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Smith, Robert (ed.) / The Wisconsin engineer
Vol. 70, No. 4 (January 1966)

Jost, Larry
Earthquakes!: their causes and characteristics,   pp. 14-18


Page 17


Modified Mercalli Scale (after Hodgson)
   1. Not felt except by a very few under especially favorable
     circumstances.
  II. Felt by only a few persons at rest, especially on upper floors
     of buildings. Delicate suspended objects may swing.
 III. Felt quite noticeably indoors, especially on upper floors of
     building, but many people do not recognize it as an earth-
     quake. Standing motor cars may rock slightly. Vibration like
     passing truck. Duration estimated.
 I V. During the day felt indoors by many, outdoor by few. At
     night some awakened. Dishes, windows, doors disturbed; walls
     make creaking sound. Sensation like heavy truck striking
     building. Standing motor cars rocked noticeably.
  V. Felt by nearly everyone: Many awakened. Some dishes, win-
     dows, etc., broken; a few instances of cracked plaster; un-
     stable object overturned. Disturbances of trees, poles, and
     other tall objects sometimes noticed. Pendulum clocks say stop.
 VI. Felt b)y all; many frightened and run outdoors. Some heavy
     furniture moved; a few instances of fallen plaster or damaged
     chimneys. Damage slight.
VII. Everybody runs outdoors. Damage negligible in buildings of
      good design and construction; slight to moderate in well built
      ordinary structures; considerable in poorly built or badly de-
      signed structures; some cbimneys broken. Noticed by persons
      driving motorcars.
VIII. Damage slight in specially designed structures; considerable
     in ordinary substantial buildings, with partial collapse; great
     in poorly built structures. Panel walls thrown out of frame
     structures. Fall of chimneys, factory stacks, columns, monu-
     ments, walls, heavy furniture overturned. Sand and mud
     ejected in small amounts. Changes in well water. Disturbs
     persons in motor cars.
 I X. Damage considerable in specially designed structures; well-
     designed frame structures thrown out of plumb; great in sub-
     substantial buildings, with partial collapse. Buildings shifted
     off foundations. Ground cracked conspicuously. Underground
     pipes broken.
  X. Some wvell-built wooden structures destroyed; most masonry
     and frame structures destroyed with foundations; ground
     badlv cracked. Rails bent. Landslides considerable from river
     banlks and steep slopes. Shifted sand and mud. Water splashed
     over banks.
 XI. Few, if any, (masonry) structures remain standing. Bridges
     destroyed. Broad fissures in ground. Underground pipelines
     completely out of service. Earth slumps and land slips in soft
     glround. Rails bent greatly.
XII. Damage total. Waves seen on ground surfaces. Lines of sight
     and level distorted. Objects thrown upward in the air.
Figure 6.-Model of ground motion during the first 20 seconds of the Japanese
      earthquake of February 15, 1887. The model is greatly enlarged.
18
       Distribution and Frequency
     There are four main belts of
   earthquakes around the earth:
     1. The circum-Pacific belt with
       many branches.
     2. Mediterranean and trans-
       Asiatic belt.
     3. Narrow belts through the
       Arutic and Atlantic oceans
       and East-African underwater
       valleys.
     4. Scattered belts of only local
       significance.
   It is estimated that over one mil-
   lion earthquakes occur annually
   with the majority of them occur-
   ring in one of these four regions. In
   Hawaii alone in the months of
   September and October of 1959
   over 22,000 earthquakes were re-
   corded. Most of them were too
   small to be felt but were recorded
   with sensitive instruments.
       Strong-Motion Instruments
     There are many types of instru-
   ments that are used in the detec-
   tion and classification of earth-
   quakes. Possibly one of the most
   interesting of these is the strong-
   motion instrument. Designed by
   the United States Coast and
   Geodetic Survey, it records ground
   acceleration and the ground dis-
   placement on a sheet of photo-
   graphic paper. The actual displace-
   ment of the ground is small and
   seldom exceeds a fraction of an
   inch but it can leave some wierd
   patterns as illustrated by Fig. 6.
             CONCLUSION
    The study of earthquakes is
  based on a progression of theories.
  The elastic rebound theory is sup-
  ported by the earth failure
  theories, which are based on the
  theory of how the earth was
  formed. From this it can be seen
  that there is really no firm founda-
  tion for the study of earthquakes,
  even though there has been much
  work done in this field.
               REFERENCES
  Byerly, Perry. Pacific Coast Earth qakc6s.
    Eugene Oregon: Oregon State System
    of Higher Education, 1952.
  Hobbs, Herbert H. Earthquakes. New
    York: D. Appleton and Co., 1907.
  Hodgson, John H. Earthquakes and
    Earth Structure. Englewood Cliffs,
    N.J.: Prentice-Hall, 1964.
  Jeffreys, Harold. Earthquakes and
    Mountains, London: Methuen and Co.
    Ltd., 1925.
THE WISCONSIN ENGINEER


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