In most top earthquake risk states in the U.S., Alaska, California, Hawaii, Nevada, and Washington top the list.
Alaska is the most seismically active region in the United States. But the lower population means there isn’t as much devastation to report.
That said, the second-largest earthquake ever recorded shook southern Alaska in 1964 with a magnitude of 9.2. In 2002, a magnitude 7.9 earthquake struck the Denali Fault in central Alaska. Another 7.9 hit on January 23, 2018, near Kodiak Island. Since 1899, six additional quakes in the magnitude 8 range have struck. Overall, Alaska has about 11 percent of the world’s recorded earthquakes. Three of the eight largest earthquakes were in Alaska. Narrowing earthquakes to the United States, seven of the ten largest were in Alaska.
Southern Alaska is near the Alaska-Aleutian Megathrust Fault Zone, which is one of the most active and dangerous on Earth.
On average, since 1900, Alaska experiences a magnitude 8.0 or larger earthquake every 13 years. The following shows how many earthquakes (on average) and their corresponding magnitudes the state experiences every year.
Magnitude range # tremors
The good news is, with improved building codes since the 1964 earthquake, a relatively low population, and the lack of high-rise buildings, it is likely that large earthquakes in Alaska will have fewer fatalities than similar-size tremors in other locations.
For the contiguous United States, California has the highest earthquake risk. And, with its high population and large cities, the risk of damage from earthquakes is the highest.
California is overdue for a large earthquake, magnitude 7.0 or stronger. In the 1800s, 14 earthquakes of magnitude 6.0 or larger occurred. Since the 1906 Great Earthquake in San Francisco, there have been only three. According to a study on paleoseismic data, there has essentially been an earthquake drought since 1906 (Biasi & Scharer, 2019).
The San Andreas Fault is the most famous (or infamous) fault in California, but there are many others. In northern California, the Hayward Fault is the biggest threat. In 2015, the third Uniform California Earthquake Rupture Forecast (UCERF3) was developed to provide updated earthquake forecasting. Compared to the previous forecast, the likelihood of moderate earthquakes (magnitude 6.5 to 7.5) has gone down, but the probability of larger ones has gone up. Tables 9–11 summarize some of the probabilities.
The following table lists the likelihood of one or more earthquakes in a 30-year period, beginning 2014, for faults near San Francisco.
Fault M6.7+ M7.5+ M8.0+
Northern San Andreas 6.4% 5.7% 2.1%
Calaveras 1.1% — —
Hayward 14.3% 3.6% <0.1%
The next table shows the chances of one or more earthquakes in a 30-year period, beginning 2014, for faults near Los Angeles.
Fault M6.7+ M7.5+ M8.0+
Southern San Andreas 19.0% 17.3% 6.8%
San Jacinto 5.0% 4.9% 2.7%
Elsinore 3.8% 1.0% <0.1%
Combining smaller areas into larger regions, without considering specific faults, shows a more ominous earthquake probability. On a more regional basis, the following is the probability of one or more earthquakes in a 30-year period, beginning 2014.
Region M5+ M6+ M6.7+ M7+ M7.5+ M8+
California as a whole 100% 100% >99% 93% 48% 7%
Southern California 100% 100% 93% 75% 36% 7%
Northern California 100% 100% 95% 76% 28% 5%
San Francisco Region 100% 98% 72% 51% 20% 4%
Los Angeles Region 100% 96% 60% 46% 31% 7%
Simply stated, the UCERF3 documents estimate that California has a 93 percent probability of a magnitude 7.0 or larger earthquake by 2045. The San Andreas Fault system has the highest chances of a high-magnitude earthquake.
Looking more specifically at the Los Angeles area faults, a team of geophysicists at Caltech “created a new method for determining earthquake hazards by measuring how fast energy is building up on faults in a specific region, and then comparing that to how much is being released through fault creep and earthquakes” (Perkins R., Fast, Simple New Assessment of Earthquake Hazard, 2019
With this new prediction method, the faults under central Los Angeles are predicted to have the strongest earthquakes in the magnitude 6.8 to 7.1 range. These quakes should strike about every 300 years on average. Previous estimates put the large quake average on the south San Andreas Fault at about every 150 years, with the last big one happening more than 300 years ago.
