record the energy emitted by seismic or earthquake waves that radiate
from the focus (point at which earthquake begins). Seismograms are
created by machines called seismographs. Seismograms are used by
seismologists (scientists who study earthquakes) to mathematically
describe the magnitude of an earthquake.
Earthquakes generate many different types of
seismic waves. Two major types are P (push/pull, compressional, or
primary) and S (shear or secondary). Seismograms will record P-wave
arrivals before S-wave arrivals. Surface waves are also generated that
travel on the surface of the crust.