This recording was taken about 90 miles from the Japanese earthquake’s epicenter. There are two distinct sound waves. Both are caused by the mainshock. A “pop” is heard 90 seconds (in actual time) after the main event. This pop wasn’t recorded at any other nearby stations, leading Georgia Tech Associate Professor Zhigang Peng to believe that either the ground shifted immediately under the measuring station, or the hill slope where the station sits in helps to amplify the shaking. It was the strongest reading he found – a ground acceleration of nearly three g.