grade levels: 9-12 achievement standards for this lesson
Earthquakes release energy in two different forms that travel
through the earth differently, and which have different energies
associated with them. Earthquake energy is measured by an
instrument known as a seismograph which measures the back-and-
forth and side-to-side motion of the earth's surface as seismic
waves pass by. Seismic body waves move through rock in a
compression - relaxation mode, analogous to sound waves
traveling through air. These waves travel fastest through rock.
Earthquake magnitude determined from measurement of body wave
motion (MB) is based on the maximum observed ratio of wave
amplitude to period for body waves arriving in the first 5
seconds of a record (U.S. Geological Survey practice). Seismic
surface wave energy is carried in a side-to-side motion and
arrives slightly later than body waves. Large earthquakes tend
to generate more surface wave amplitude relative to body wave
amplitude. The opposite is true for small events. T