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T-wave / P-wave:

When an earthquake occurs in the Earth's crust under the ocean, it produces two types of seismic phases. The first is called a P-wave, which stands for "Primary" waves, and the second is an S-wave, which stand for "Secondary" waves. In addition, part of this seismic energy goes upward into the ocean where it converts into sound (acoustic) energy making a T-wave . A T-wave is an ocean sound wave from an earthquake that travels through the water and is recorded on a hydrophone. The "T" stands for "tertiary", because they travel the slowest and so arrive third and are the last arriving phases recorded from an earthquake. Not all earthquakes generate T-waves because to make T-waves the earthquake has to be underwater.

Honshu Japan Mw 9.0 Earthquake:
March 11, 2011

The Japan earthquake was the largest source of ocean sound ever recorded on our hydrophone arrays. This unique record gives us insight into the physics behind how sound is transmitted from the Earth's crust into the ocean and then propagates through the Pacific Ocean basin. These insights help us gauge the size and scale of other submarine earthquakes and volcanic eruptions we record that were not detected by other sensor networks. We also plan to analyze the hydrophone record for evidence of a pressure signal from the tsunami, which will provide additional information and help our understanding of tsunami propagation in the ocean.

 

Below are 2 sound files and an example spectrogram from hydrophones that recorded the seismic and acoustic arrivals for the Honshu Japan Mw 9.0 earthquake. The first sound file is from a hydrophone located in the central north Pacific, the second sound file and spectrogram is from a hydrophone located near the Aleutian Islands.

 

 

View YouTube movie about the undersea hydrophone and listen to the event:

 

 

Listen to the earthquake (sound has been sped up 16 times):

P-wave sound (sendai-pwave-sound.wav)

 

P and T-wave sounds: (sendai-ptwave-sound.wav)

 

 

The P-wave is the initial burst visible on the image above (left half) and the T-wave follows

in time (right half).