The elephant that receives the message also exhibits specific behavior. It also may hear airborne sounds emitted by the sender, so it spreads out its ears to triangulate the signal, thus determining the direction it is coming from.
O'Connell-Rodwell noticed during her many hours of observation that sometimes elephants would freeze simultaneously, even if they were in mid-stride, and press their front feet onto the ground. They might also lift one leg, or roll one foot forward to press the toes against the ground.
The African elephants responded in the same way to signals sent by the researchers, even if the signals were sent only through the ground and there was no audio soundtrack. If the signal indicated danger, the elephants would crowd together in their defensive posture, but only if it sounded like it was from an elephant that lives in the same area.
Further study of elephants has shown that they are particularly well equipped to communicate through seismic waves. Their feet, for example, have large fat pads beneath the heel that scientists believe may facilitate transmitting the waves to the elephant's brain. Similar fat pads are found in other mammals, including cats, suggesting that many animals may be using seismic waves to communicate.
An insect may not be able to send a seismic wave very far, but its potential mate may be near. And cats, as many folks know, seem to have an uncanny ability to sense danger.
O'Connell-Rodwell thinks humans may have similar talents, but they probably lie dormant in most of us. Centuries ago, when our ancestors gathered around the camp fire to listen to the village drummer send out both auditory and seismic messages, they may have been a little more attuned to the vibrations in the soil beneath their feet. That could come in handy during a hunt, by the way.
There's some evidence that humans with hearing impairment are also more aware of seismic signals.
"They are much better at detecting vibrations than humans with normal hearing," said O'Connell-Rodwell, who is working in that field as part of her research in Stanford's department of otolaryngology, or head and neck surgery. "Schools for the deaf have wooden dance floors and they play music through the floors and people describe it as hearing the music through their feet."
I knew a teenage girl years ago who was completely deaf, but could dance as well as anyone on the floor. She knew the music, even if she couldn't hear it.
So if elephants and insects, and some people, can sense seismic signals, is there really anything to those stories we hear about animals detecting an earthquake before it happens?
O'Connell-Rodwell thinks there's probably something to it, but she says the evidence is still woefully lacking.
After the 2004 tsunami disaster struck Southeast Asia there were reports that elephants in Thailand had become agitated and raced to higher ground even before the wave struck the beaches. They may have sensed minor precursor shock waves before the major shock hit the area, but maybe not.
O'Connell-Rodwell points out that a group of elephants in a national park in Sri Lanka had been equipped with satellite collars, and they didn't move at all before the tsunami struck. But seismic waves behave differently in different soils, so it's possible the precursors could be detected in Thailand, but not Sri Lanka.