(526 kb) Quakes in order of travel time to a spot on the subduction interface
In February 1990 a magnitude 6.2 earthquake occurred deep below the southeast coast of the North Island. It was followed by hundreds of aftershocks having magnitudes up to 5.5. This recording presents 164 of the aftershocks as they were heard at one seismic recording station. You will hear all the events in rapid sequence, without the true time intervals between them. Some quiet space was created between each event to prevent clicks due to the splicing.
The main shock was at a depth of 30 km, 10 km below the interface between the subducting Pacific Plate and the overriding Australian Plate. You will hear for each earthquake its sharp P-wave arrival rapidly followed by the less sharp S-wave arrival. The effect is like an echo, except that the S-wave echo is louder than the primary wave.
The recording has been speeded up by a factor of 50, so each event passes in just 0.4 second instead of the true 20 seconds. The recoding below proceeds not in the chronological order of the earthquakes, but in the order of the travel time to a certain spot on the plate interface. Any echo off this ``bright spot'' will have less amplitude than the primary, and make the event sound more reverberatory. In order of distance to the bright spot, the reverberation should become clearer toward the end of the recording.
The next recording is in decreasing order of the depth of the earthquakes. Note how the signals become more impulsive as their sources get shallower.
The final recording is in the earthquakes' time order of occurrence. Generally, the larger-magnitude aftershocks occurred earlier in the sequence. Many of the larger events sound very noisy.
The plot below is in the order of the first recording, from left to right, of increasing bright-spot travel time. Time increases downward in this plot:The interface-ordered recording is also available as a Podcast episode for Apple iTunes and iPod listeners. Subscribe to http://www.seismo.unr.edu/sounds/sound-of-seismic.xml. A new episode will be posted each month.
