Why do we hear an earthquake before it arrives?
AnswerThe original question was as follows:
Why do we hear an earthquake before it arrives? I was surprised to hear that earthquakes travel many times faster than sound because you always hear the boom of an earthquake before it hits.The speed of sound in air is 330 meters per second. Certainly, that is a great speed in many contexts but not always. Thus, it means that it takes 300 seconds or 5 minutes for the sound to travel a hundred kilometers. If earthquakes would only travel at this speed many things about them would be different. In reality, sound which may form in the air above the point of origin of the earthquake is transmitted much slower than the seismic waves of the quake and is also strongly damped by distance. The speed of seismic waves in the earth can not be described by a single number because it depends heavily on the material through which the waves are passing. Besides, these waves are of at least four different kinds with different speeds, not simply related to each other. The fastest waves are called P-waves. Their speed in the uppermost layers of Iceland is 2-3 km per second, whereas they may travel about 6,5 km per second in lower layers. The travel time of these waves from the point of origin to a place in the neigbourhood is determined by the first figure, but if the places are for instance 100 km apart the fastest wave will go deeper and the travel time will then correspond to a speed of 6,5 km/s. Thus it takes the wave only about 15 seconds to travel 100 kilometers. It is well known that sound is created in air when an oscillating object is in contact with it. The sound really is oscillations in pressure, displacement and speed of the air, in concordance with the object creating them. For instance, the frequency or the pitch of the sound is determined by the frequency of the oscillations of the source. But you also have to keep in mind that the human ear only detects oscillations in a certain interval as sound. For many people this interval will stretch from base tones at 20 Hertz (oscillations per second) to high pitch at for instance 20.000 Hertz. Now, the seismic waves themselves include oscillations of the surface of the earth which is in contact with the air. Therefore, they cause oscillations in the air. However, the frequency of these oscillations is so low that we hear the least part of them as sound. Still, the frequency of the fastest waves, the P-waves, may be more than 30 Hertz and thus be audible as sound. The sound induced by seismic waves in the air have been compared to the boom in underground stations when a train is approaching. We normally sense P-waves as an earthquake to a lesser degree than for instance the S-waves following them. If an earthquake has not been very strong or we are reasonably far away from its center we will not at all sense the P-waves as an earthquake but only hear the sound induced by them in the air. Still, we may feel the S-waves quite clearly and then other waves which often arrive later, especially at a distance. The reason for this incoherence of sensation is thus found in the different sensitivity of our sensing organs. When Professor Ragnar Sigbjörnsson was talking to a lady near to the center of the National Day earthquake of year 2000 she said she had realized at once that the epicenter was close to her because there was no sound ahead of the quake. This observation is quite to the point according to the text above. Firstly, the P-waves will have been felt as an earthquake at such a place in this event and, secondly, they will not have been far ahead of the S-waves. Sources:
Páll Einarsson, 1985. "Jarðskjálftaspár." Náttúrufræðingurinn, 55 (1), pp. 9-28.
Páll Einarsson, 1991. "Jarðskjálftabylgjur." Náttúrufræðingurinn, 61 (1), pp. 57-69.
Um þessa spurningu
Þorsteinn Vilhjálmsson. „Why do we hear an earthquake before it arrives?“. The Icelandic Web of Science 9.3.2005. http://why.is/svar.php?id=4825. (Skoðað 23.3.2017).
Þorsteinn Vilhjálmssonprofessor emeritus, editor in chief of Vísindavefurinn 2000-2010 and editor in chief of Evrópuvefurinn 2010-2011