Q: In school we learned that primary waves can travel through solids, liquids, and gases. My question is: what would happen if a P-wave was traveling through the ground and it reached a canyon and a bird was in the air when the wave started traveling through the air? Would the bird be affected? (Teddy, Charlottesville, Virginia)

A magnitude 6.9 earthquake tumbled this cabin in the Santa Cruz Mountains about 60 miles from San Francisco. [J.K. Nakata, U.S. Geological Survey]

A: The bird would probably not notice the p-wave. Here’s a likely scenario based on information from experts:

Offshore of central California, a fault along the ocean floor cracks as the North American and Pacific plates move minutely. A small (magnitude 3.4) earthquake results and waves flow through the Earth and along its surface, releasing the fault’s pent-up energy.

The P-wave travels fast: reaching San Francisco in 40 seconds, Wichita, Kansas in 4 minutes, and rattling through the Earth’s core to reach Nairobi, Kenya in 16 minutes. The wave hustles along like a sound wave, moving dirt and rock only a tiny distance as the disturbance zips past.

Back at California, near its source: The wave first travels down into the earth and bends upward as it gets into Earth’s deeper layers. As the wave moves up towards the surface, Earth absorbs and dissipates frequencies high enough for most animals to hear. The almost-vertically traveling wave makes it to the surface at the canyon. It bounces back from Earth’s surface. Only a small fraction of its energy crosses into canyon air. A low amplitude, low frequency (perhaps 1 to 20 Hz) wave continues. The canyon remains quiet (except for a small whisper of oak leaves as trees tremble). A band tailed pigeon, beating her wings upward, hears the incredibly low sound but pays no attention. She hears too much non-threatening noise at these frequencies to listen.

The experts who contributed to this answer are: Andrew Michael geologist at Menlo Park, California Earthquake’s Hazards Team of the U.S. Geological Survey and Charles Walcott, neurobiology professor and faculty dean at Cornell University.

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