The Ridgecrest earthquake shook high-rise buildings in Los Angeles by up to 4 times more than expected

Earth’s motions in the Los Angeles basin show four times greater shaking than expected from the Ridgecrest earthquake.

Written by Laura Vataroso, MA, PhD. Candidate, UMass Amherst (labtalk_laura)

Quote: Vataroso, El, 2020, Ridgecrest earthquake shook tall buildings in Los Angeles by as much as 4 times more than expected, Tumblr, http://doi.org/10.32858/temblor.128

Buildings shook in Los Angeles during the 2019 magnitude 7.1 Ridgecrest earthquake. Some shook more than others. Credit: David Mark from Pixabay

A network of more than 500 sensors spread across Los Angeles picked up shaking during the Ridgecrest July 6, 2019, magnitude 7.1 earthquake. Although the epicenter was about 124 miles (200 km) away, the vibration was felt throughout downtown Los Angeles, and data from those sensors revealed that the vibration was amplified by up to four times the intensity expected in some parts of Los Angeles. Angeles, according to a just report in Earthquake Research Letters.

“Observations recorded on this dense cluster allowed us to see the variation in Earth’s vibration on a previously unobserved scale of an earthquake as large as the one in the Los Angeles basin,” explains Monica Kohler, a seismic engineer at the California Institute of Technology and lead author. Of the report.

Los Angeles is like a bowl of jelly

Risk assessments do consider amplifying vibration from sinks, but why some areas vibrate more than others isn’t well understood. An analogy often used to understand the effects of a tank is a bowl of jelly – when it vibrates, the gel vibrates more than the bowl. Los Angeles is built on a large, deep sedimentary basin that can amplify earthquake waves just like jelly, especially longer waves affecting large buildings and taller heights.

The study authors found that factors often considered in these risk assessments – basin depth and seismic shear velocity in shallow soils – are not always the best predictors of vibration amplification. This could mean less vibration in large buildings.

Ridgecrest shook Los Angeles

During the Ridgecrest earthquake, high altitudes took a lot of movement but not much damage. Although the basin is six miles (nine kilometers) deep below downtown Los Angeles, the largest swells that shook were seen in western Los Angeles and the San Fernando Valley.

“We thought the biggest amplification would be in the deepest part of the basin, but it turns out that oversimplifies things,” explains Kohler.

At sites across the Los Angeles Basin, seismometers recorded shaking during the Ridgecrest earthquake. The amplification of the largest vibration (the red diamond) of the long period waves (six seconds in this example) did not occur in the deepest parts of the basin – the darkest brown colors in this map – as the researchers expected. Credit: Kohler et al. , 2020

“It’s pretty cool – for years we’ve been talking about amplification from a very deep basin in Los Angeles. This is likely to change our thinking,” says Robert de Groot, communications, education and outreach coordinator for the US Geological Survey’s ShakeAlert Earthquake Early Warning System. He says cutting-edge research like this is always being incorporated into early warning technology and will likely inform future updates of those systems.

In the event of a future earthquake like Ridgecrest, a tall building in those areas could be subjected to four times greater vibration than a building in downtown Los Angeles, and eleven times what is felt on the bedrock surrounding the basin. In a 52-storey building, this can mean up to three feet (one meter) of oscillation on the upper floors, and up to two meters in a 7.6 magnitude earthquake, which can stress the building’s structural integrity.

Build to withstand vibration

The study has important implications for building codes, as long-amplitude waves can affect tall buildings as well as large fuel storage facilities and long-range bridges. “The building code currently has a very simplified version of what basin effects are, so we want to show how it should be modified or expanded for the next generation of code,” says Kohler.

Building codes take into account a simplified understanding of how vibration in the tub is amplified. Credit: Masbet Christiano from Pixabay

While this is a major step toward better risk assessment in the Los Angeles basin, many questions remain unanswered. Would a different earthquake produce the same vibration patterns observed during Ridgecrest? Kohler says the 2010 El Mayor Cocaba earthquake of magnitude 7.2 produced similar patterns. Santa Monica in western Los Angeles – both within the high amplification region of this study – was severely damaged during the 1994 Northridge earthquake. This indicates that the observed pattern of the Ridgecrest earthquake may be applicable to most others, but more data is needed.

Community Seismograph Group

The majority (360) of seismic stations that recorded this data were part of the Seismic Community Network – a decade-long partnership between the California Institute of Technology and the community to install these sensors. This project was a collaboration between civil engineers, seismologists, and computer scientists. Most community sensors are housed in public schools in the Los Angeles County Unified Education System. This network uses Raspberry Pis – credit card sized computers – to process data at every location, and from there it is sent to the cloud.

Other cities on the West Coast would benefit from deploying this kind of dense seismic network – major earthquakes in cities like San Francisco and Seattle are also affected by the amplification of the basin, but without the dense data provided by hundreds of sensors, these effects are not well understood.

“There are no downsides to that, it’s just a matter of getting the money. Seattle is one example of an active seismic area that really needs more stops like this,” says Kohler.

Check earthquake hazards in Temblor.

Further reading

Kohler, MD, Filippitzis, F., Heaton, T., Clayton, RW, Guy, R., Bunn, J., & Chandy, KM (2020). The 2019 Ridgecrest earthquake reveals areas in Los Angeles that increase elevation tremors. Earthquake Research Letters.

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