouri, there’s a geological enigma that has produced some of the largest quakes on record for the U.S.
However, this region in America has yet to be fully explained by scientists as New Madrid is about as far from a plate boundary as you can get.
Salt Lake City
Salt Lake City is known for its beautiful landscape, but the source of this beauty is also a very serious and potentially deadly seismic hazard.
The Wasatch Fault lies underneath Salt Lake City which is home to 1.6 million people.
Evidence shows that the area is due for an earthquake.
While Hawaii is a well-known volcanic hazard, not many people realize that it’s also susceptible to major earthquakes.
For example, in 1868 a 7.9 magnitude quake killed 77 people.
Because evidence of prehistoric quakes is difficult to find, it makes it hard to predict when the next one will hit.
All earthquakes in Hawaii are associated with the processes that form volcanoes.
Alaska was home to the second-largest earthquake ever recorded.
This quake occurred in Prince William Sound.
The magnitude of the 9.2 quake killed 128 people.
Most of these deaths were the result of a tsunami that reached heights of around 220 feet locally.
The mega-thrust quake was the product of the collision of two tectonic plates.
The ocean plate was forced beneath the continental plate.
Alaska is an earthquake hotspot because of the Denali fault.
Fault lines are spread throughout the United States.
Many are located in California and the American west, but there are some that exist in the central and eastern regions of the U.S. – they just haven’t been as active.
Understanding where these fault lines are located can help you be more prepared for the seismic activity should it occur.
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