Measuring Earthquakes Chapter 5.2-5.3

Where do Earthquakes begin? What is the difference between the focus and epicenter? Earthquakes always begin in rock below the surface, usually within 100 km of Earth’s surface. Focus: The point where rock that is under stress breaks, triggering an earthquake. Epicenter: The point on the surface directly above the focus. Epicenter/focus animation

What are seismic waves? Where are the seismic waves the strongest? Seismic Waves: vibrations that travel in a ripple through Earth, carrying the energy released during an earthquake. .

Three Categories of Seismic Waves P Waves (Primary Waves) - First to arrive at a seismograph - compress and expand ground

Three Categories of Seismic Waves (cont.) 2) S Waves (Secondary Waves) - Vibrate from side to side AND up and down - 2nd to arrive at seismograph Animation of P and S waves Video on P and S Waves

3) Surface Waves. Produce the most severe ground movements. Three Categories of Seismic Waves (cont.)

How do we detect seismic waves? How do we measure earthquakes? Seismograph: a machine that records the ground movements caused by seismic waves as they move through the earth. seismograph animation Magnitude: measurement of earthquake strength based on seismic waves and movement along faults.

3 types of rating scales (continued) Mercalli Scale – Not a precise measurement The same earthquake can have different ratings

2) Richter Scale Rates size of seismic waves with a mechanical seismograph Accurate for small, nearby earthquakes Not accurate for large or distant earthquakes. 3) Moment Magnitude Scale Estimates total energy released by earthquake Used to accurately rate all earthquakes of all sizes, near or far What we currently use Which scale do we use today?

How many seismographs do you need to determine an epicenter Locating the epicenter: You need 3 seismographs to determine an epicenter. Locating an Epicenter Video