Large Variations in Shaking in L.A. Basin After Ridgecrest Earthquake

The 2019 Ridgecrest earthquake sequence has revealed areas of the Los Angeles basin where the amplification of shaking of high-rise buildings is greatest, according to a new report in Seismological Research Letters. The 6 July 2019 magnitude 7.1 earthquake, located 200 kilometers (124 miles) north of Los Angeles, did not cause structural damage in the city. But there was significant shaking in some high-rise buildings in downtown Los Angeles—so much that their residents reported feeling nauseous from the movement.

All buildings have a natural "vibration" or sway, which civil engineers and seismologists refer to as the building's longest natural period since it marks the amount of time it takes for a building to move back and forth in one cycle in a plane parallel to the ground. High-rise buildings of 15 floors or more, long-span bridges and large diameter fuel storage tanks, among other structures, typically have natural periods of three seconds or more.

Using data from a network of seismic stations across the L.A. basin Caltech scientists (including) Monica Kohler and Filippos Filippitzis of the Department of Mechanical and Civil Engineering,Tom Heaton, Rob Clayton and Richard Guy of the Seismological Laboratory, Julian Bunn of the Department of Astronomy, and Mani Chandy of Computing and Mathematical Sciences) determined that long-period buildings experienced the most amplification of shaking from the Ridgecrest earthquake. The study used data from the densely spaced Community Seismo Network.

But the effect was not the same throughout the basin. At six- and eight-second periods, the maximum amplification occurred in the western part of the L.A. basin and the south-central San Fernando Valley.

In the event of a future earthquake similar to Ridgecrest, a high-rise building in those areas could experience shaking four times larger than a building located in downtown Los Angeles, the researchers concluded. In a 52-story building, this means that the upper floors might sway back and forth as much as one meter (about 3 feet)—or as much as two meters in a magnitude 7.6 earthquake, straining the building's structural integrity.

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