Did you know that Oklahoma has more earthquakes than California? Or Wyoming and Nevada record almost 200 earthquakes with magnitudes above or equal to 3 (M ≥ 3.0) per year?
How about of the five largest earthquakes (M ≥ 5.8) in the contiguous US in the past decade, 3 were not in CA, and 2 were not even in the West? [i] If you do, then you would likely agree that seismic risk is not just a concern for Californian properties. This week’s post, the final in our series on seismic risk assessments (SRAs), will look at seismic hazards outside of California.
The fact is that California does have the most damaging earthquakes, so there is a lot of focus on risk mitigation in the state. However, that doesn’t mean that earthquakes outside of California aren’t causing damage to buildings and infrastructure, especially if they were built without consideration of seismic standards. Take the following examples:
In 2011, an M5.8 earthquake in Virginia resulted in damage as far as Washington, D.C., 97 miles away. [ii]
An M5.8 quake caused damage to buildings and homes in Pawnee, OK in 2016. [iii]
Wells, Nevada businesses faced extensive damage after an M5.9 earthquake in 2008. [iv]
A month later, a M5 earthquake in Cushing, OK, 25 miles from Pawnee, caused damage to over 40 buildings. [v]
Damage typically occurs from earthquakes at or above magnitude 4, but as the USGS states, “there is not one magnitude above which damage will occur. It also depends on other variables, such as the distance from the earthquake, what type of soil you are on, etc.” [vi]
For instance, earthquakes that occur in the east actually have the potential for much greater damage than those in the west because of the east’s older continental crust. Its age makes it more rigid, and therefore can efficiently propagate seismic energy over hundreds of miles, whereas newer crust in the west is warmer and can absorb the seismic energy. For example, a historic M≥8.5 that occurred in the Mississippi Valley (discussed further below) had “shocks felt strongly over an area of 50,000 square miles; moderately over an area between 965,000 and one million square miles; and at least lightly over an area of approximately one million square miles. By comparison, the San Francisco earthquake of 1906 was felt moderately over only 60,000 square miles.”
Seismologists have also recently noticed that in the east, earthquakes often occur along reverse faults where the ground shifts vertically against the pull of gravity, which release greater energy than strike-slip faults of the west which slide horizontally past each other. It’s just not as common for very large earthquakes to occur in some parts of the east as they do throughout the west, although historically, they have occurred.
Historic Earthquakes East of the Rockies
Some of the largest earthquakes in the United States since its settlement by Europeans occurred in the Mississippi Valley during 1811-1812 in an area known as the New Madrid seismic zone (NMSZ). They were by far the largest east of the Rocky Mountains in the U.S. and Canada, with estimated magnitudes between 8.5-8.7 and ground motions so powerful that they caused permanent changes in the course of the Mississippi River, which flowed backwards temporarily. [vii] The NMSZ, extending from northeast Arkansas, through southeast Missouri, western Tennessee, and western Kentucky to southern Illinois, has experienced several other massive earthquakes, including a M6.8 in Charleston, MO in 1895, and a M5.8 in Blacksburg, VA two years later.
New York has experienced several large earthquakes (M ≥ 4.0), especially in the northern part of the state and along the Canadian border. The town of Attica suffered widespread damages after a M5.2 quake in 1929. 250 miles northeast along the St. Lawrence River, Massena, New York was hit with a M5.8 quake that caused $2 million in damages, relative to the time in 1944. The area surrounding Lake George experienced building damages after a M4.7 quake in 1931, and New York City suffered damaging M5.2 earthquakes twice, in 1737 and 1844. [viii]
In the southeast, Charleston, South Carolina experienced a massive M7.7 earthquake in 1886, damaging thousands of buildings, some as far as 100 miles away, and a year later Virginia experienced a M5.8 earthquake felt in 13 surrounding states.
Earthquakes as Indicators
What is the modern significance of these historic earthquakes? Seismologists have learned that earthquakes occurring east of the Rockies are as likely, if not more likely, to occur on unknown fault lines. Therefore, fault lines east of the Rockies are an unreliable guide for determining earthquake probability. The best guide, according to the USGS, are the earthquakes themselves, “This doesn’t mean that future earthquakes will occur exactly where past ones did, although that can happen. It means that future earthquakes are most likely to occur in the same general regions that had past earthquakes… Most earthquakes tend to occur in the same general regions that are already known to have earthquakes.” [ix]
This has proven to be the case, as every region where the aforementioned historic earthquakes have occurred have since experienced significant earthquakes (M ≥ 4.5). Some of the most recent of this magnitude include a M5.8 in Virginia (2011), M4.7 in Arkansas (2011), M4.8 in SE Missouri (1990), M4.6 in NE Alabama (2003), M5.2 in SE Illinois (2008) and M5.3 in NE New York (2002).
USGS Map of M ≥ 4.0 earthquakes recorded since 1900 [x]
Natural & Industry-Induced Earthquakes
New research is being done to understand the correlation of industrial practices, particularly fluid injection from oil and gas extraction, and a region’s seismic activity. Midwest and Central US regions where these practices occur saw the average number of M≥3 increase six-fold in 2011 over 20th century levels (160-fold in Oklahoma), and some regions with zero seismic activity on record, like Dallas-Fort Worth, TX, are now experiencing an average of 20 earthquakes every year.
Induced earthquakes typically occur as a result of “wastewater disposal,” a practice used in the oil and gas extraction process in which fluid waste from production is injected deep underground far below ground water or drinking water aquifers. Earthquakes occur when the injected wastewater counteracts the frictional forces on faults (tectonic stress and the weight of rock which keeps the faults from slipping) and, in effect, “pries them apart”, thereby facilitating earthquake slip. [xi] This can occur miles from the injection site as the pressure travels underground.
An induced earthquake versus a natural earthquake of the same magnitude will have the same effect and potential for damage. Several large induced earthquakes M≥4 have occurred in Texas, Colorado, Oklahoma, and Ohio resulting in damages. The largest induced earthquake recorded thus far was a M5.7 in Prague, OK, but seismologists have not ruled out the possibility of larger M≥7 induced earthquakes. The question as to who is liable for the resulted damages still remains up to the Supreme Court to decide.
Rethinking Your Property’s Seismic Risk?
Regions where earthquakes aren’t the norm typically face greater devastation because people are unprepared and buildings do not have the engineering to withstand ground shaking. It doesn’t have to be expensive to retrofit a building to meet seismic standards – in some cases, anchors or braces are all that is needed to protect a building façade from seismic damage. Whether you’re a lender, investor, buyer or owner, if your property is located in a region seeing historic or recent seismicity, working with a licensed engineering consultant can help you understand potential seismic hazards and make informed business decisions.
For more information on seismic risk assessments, read more on the EBI website or contact us.