When earthquakes happen, Prof. Katie Keranen of Cornell’s Earth and Atmospheric Systems, watches … measures … and, monitors.
Keranen images the Earth’s crust by studying the propagation of energy waves through rock, a field known as seismology. The sources of these waves are subdivided into two categories: natural or passive sources, such as movement caused by earthquakes, and active sources, which are manmade.
Seismologists are interested in the ground motion that results from both types To generate an active source, Keranen drills a shallow hole in the Earth’s surface, loads it with explosives and detonates them, causing ground motion.
The shaking is picked up by the web of sensors, or seismometers, deployed around the origin point of the explosives. The data from the wave pattern yields high resolution images of the geological features of Earth’s interior several kilometers deep, Keranen said.
Using waves to image the interior a solid object by breaking it into multiple slices is known as tomography. This technique has a variety of applications, including medical X-ray imaging.
“Just like we can get a medical tomogram to look inside the human body, we can look inside the earth using seismology,” Keranen said.
Seismometers pick up on a variety of wave-generating events – explosions or nuclear blasts; implosions such as collapsing underground caves; earthquakes; and volcanoes. Seismologists distinguish these events by analyzing each unique wave behavior, according to Keranen. The patterns yield valuable information about the source, magnitude and other particulars of an event.
A typical earthquake pattern consists of three wave fronts, Keranen said. The P wave is the fastest, has the lowest amplitude and is the least dangerous. It often triggers earthquake early warning systems, alerting people that the next two (and more destructive) waves are on their way. The S wave causes sideways shearing movement. Surface waves are responsible for general motion close to Earth’s surface.
(Story credit: Jacqueline Carozza, Cornell Daily Sun)