Scientists take a step toward warning those in tall buildings during earthquakes

Some buildings sway more than others in an earthquake. Scientists are exploring how to incorporate this difference into earthquake early warning systems.

Written by Jing Han Chung, University of New Mexico

Citation: Chung, JH, 2021, Scientists take a step toward warning those in tall buildings during earthquakes, Temblor, http://doi.org/10.32858/temblor.233

When an earthquake strikes, people in tall buildings can experience more shaking than others on the street. Alert systems in the United States currently do not take this into account. A team of researchers from the University of California, Los Angeles (UCLA) and the US Geological Survey (USGS) is working on a potential solution that improves earthquake early warning systems to warn people in tall buildings differently from those on the ground. The team presented its latest findings at the 2021 fall meeting of the American Geophysical Union.

Ridgecrest earthquakes

When the 2019 Ridgecrest earthquake sequence hit southern California, people in downtown Los Angeles felt some shocks, particularly the 6.4-magnitude initial tremor and the 7.1-magnitude main quake. Suffered a lot of weak vibration – Modified Mercalli Intensity (MMI) level three. Fortunately, Los Angeles did not suffer any major infrastructure damage because the earthquakes were more than 100 miles (about 160 kilometers) away.

The Ridgecrest earthquakes were the largest in California since the implementation of ShakeAlert – the earthquake early warning system operated by the United States Geological Survey (USGS) and several partner organizations. At the time, a public alert via ShakeAlert was only available to Los Angeles County, through the ShakeAlertLA app, which informed those who expected to experience vibration intensity greater than MMI IV, and encouraged them to seek cover.

ShakeAlert underestimated the magnitude of the two large Ridgecrest earthquakes (Chung et. al., 2020), resulting in an unpredictable Los Angeles vibration intensity, especially for a 7.1 main shock, which was initially estimated to be a magnitude 6.3 event. . This disparagement meant that many of the people who felt the shaken received no warning. Some criticized the system for the accident because the shaking was obvious, especially for people in tall buildings.

Tall buildings respond differently

Buildings have a natural, or rate, frequency with which they oscillate back and forth – known as resonant frequency. When seismic waves of the same frequency pass a building, they are amplified, causing a stronger vibration. In this way, tall buildings are particularly vulnerable to large, long-range earthquakes, said Fred Gary, study co-author and UCLA research scientist.

Damping describes how something swaying – like a building – will dissipate the energy of that rhythmic movement. Gary said that taller buildings tend to experience more long jolts than shorter ones because they often have less damping.

Body waves from earthquakes travel faster through the Earth than slower, more destructive surface waves. By detecting the first body waves – the P waves – earthquake early warning systems like ShakeAlert can quickly estimate the magnitude, intensity and location of the vibration.

Once this calculation is made, ShakeAlert creates a ShakeMap that alert systems use to notify people that seismic waves are on their way. However, ShakeAlert only takes into account ground level vibration, which means that building height is not explicitly taken into account. And in the event of a large earthquake at a great distance, the vibration in tall buildings can exceed the alert threshold, even if it does not occur on the ground.

To solve this problem, Gary and the team are identifying ways to quickly predict the vibration of tall buildings.

Determine the appropriate solution

Since every building will respond differently during an earthquake, Gary and the team needed to find a way to estimate a building’s floor-peak acceleration—a measure of the greatest acceleration a building experiences during an earthquake.

They tested two simplified mathematical solutions to calculate PFA from peak ground acceleration (PGA). These came from two sets of building design manuals used by engineers – FEMA P-58 and American Society of Civil Engineers (ASCE) 7-16. The team also studied a more complex solution they called the “model solution.”

The team found that for buildings in Los Angeles affected by distant earthquakes, the FEMA P-58 solution underestimated the observed floor-peak acceleration. The FEMA P-58 solution was developed for buildings less than 15 stories high and earthquakes less than 50 kilometers (about 30 miles) away, Gary says, which may explain the discrepancy. On the other hand, the ASCE 7-16 solution performed better for the distant earthquake although the intensity of the shaking was overestimated for the near seismic cases.

A more complex model solution results in a better approximation for floor-peak acceleration than both other simpler solutions because it takes into account the specific characteristics of a given building, such as the building’s buzzing frequency, the shape of the situation (how the different floors move with respect to each other in a specific time period) and the damping ratio (a measure of for the building’s dissipation energy capacity).

Skyscrapers in downtown Los Angeles. Credit: BDS2006, via Wikimedia Commons (CC-SA 3.0)

Since earthquake early warning systems work best for earthquakes that are farther away due to the computational and alerting time (in the order of seconds), Gary says he and his team prefer an ASCE 7-16 solution. Furthermore, gathering the building-specific information needed for a typical solution for all tall buildings in each city can be difficult, Gary points out.

The peak ground accelerations predicted by the ASCE 7-16 solution compare well with the peak ground accelerations measured during the Ridgecrest mainshock, says Dustin Cook, a postdoctoral fellow at the National Institute of Standards and Technology who was not involved in this study. He says the team’s preference for an ASCE 7-16 solution makes sense, because a FEMA P-58 solution was used outside the intended range in the Ridgecrest case.

future alerts

Ultimately, using an accurate estimate of how the ground moves during an earthquake (the acceleration of the Earth’s peak) will help scientists predict the responses of buildings before the seismic waves arrive, says Gary. Another way forward is to combine artificial intelligence with a wealth of data for the response of simulated buildings from different earthquake scenarios, he says.

Currently ongoing studies are looking at creating detailed build databases using different methods, Cook says, which will enhance the utility of the model solution.

Another complication Gary says:[is that] Various buildings have been observed [shake] differently even when they are adjacent.” Since residents of the same building on different floors can also experience vibration differently, he says, an ideal future for earthquake early warning systems is one that can warn people based on their location within the building.

references

Chung, A.I., Meier, MA, Andrews, J., Böse, M., Crowell, BW, McGuire, JJ, Smith, D.E., (2020), Performance of the ShakeAlert earthquake early warning system during the 2019 Ridgecrest Earthquake Sequence. Bulletin of the American Seismological Society, 110 (4), 1904–1923. doi: https://doi.org/10.1785/0120200032

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