Glad You Asked: What is an Earthquake Early Warning System, and does Utah Have One?
By Michael Hylland
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Can earthquakes be predicted? In a word, no.
Of course, our understanding of earthquake processes and occurrence has increased tremendously since the first earthquake-detection instrument was invented in China nearly 2,000 years ago, and even since 1935 when Charles Richter helped develop the earthquake magnitude scale bearing his name. Our current state of knowledge allows us to forecast the likelihood of a damaging earthquake in a particular region within some given amount of time. But, we remain unable to predict specific earthquakes in a time frame that allows actions to be taken to reduce damage, injuries, and loss of life.
Although earthquakes cannot be predicted, technology now exists that can detect earthquakes quickly and predict the arrival times of ground motions (shaking), and the severity (intensity) of shaking, at sites in the region of the earthquake epicenter. These Earthquake Early Warning (EEW) systems are designed to send alerts that can prompt actions to protect life and property before strong shaking arrives.
EEW systems work on the principle that electronic warning signals can be transmitted almost instantaneously, whereas earthquake ground motions travel through the Earth’s shallow crust at speeds of around 0.5 to 3 miles per second. An earthquake begins when movement occurs along a fault in the Earth’s crust; for large earthquakes in Utah, this movement typically initiates about 10 miles beneath the ground surface. The movement occurs when crustal stresses build up and finally exceed the frictional forces that normally hold the bedrock in place along the fault. All that built-up energy is released suddenly and radiates outward in waves, like the ripples on a pond that radiate outward from the rock that was thrown in.
An earthquake produces different kinds of seismic waves that move at different speeds. The fastest waves, P waves (primary waves), are compressional waves that travel through the interior of the Earth similar to how a sound wave travels through the atmosphere. These waves are the first to arrive at a site, produce the smallest ground motions, and are typically felt as a “bump.” The next waves to arrive are S waves (secondary, or shear waves), which travel through the interior of the Earth with a side-to-side motion (like the motion produced by snapping a garden hose). Because these waves have a higher amplitude than P waves, they are felt more strongly. The last waves to arrive at a site are waves that travel along the Earth’s surface. These surface waves generally cause the most damage during an earthquake.
The first component of an EEW system is a dense network of sensors that can detect P waves and then trigger the alert. An alert center that nearly instantaneously receives signals from the sensors can use computer algorithms to quickly estimate the earthquake’s location and magnitude, map the resulting intensity in the region of the earthquake, and calculate the arrival times of damaging ground motions. This information can then be sent in a mass communication via public emergency alert systems, smartphone apps, social media, and other electronic alert technologies before strong shaking begins.
Depending on how far a site is from where the earthquake occurred, an EEW system can provide seconds to minutes of advance warning. Even a few seconds of warning can be enough to allow property- and life-saving actions to be set in motion. For example, schoolchildren and others can be alerted to “drop, cover, and hold on,” industrial workers can move away from dangerous machines or chemicals, and surgeons can suspend delicate operations. Automated responses can also be triggered, such as slowing down or stopping trains or taxiing airplanes, opening doors at ambulance and fire stations, and initiating safety and back-up protocols at power plants.
Several countries, including Japan, Mexico, and Turkey, have EEW systems in place and have demonstrated their effectiveness. A similar system has not yet been fully implemented in the U.S., but a demonstration EEW called ShakeAlert has been developed for California, Oregon, and Washington by the U.S. Geological Survey (USGS) and university partners. ShakeAlert has been sending alerts to test users, including the San Francisco Bay Area Rapid Transit (BART) system, since 2012. During the magnitude (M) 6.0 South Napa earthquake on August 24, 2014, the shaking intensity in the BART service area was not sufficiently high to prompt emergency actions, but the BART offices received an alert 10 seconds before shaking began.
Utah does not currently have an EEW system, but several factors make a future system attractive for Utah’s Wasatch Front, including the likelihood of large earthquakes (M 7); a high population density; heavily used transportation systems including freight, commuter, and light rail, and a major international airport hub; and numerous vulnerable commercial and service facilities such as oil refineries and hospitals.
At the same time, several factors present significant challenges, including the fact that about 80 percent of Utah’s 2.9 million residents live within 15 miles of the Wasatch fault (the most geologically active fault in Utah), meaning that alert times related to a Wasatch fault earthquake could be very short. Also, costs associated with implementation, operation, and maintenance would likely be in the tens of millions of dollars. Still, numerous other sources of large earthquakes exist in the Wasatch Front region, and as the saying goes, “an ounce of prevention is worth a pound of cure.” Conceivably, an effective Wasatch Front EEW system could save Utah billions of dollars, as well as many lives, when the Big One happens.
Survey Notes, v. 47 no. 3, September 2015