Earthquakes

An earthquake is a sudden, rapid shaking of the ground caused by the shifting of rocks deep underneath the earth’s surface. Earthquakes can cause fires, tsunamis, landslides or avalanches.

While they can happen anywhere without warning, areas at higher risk for earthquakes include Alaska, California, Hawaii, Oregon, Puerto Rico, Washington and the entire Mississippi River Valley.

The seven steps of earthquake safety.

Prepare

  • Secure items such as televisions and objects that hang on walls. Store heavy and breakable objects on low shelves. 
  • Practice “Drop, Cover, and Hold On” with family and coworkers. Drop to your hands and knees. Cover your head and neck with your arms. Crawl only as far as needed to reach cover from falling materials. Hold on to any sturdy furniture until the shaking stops. 
  • Create a family emergency communication plan that has an out-of-state contact. Plan where to meet if you get separated. 
  • Make a supply kit that includes enough food and water for at least three days, a flashlight, a fire extinguisher, and a whistle. Consider each person’s specific needs, including medication. Do not forget the needs of pets. Have extra batteries and charging devices for phones and other critical equipment. 
  • Consider earthquake insurance policies. Standard homeowner’s insurance does not cover earthquake damage. 
  • Consider a retrofit of your building if it has structural issues that make it vulnerable to collapse during an earthquake. Also, get professional help to repair defective electrical wiring, leaky gas lines, and inflexible utility connections.
Take steps in advance to prepare for potential earthquakes.

Stay Alert

Familiarizing yourself with these terms will help you to understand earthquake hazards:

Earthquake: a sudden slipping or movement of a portion of the earth’s crust, accompanied and followed by a series of vibrations.

Aftershock: an earthquake of similar or lesser intensity that follows the main earthquake.

Fault:  a fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture causing an earthquake. The slippage may range from less than one inch to more than 10 yards in a severe earthquake.

Epicenter: the place on the earth’s surface directly above the point on the fault where the earthquake rupture began. Once fault slippage begins, it expands along the fault during the earthquake and can extend hundreds of miles before stopping.

Seismic Waves: vibrations that travel outward from the earthquake fault at speeds of several miles per second. Although fault slippage directly under a structure can cause considerable damage, the vibrations of seismic waves cause most of the destruction during earthquakes.

Magnitude: the amount of energy released during an earthquake, which is computed from the amplitude of the seismic waves. A magnitude of 7.0 on the Richter Scale indicates an extremely strong earthquake. Each whole number on the scale represents an increase of about 30 times more energy released than the previous whole number represents. Therefore, an earthquake measuring 6.0 is about 30 times more powerful than one measuring 5.0.

There are a number of different aspects to earthquakes.

During an Earthquake

If you feel an earthquake you should minimize your movements and find a nearby safe place. Stay indoors until the shaking has stopped and you are sure exiting is safe.

If you are indoors:

  • Drop, Cover, and Hold On. Drop to your hands and knees. Cover your head and neck with your arms. Hold on to any sturdy furniture until the shaking stops. Crawl only if you can reach better cover without going through an area with more debris. 
  • Take cover under a desk, table, bed or doorframe. Stay clear of windows and other glass. 
  • Stay put until the shaking stops. Do NOT run outside. Most injuries during earthquakes occur when people are hit by falling objects while entering into or exiting from buildings.
  • If in bed, stay there and cover your head and neck with a pillow.
  • Be aware that the electricity may go out, or the sprinkler systems or fire alarms may turn on. Do not use the elevators.

If you are outdoors:

  • Stay outside.
  • Find an open area clear of buildings and power lines.

If you are in a moving vehicle:

  • Stop your vehicle in an open area that is away from buildings, trees, overpasses, underpasses, or utility wires. 
  • Stay inside with your seatbelt on until the shaking stops.
  • Proceed cautiously once the earthquake has stopped, and watch for road and bridge damage.
  • Be alert for falling rocks and landslides, especially if you are near slopes, cliffs, or mountains.
  • If a power line falls on your vehicle, do not get out. Call 911 and wait in your vehicle for assistance.

