As earthquake waves travel along the surface of the Earth, they cause the ground to move. The ground motions can be captured and displayed as a movie, providing a visual demonstration of these often indiscernible movements. Charles Ammon, a professor at Penn State University, has developed animations of earthquake waves using actual earthquake data. The visualizations show how the ground moves as seismic waves sweep across about 400 earthquake recording stations in the Transportable Array. These innovative visualizations have been created for selected large earthquakes that have taken place in the US and around the world.
The visualizations illustrate how seismic waves travel away from an earthquake. Because the array’s seismometers are closely spaced in a grid pattern with unprecedented density, the recorded wave amplitudes at each seismometer clearly show through time how wave after wave progresses along the great circle path from the earthquake’s epicenter.
Earthquake Near Wells, Nevada
February 21, 2008 - Magnitude 6.0
This earthquake occurred right in the center of the Transportable Array. The visualization clearly shows seismic waves radiating outward from the epicenter like ripples spreading outward when a pebble is dropped in a pond.
Earthquake Off the Coast of Southern Sumatra
September 12, 2007 - Magnitude 8.4
This visualization clearly shows vertical ground displacement sweeping through the Transportable Array. Surface waves from this earthquake, which occurred on the other side of the Earth, can be seen arriving at the earthquake recording stations after travelling the shortest distance from the epicenter as well as those surface waves that travelled in the opposite direction.
The circles in the visualization represent earthquake recording stations and the color of each circle represents the amplitude, or height, of the earthquake wave detected by the station’s seismometer. The color of the circle changes as waves of differing amplitude travel past the seismometer. Blue represents downward ground motion, red represents upward ground motion, and darker colors indicate larger amplitudes.
In each visualization, a representative seismogram from the earthquake is displayed. As the movie plays, a time bar moves across the seismogram and reveals first the P waves, then the S waves, and finally the surface waves as they traverse the array. Since surface waves generated from strong distant earthquakes can travel very long distances, seismometers can detect surface waves that take the shortest path to the earthquake recording station as well as surface waves that take a longer path, traveling around the surface of the Earth in the opposite direction. In the visualizations, the surface waves that travel the longer path around the Earth are apparent because, as time progresses, the waves begin to arrive from the opposite direction.