Elephants appear to use vibrational communication for a number of purposes. An elephant running or mock charging can create seismic signals that can be heard at great distances. Vibrational waveforms produced by locomotion appear to travel at distances of up to while those from vocalizations travel . When detecting the vibrational cues of an alarm call signaling danger from predators, elephants enter a defensive posture and family groups will congregate. Vibrational cues are also thought to aid their navigation by use of external sources of infrasound. After the 2004 Boxing Day tsunami in Asia, there were reports that trained elephants in Thailand had become agitated and fled to higher ground before the devastating wave struck, thus saving their own lives and those of the tourists riding on their backs. Because earthquakes and tsunamis generate low-frequency waves, O'Connell-Rodwell and other elephant experts have begun to explore the possibility that the Thai elephants were responding to these events.