Some buildings sway more than others in an earthquake. Scientists are exploring how to integrate this variation into Earthquake Early Warning systems.
By Jeng Hann Chong, University of New Mexico
Citation: Chong, J.H., 2022, Scientists take 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 out on the street. Alerting systems in the U.S. currently do not account for this. A team of researchers from the University of California, Los Angeles (UCLA) and the U.S. Geological Survey (USGS) are working on a potential solution that optimizes Earthquake Early Warning systems to warn people in tall buildings differently than those on the ground. The team presented their latest results at the 2021 Fall Meeting of the American Geophysical Union.
When the 2019 Ridgecrest earthquake sequence struck southern California, people in downtown Los Angeles felt some of the shocks, particularly the magnitude-6.4 foreshock and magnitude-7.1 mainshock. Many experienced weak shaking — Modified Mercalli Intensity (MMI) level III. Fortunately, Los Angeles sustained no significant infrastructure damage because the earthquakes nucleated 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 run by the U.S. Geological Survey (USGS) and several partner organizations. At the time, public alerting via ShakeAlert was only available to Los Angeles County, through the ShakeAlertLA app, which notified those expected to experience a shaking intensity greater than MMI IV, and encouraged them to seek cover.
ShakeAlert underestimated the magnitudes of the two large Ridgecrest earthquakes (Chung et. al., 2020), resulting in an underprediction of shaking intensities for Los Angeles, especially for the magnitude-7.1 mainshock, which was initially estimated to be a magnitude-6.3 event. This underestimate meant that many people who felt shaking received no warning. Some criticized the system for the mishap because the shaking was pronounced, especially for people in tall buildings.
Tall buildings respond differently
Buildings have a natural frequency, or rate, at which they will sway back and forth — known as a resonant frequency. When seismic waves with the same frequency pass a building, they are amplified, causing stronger shaking. In this way, tall buildings are particularly vulnerable to large and long-distance earthquakes, said Farid Ghahari, study coauthor and a research scientist at UCLA.
Damping describes how something that’s oscillating — like a building — will dissipate the energy of that rhythmic movement. Taller buildings tend to experience more prolonged shaking than shorter ones because they often have lower damping, said Ghahari.
Body waves from earthquakes rapidly travel through the ground compared to the slower, more destructive surface waves. By detecting the first body waves — the P-waves — Earthquake Early Warning systems like ShakeAlert can rapidly estimate magnitude, shaking intensity, and location of shaking.
As soon as this calculation is made, ShakeAlert creates a ShakeMap that alerting systems use to inform people that seismic waves are en route. However, ShakeAlert only considers ground-level shaking, which means that building height isn’t explicitly taken into account. And for a large, distance earthquake, shaking in tall buildings could exceed the alert threshold, even if it doesn’t on the ground.
To resolve this issue, Ghahari and the team are identifying ways to rapidly predict shaking for tall buildings.
Identifying a suitable solution
Because each building will respond differently during an earthquake, Ghahari and the team needed to find a way to estimate a building’s Peak Floor Acceleration — a measure of the greatest acceleration that the 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 — Federal Emergency Management Agency (FEMA) P-58 and American Society of Civil Engineers (ASCE) 7-16. The team also examined a more complex solution that they termed the “modal solution.”
The team found that for buildings in Los Angeles affected by distant earthquakes, the FEMA P-58 solution underestimates the observed Peak Floor Acceleration. The FEMA P-58 solution was developed for buildings less than 15 stories tall and earthquakes less than 50 kilometers (about 30 miles) away, says Ghahari, which could explain the discrepancy. On the other hand, the ASCE 7-16 solution performed better for distant earthquake despite overpredicting the shaking intensity for nearby temblors.
The more complicated modal solution yields a better approximation of Peak Floor Acceleration than both other, simpler solutions because it considers specific properties of a given building, like the building’s resonant frequency, mode shape (how different floors move with respect to one another at a specific period) and damping ratio (a measure of the building’s dissipation energy capability).
Because Earthquake Early Warning systems work best for earthquakes located some distance away due to the time needed for calculations and alerting (on the order of seconds), Ghahari says he and the team prefer the ASCE 7-16 solution. Moreover, gathering building-specific information needed for the modal solution for all tall buildings in each city can be difficult, Ghahari notes.
The Peak Floor Accelerations predicted by the ASCE 7-16 solution compare well to observed Peak Floor 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. That the team favors the ASCE 7-16 solution makes sense, he says, because the FEMA P-58 solution was used outside the intended range in the case of Ridgecrest.
Eventually, using an accurate estimate of how the ground will move during an earthquake (Peak Ground Acceleration) will help scientists predict buildings responses before the arrival of seismic waves, says Ghahari. Another way forward involves combining artificial intelligence with a large dataset of simulated building response from different earthquake scenarios, he says.
Ongoing studies are currently looking at creating detailed building databases using different methods, says Cook, which would enhance the utility of the modal solution.
Another complication, says Ghahari, “[is that] different buildings have been observed to [shake] differently even when they are adjacent to one another.” Because residents in the same building at different floors can also experience shaking differently, he says, an ideal future for Earthquake Early Warning systems is one that can warn people based on their location within a building.
Chung, A. I., Meier, M-A, Andrews, J., Böse, M., Crowell, B. W., McGuire, J. J., Smith, D. E., (2020), ShakeAlert Earthquake Early Warning System Performance during the 2019 Ridgecrest Earthquake Sequence. Bulletin of the Seismological Society of America, 110 (4), 1904–1923. doi: https://doi.org/10.1785/0120200032
Ghahari, S. F., Baltay, A., Çelebi, M., Parker, G. A., McGuire, J. J., & Taciroglu, E. (2022, January 25). Earthquake early warning for estimating floor shaking levels of Tall Buildings. Bulletin of the Seismological Society of America. doi: https://doi.org/10.1785/0120210224