This recording of ground motion comes from West Island, a tiny Indian Ocean coral atoll in the Cocos (Keeling) Islands.
Listen for two distinct sounds in the background: the nervous fluttering and chattering of the ocean surf breaking on the shore of this island and its neighbors, and a steady deep roar — the cumulative effect of surf breaking on all the world’s coastlines. The sounds that resemble someone scraping and banging on a rusty oil drum are, in fact, those of the Earth’s crust vibrating in response to a series of earthquakes south of Java, some 1,000 km (600 miles) away.
This recording begins at about 1pm local time on Monday, July 17, 2006 and runs through the entire afternoon and night, ending at about 5am Tuesday morning. A holiday vacationer, tuned to sounds on the human timescale, would doubtless have been lulled to sleep by the gentle rhythm of the waves in this balmy tropical paradise, oblivious to the seismic drama unfolding below.
Technical note
Time-acceleration = 245 (1 second = ~4 minutes Earth time; this 4-minute recording captures about 16 hours of real Earth time). This stereo recording is based on data obtained from three seismometers located in a borehole 70 meters underground. The North-South channel is panned left, East-West is panned right, and the vertical component is at the center. Surface waves from the Java event’s main shock (magnitude 7.5) were large enough to clip the East-component seismometer, causing terrible distortion. I therefore edited out the main event.
