EARTHQUAKE HAZARDS

A.  INTRODUCTION

        Earthquakes are one of the more costly geologic hazards, both in 
        terms of loss of life and damages to human structures.    
        

        1.  DEATHS

             The average, annual, worldwide death toll due to earthquakes in
             the 20th Century was close to 15,000 (but none in Wisconsin).

             There have been at least 25 earthquakes that killed  >100,000  at
             one time, and it has been estimated that 100 million people have
             died from earthquakes in the course of human history! 

         2.  DAMAGES

              FEMA (the Federal Emergency Management Agency) estimates
              that the annual costs to repair earthquake damages in the U.S.
              is roughly $4.4 billion.
                     

B.  SEISMIC SHOCK WAVES

        Fault movement causes shock waves to move out from the focus
        in all directions.   There are two main categories of shock waves:

        1.  BODY WAVES

             Body (P and S) waves travel within the earth and have
             the highest velocities.  P waves travel faster than sound
             and often create noises.  S waves, though slower than P 
             waves, are more damaging to human structures.
             
                          

        2.  SURFACE WAVES

             Surface waves travel along the earth's surface.   They
             have the lowest velocities, the highest amplitudes, and
             usually cause the most damage to human structures.   

C.  EARTHQUAKE MAGNITUDE

         1.  RICHTER SCALE

              The Richter magnitude scale measures the energy released 
              during an earthquake based on the maximum amplitude of
              ground motion caused by the surface waves (and corrected
              for distance from the epicenter).  The Richter scale number
              is the log₁₀ of the corrected ground motion in microns.

         2.  MOMENT MAGNITUDES

              This scale, which is used by seismologists, provides a better
              measure of the total energy released during an earthquake
              because it considers the amount of movement along the fault,
              the rupture area, and the rigidity of the faulted rock. 

D.  EARTHQUAKE DAMAGE 

       1.  MERCALLI SCALE

             The Mercalli scale shows earthquake intensity, which is a
             measure of an earthquake's effects on humans and human
             structures (buildings, roads, bridges, railroad, utility lines,
             dams, ground failures, etc.)
 

       2.  SPATIAL PATTERNS

            One might expect that earthquake intensity should decrease as
            distance from the epicenter increases.  While this is generally
            true, intensity maps show that there must also be other factors 
            controlling the patterns of earthquake damage.  For example: 

            The question is: why do intensity maps not look like targets with 
            epicenters located at their bulls' eyes?   Hypotheses to consider:
  

             a.  Type of Geologic Material  

                  Amplification in the Los Angeles region

                  Liquefaction:  picture, diagram

             b.  Topographic Setting

             c.  Building design and construction 

                  The principle of resonance