In August 2004 a UNR group set out a line of 400 seismometers extending from Fresno, Calif. north across the Sierra Nevada range, the Long Valley volcanic caldera, and central Nevada to the Idaho border. These seismometers monitored earthquakes in the caldera, and several large (>100,000 lb ANFO) mining blasts at huge open-pit gold mines in northern Nevada for one week.
This record of 411 seismograms shows the data from one of these mining blasts, from the south end at Fresno on the left, to the Idaho border with Nevada on the right, more than 500 km (300 mi) north. Time on each one of these four-minute-long seismograms increases downward, as deeper structures tend to affect the seismograms at larger time. The blast was located near the right side about 100 km south of the Idaho border.
The waves fanning out from the blast are the P-wave, above and arriving first, and the S-wave second. The blast itself was a single event. The two wave arrivals are the result of the natural separation of fast compressional waves and slow shear waves in elastic media. The compressional wave is equivalent to the sound wave, although in the earth's crust and mantle P-waves travel up to 25 times faster than sound waves travel in air. Shear waves travel in solids but not in zero-rigidity fluids like water or air. Small variations (on this plot) in the arrival time of the P-waves are studied for their information on the configuration and thickness of the earth's crust.
The recording has been speeded up by a factor of 100, so each pair of seismograms (rendered in the left and right channels) passes in just 2.4 seconds instead of the true 240 seconds. The recoding below proceeds in the order of the seismogram recordings from south to north. The total time of the recording is 7 minutes 41 seconds. Every 2.4 seconds you listen to the same absolute time interval, from pairs of recorders that continually step from south to north at intervals of about 3 km.
At the beginning of the recording you are 500 km from the blast, hearing mostly traffic, wind, and instrument noise. Near the beginning, in the area of the Long Valley Caldera, you may hear one or two small earthquakes, which are occurring continually there. By halfway through you should be hearing the double ``heartbeat'' of the passing P- and S-waves. These grow more distinct as you get closer to the blast site. The recordings closest to the blast are at about 6:34, but the first 8 seconds after the blast (of real time) are cut off, so you don't hear the raw blast. Turn the volume up to hear the ``heartbeat'' better.
There are many layers of noise in these recordings. On top of the blast P and S waves we are listening for, there is scattering, attenuation, and reverberation of the waves in the crust. Some recording sites have loose, echoing properties that can vary greatly from site to site. Small earthquakes may be heard as well. Most of the sound is from traffic and wind noise, and other cultural noise such as pumps and electrical devices. There is some thermal noise from the geophone-amplifier system, and on top of everything are series of clicks and buzzes from poorly functioning recorders, and some added noise from the amplitude balancing procedures used in preparing the recording.
Gold Mine Blast Across 500 km (9.0 Mb)
This recording is also available as a Podcast episode for Apple iTunes and iPod listeners. Subscribe to http://crack.seismo.unr.edu/sounds/sound-of-seismic.xml. A new episode will be posted each month.The plot below is in the order of the seismometer deployment, from left to right, south to north. Time increases downward in this plot: