Objectives Learn how energy of an earthquake travel through Earth. Differentiate between the kinds of seismic waves. Understand the different type of scales used to measure the strength of an earthquake.
Vocabulary Words Focus Epicenter P Waves S Waves Surface Waves Seismographs Magnitude Mercalli scale Richter scale Moment magnitude scale
Measuring Earthquakes The point beneath Earth’s surface where rock under stress breaks and causes an earthquake is called the focus. The point directly above the focus is called the epicenter.
Seismic Waves Seismic waves are vibrations produced during an earthquake. Seismic waves carry the energy of an earthquake away from the focus, through the Earth’s interiors, and across the surface.
Types of Seismic Waves P-Waves: compress and expand the ground like an accordian. Arrives at a seismograph first. S-Waves: vibrate from side-to-side and up and down. Arrives at a seismograph second. When P and S Waves reach the surface, they are changed into surface waves. Surface Waves move more slowly than P and S Waves. Surface waves produce the most severe ground movements.
Seismic Recording Instruments Seismographs are instruments scientists use to measure and record the vibrations of seismic waves. Magnitude measures the strength based on seismic waves and movement along the faults.
Measurement Using Rating Scales 3 rating scales are used: Mercalli Scale Richter Scale Moment Magnitude Scale
The Richter Scale The Richter scale rates earthquakes according to the size of seismic waves as measured by a seismograph.
Mercalli Scale The Mercalli scale rates earthquakes by describing their effects on people, buildings, and the land surface in a given area.
Moment Magnitude Scale The Moment Magnitude Scale rates earthquakes by estimating the total energy they release. The MMS can be used to rate earthquakes of all sizes, near or far.