Classroom activity associated with the poster
As seismic waves radiate out from the epicenter of an earthquake and encounter Earth's surface, they cause the ground to move. Unless the earthquake is nearby, the scale of these motions are generally too small, perhaps only .001mm (a fraction of the diameter of a human hair), and occurs over a time scale, 100s of seconds, that is too long for humans to “feel” or detect. However, sensitive instruments such as seismographs are able to discern such changes and record them as seismograms. The visualization on the front of this poster uses a color scale to indicate the change in amplitude of the ground through time as recorded at nearly 400 seismic stations that make up the USArray (shown at right). Because the seismometers are distributed in a grid with unprecedented density, a spacing of approximately 70km, the collective display of the ground motion across the array is particularly informative both for teaching and scientific research. See the current status of the array.
This dense deployment of seismometers is part of a large-scale geoscience experiment, EarthScope, which is funded by the National Science Foundation. The goal of EarthScope is to explore the structure and evolution of the North American continent. This basic research contributes to the development of natural resources, the mitigation of geologic hazards, an understanding of the forces that shape our landscape and our understanding of the planets structure and inter-workings. For more information on EarthScope please visit ([url=http://www.earthscope.org]http://www.earthscope.org[/url])
The visualization on the poster was generated by first collecting the individual responses of each seismic station. These are represented by a circle. The color of the station, as shown in the key below, indicates the ground displacement at that individual station (e.g. red = down and blue = up). If you are interested in seeing similiar data from other earthquakes a collection and tutorial explaining how their interpretation is available.
Next, using a computer, the individual responses from each station are combined to create contours or connections between points of a constant value. A these contour are then colored to indicate various levels of displacement. Warmer colors like yellow, orange and red indicate an increasing ground displacement in the positive direction (up). While cooler colors from pale to dark blue indicate increasing ground displacement in a negative direction (down).
By analizing how these countours change through time (below), we see how seismic energy radiates outwards from the epicenter and spreads across the array. If the continent were homogenous the pattern on the front of the poster would be perfectly concentric like the ripples on water. However, since Earth’s crust is not homogeneous we see many subtle anomalies to the pattern. Scientists study these anomalies to infer structure and composition of the ground in which the waves are traveling to help them answer geologic questions. For example,
- What cause earthquakes to occur far from the edges of tectonic plates?
- What happens to plates after they have been subducted?
- What is the role of hotspots in evolution of the continents?
(Visulizations play 75x faster than actual)
When examining this data, it is important to note that the visualizations shown on the poster have been slightly altered artistically. This was necessary to smooth out anomalies generated from the process used to create the colored data contours; a process combining data from over 400 stations. However, the video clips and still images captured from the videos (shown above) have not been altered in this way. As a result, this data appears much more "complex". For example, background "noise" can be seen as random blobs when the clip opens. Also, a prominent example of an "edge effect" can be seen by watching the upper right corner of the field in the clip. It is important to remember these as they are artifacts of the process used to generate the visualization rather than actual data representing geologic features of Earth.
