The image above shows the initial shudders of the 9.0-magnitude earthquake that devastated Japan in March, as registered by seismic sensors (yellow dots) across the world. From measurements like these, scientists can infer a wave's speed and determine if a structure within the earth slowed it down or sped it up. A network of seismometers concentrated in one region allows scientists to create detailed 3-D images of the interior beneath that area.
The readings clustered on the right side of the globe in the above image represent some of the 1,000-plus seismic stations deployed in North America. About 400 of those are part of EarthScope, a National Science Foundation-funded project that aims to understand when and how earthquakes strike.