When assessing the risk for smaller earthquakes, the 10-year probability for a magnitude 6.0 or larger earthquake is about 9 percent, with the probability of a magnitude 6.5 or greater earthquake at about 2 percent.
By the numbers, Hawaii ranks third for having the most earthquakes in the U.S. However, given its small land mass, Hawaii is where you are more likely to experience an earthquake than in any other state.
Most of the seismic threat in Hawaii is directly related to volcanic activity. Earthquakes can happen before or during an eruption. They can also be caused by magma flow that doesn’t erupt under the surface.
Large earthquakes in Hawaii that are not related to volcanic activity occur at irregular intervals.
A magnitude 6.9 earthquake shook Hawaii on May 4, 2018. Its epicenter was on the south side of Kilauea.
In April 2019, a magnitude 5.3 also rocked the island, although this tremor was caused by crustal movement and wasn’t magma related.
The largest recorded earthquake in Hawaii, magnitude 7.9, occurred in 1868, when the island was far less populated. The earthquake caused a landslide that killed 31 people and a tsunami that took another 46 lives.
After Hawaii, Nevada is the fourth most seismically active state, though some lists will rank Nevada as third. Near Las Vegas, the half-dozen faults have been mostly quiet for the past 1,000 years. However, they are capable of magnitude 7.0 ruptures.
Las Vegas, Carson City, and the Reno-Tahoe areas are all at high risk of large earthquakes, although tremors could strike anywhere in the state. A magnitude 7.0 or larger quake in the Reno/Sparks area could easily cause an excess of $1.9 billion of damage, and a rupture in the Stateline area near Tahoe could cause $590 million in damage.
At least 30 faults could potentially cause damage in the Reno and Carson City urban areas. The probability of at least one magnitude 6.5 tremor occurring in the next 50 years is between 50 and 60 percent.
The city of Fallon, in Churchill County, has a 20 to 25 percent probability of experiencing a magnitude 6.5 or larger in the next 50 years.
Faults around the Las Vegas area have a lower probability of a large magnitude tremor, but there is still a 10 to 20 percent chance of a magnitude 6.0 or larger striking the area within the next 50 years.
In 2008, a magnitude 6.0 tremor struck the small rural community of Wells. Of the 1,600 residences, 700 were damaged as a result of the earthquake. The area is still struggling to recover, as businesses closed and people left the area.
The following is a list of the biggest earthquakes in Nevada from 1915 to 2008.
Date Location Magnitude
October 2, 1915 Pleasant Valley 7.1
December 20, 1932 Cedar Mountain 7.2
January 30, 1934 Excelsior Mountains 6.5
July 6, 1954 Fallon-Stillwater area 6.6
August 23, 1954 Stillwater 6.8
December 16, 1954 Fairview Peak 7.1
December 16, 1954 Dixie Valley 6.8
February 21, 2008 Wells 6.0
Depending on the list, in the contiguous U.S., Washington has the second (or third) highest risk of earthquakes. While there are other earthquake risks, the state’s biggest risk is a megathrust quake along the Cascadia Subduction Zone. Such an event will affect the millions who live in and around the Seattle area, along with those living in other areas around the Puget Sound and Pacific Northwest.
Besides the Cascadia Subduction Zone, there are two other major earthquake threats. The Seattle Fault, which is a complex of faults branching across the Seattle area, and a much larger fault zone north of Seattle called the South Whidbey Island Fault. The three fault zones can produce earthquakes with magnitudes of 7 to over 9 (Why you should be prepared: 3 big earthquake threats in PNW
The magnitude 6.8 Nisqually quake in February 2001 was Washington’s most recent big earthquake.
A magnitude 7.0 event on the Seattle Fault would not just shake the city and surrounding communities, but it would likely cause a tsunami in the Puget Sound that would inundate the waterfront area. The last rupture was about 1,000 years ago, and scientists have no idea when the next one might happen.
The South Whidbey Island Fault could produce a 7.5 rupture, although, like the Seattle Fault, scientists have not determined any average interval.
The Top 16 Earthquake Risk States in the U.S.
Post a Comment