If you are trapped under debris:

  • Do not move about and kick up dust, or light a match.
  • Cover your mouth with a handkerchief or clothing.
  • Tap on a pipe or wall so rescuers can locate you, or use a whistle if one is available. Shout only as a last resort. Shouting can cause you to inhale dangerous amounts of dust.

Immediately after an earthquake:

  • Listen to the radio and follow instructions.
  • DO NOT enter damaged structures.
  • Check all gas lines.
  • Inspect chimney or have chimney inspected before using the fireplace.
  • Expect to feel aftershocks, which are usually smaller in size, and take the same precautions as you would during an earthquake.
  • For power outages, damaged gas lines or downed wires, call your utility company.
  • If you are inside and smell gas, get out and move as far away as possible.
  • Before leaving any building, check for debris that may fall on you.
During an earthquake, drop, cover, and hold on.

Alert Systems

Using alert systems is crucial to being informed before and during earthquakes, just as with any other disaster. Our alert systems page has resources for a number of alert options.

The Red Cross also has an Earthquake app that includes earthquake alerts and information for your area.

The Earthquake Notification Service (ENS) is a free service that can send you automated notification emails when earthquakes happen in your area.

The United States Geological Survey maintains a user-defined searchable database to see earthquakes occurring around the world which can be found here: https://earthquake.usgs.gov/earthquakes/search/

Local Earthquake Risks

There are three seismic zones in the Central U.S. that could affect Williamson County.

History of earthquakes in Williamson County

Williamson County is in close proximity to the major intraplate (within a tectonic plate) seismic zone known as the New Madrid Seismic Zone (NMSZ). The NMSZ is an approximately 120-mile long fault system that stretches across five states including Western Tennessee. 

The New Madrid Seismic Zone runs through the Central U.S.

Historically the zone is known for producing four of the largest North American earthquakes in recorded history, all of which would have been felt in Williamson County. This includes the noted three-month period between December 1811 and February 1812 that had quakes reaching Richter Scale magnitudes into the 7.0 through 8.6 ranges. 

Earthquake comparison chart of intensity versus magnitude.
The Mercalli intensity scale.

Since the 1812 earthquakes, the largest recorded quakes from this zone were the October 1895 6.6 magnitude quake (epicenter Charleston, MO) and the November 1968 5.5 magnitude quake (epicenter in Dale, IL). From the time when seismic measurement instruments were installed in and around the zone in the 1970s, more than 4,000 small earthquakes have been recorded, with the vast majority being too small to be felt.

Map of NMSZ history and possible future activity.

According to a FEMA report filed in 2008, a serious earthquake in the NMSZ could result in the highest economic loss due to a natural disaster in U.S. history, causing widespread and catastrophic damage across a seven-state radius with most of the worst impacts taking place in Western Tennessee (includes Williamson County). Based on this report, a 7.7 magnitude quake in the NMSZ would result in thousands of fatalities, damage to structures, negative social impacts, economic loss, and total disruption of vital infrastructure in Western Tennessee. 

Williamson County is not part of the 20-county impact zone expected if there is a large earthquake along the New Madrid Seismic Zone. However, Williamson County has the potential for large damage. Furthermore, Williamson County would most likely provide shelter and assistance to those who experience damage and loss due to the earthquake. 

The earthquake problem, danger, and solution in the Central U.S.

The current lack of apparent land movement along the NMSZ has long puzzled scientists. Currently GPS measurements show that the NMSZ faults are moving no more than 0.0079 inches a year. In contrast the San Andreas Fault in California moves up to 1.5 inches a year. This has led some researchers to believe that the fault may be “shutting down” while others say it is a “sleeping giant.” These differing views have made it difficult for public policymakers to decide on if and how much to prepare for and spend on mitigating a potential large-scale earthquake. 

Ranking Local Vulnerability

Williamson County uses a simple system called a vulnerability calculator to determine each jurisdiction’s vulnerability to hazardous events, as shown in the charts below.

Earthquake Vulnerability Calculator for Williamson County.

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