Resonance implicated in Nimitz tragedy.

Resonance implicated in Nimitz tragedy

Seismologic studies following October's earthquake in northern California suggest vibrational resonance played a role in the collapse of the double-decked Nimitz Freeway in Oakland. Mud sediments beneath the structure amplified specific vibrations that matched the inherent resonant frequencies of the highway, report seismologists from the Lamont-Doherty Geological Observatory in Palisades, N.Y., and from the U.S. Geological Survey.

Within 72 hours after the main shock, the investigators had stationed seismometers at locations around the collapsed section of the highway. This portion of the Nimitz rests on fine-grained sediments washed in with mud from the San Francisco Bay, while other, uncollapsed sections sit on alluvium deposits that better resist shaking. Measurements of the different areas showed that the fine-grained sediments amplified the shaking more than the alluvium, which explains why so many nearby chimneys fell, says Lamont-Doherty's Robert W. Busby.

But why was this highway section hit harder than most other structures on mud sediments? To address that question, many engineers have focused on design flaws in the freeway supports. The seismologic studies, however, point to resonance as a contributing factor. Measurements, including some made on the highway itself, show that the mud sediments amplified frequencies of around 2 cycles per second, which are close to the highway's natural frequency. Like a child who pumps her legs to swing higher, the highway experienced resonating vibration that caused it to shake more violently than surrounding structures, say Busby and his colleagues.