Zoo Station

The Earth Institute at Columbia University is known primarily for the work Jeff Sachs and his colleagues do in Africa. But, there are excellent research centers within the Institute dedicated to climatology, geo-physics and the like. The most prominent of these is the Lamont-Doherty Earth Observatory. Researchers at LDEO have now managed to create a 'soundtrack' for the Dec 26th Sumatra quake that unleashed the killer Tsunami. But first, some background.

Recently, researchers at Columbia University's Lamont-Doherty Earth Observatory analyzed recordings of the underwater sound produced by the magnitude 9.3 earthquake. Their unique approach enabled them to track the rupture as it moved along the Sumatra-Andaman Fault.

When an earthquake occurs underwater, part of its energy is released in the form of sound, known as a tertiary or T wave, which travels great distances through the ocean. T waves travel significantly slower than primary and secondary (P and S) waves, both of which are often recorded in an overlapping pattern that can obscure important parts of the seismic record. As a result, oceanic sound energy sometimes provides a more direct look at the entirety of a large underwater earthquake. "It's like the hare and the tortoise" said Tolstoy, "The tortoise is moving a lot more slowly, but it gets the right answer in the end". The T waves for the Sumatra earthquake were captured by underwater microphones located at Diego Garcia, more than 1,700 miles from the epicenter. These microphones are part of arrays known as hydroacoustic stations that are scattered throughout the world's oceans to listen for the telltale sound of an atomic blast.

What is surprising, however, is the fact that the earthquake appeared to occur in two distinct phases. The first phase encompassed the first three minutes of the eight-minute earthquake, during which the rupture proceeded north at about 1.7 miles per second (2.8 km/sec) from the epicenter. During the second phase, the rupture slowed to 1.3 miles per second (2.1 km/sec) and continued north for another five minutes until it reached a plate boundary where the fault changes from subduction to strike-slip, where the two plates push past one another in opposite directions. This suggests that had the subduction continued, this longest ever recorded earthquake might have been even longer.

You can listen to the earthquake here. Turn up the volume to maximum, listen carefully, and give it a few seconds before the rumbling begins.