Seismic energy transmits most efficiently between the 10 and 40 Hz - in the same range as the fundamental frequency and 2nd harmonic of an elephant rumble. It turns out that when an elephant rumbles a replica of the airborne sound is also transmitted through the ground. Elephant sounds have been measured as traveling at about 309 m per second through air and at about 248-264 m/sec through the ground.
Experiments carried out by Caitlin O'Connell and colleagues have shown that elephants are able to pick up these seismic signals, to orient in the direction that the vibrations come from and even to respond to them appropriately.
Elephants may be able to detect these seismic vibrations, or rayleigh waves, through two possible means, bone conduction and the use of massive ossicles of their middle ears or possibly by mechano-receptors in the toes or feet that are sensitive to vibrations.
The tip of an elephant's trunk has layers of cells called Pacinian corpuscles that are extremely sensitive to vibrations and it thought to be able to detect movement as subtle as Brownian motion. Pacinian corpuscles have also been found in the elephant foot - concentrated in the front and back (toes and heel area) dermal layer. Movements or vibrations deform the layers of Pacinian corpuscles, sending a nerve signal to the brain. Although these corpuscles are found in other mammals, too, they are particularly densely packed in the tip of an elephant's trunk.
You can find an interesting article about seismic signals among elephants here, and find several others if you check out the result of a Google search on “Seismic signals from elephants”. You will also enjoy hearing Dr. Caitlin E. O'Connell talk about this exciting topic on TEDx Talks in 2013.