Reason Magazine 2014 Quartal 2 Seite 35

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A DESCRIPTION OF MODIFIED MERCALLI INTENSITY MMI SCALES I Not felt except by a very few under especially favorable conditions II Felt only by a few persons at rest especially on upper floors of buildings III Felt quite noticeably by persons indoors especially on upper floors of buildings Many people do not recognize it as an earthquake Standing motor cars may rock slightly Vibrations similar to the passing of a truck Duration estimated IV Felt indoors by many outdoors by few during the day At night some awakened Dishes windows doors disturbed walls make cracking sound Sensation like heavy truck striking building Standing motor cars rocked noticeably V Felt by nearly everyone many awakened Some dishes windows broken Unstable objects overturned Pendulum clocks may stop VI Felt by all many frightened Some heavy furniture moved a few instances of fallen plaster Damage slight VII Damage negligible in buildings of good design and construction slight to moderate in well built ordinary structures considerable damage in poorly built or badly designed structures some chimneys broken VIII Damage slight in specially designed structures considerable damage in ordinary substantial buildings with partial collapse Damage great in poorly built structures Fall of chimneys factory stacks columns monuments walls Heavy furniture overturned IX Damage considerable in specially designed structures well designed frame structures thrown out of plumb Damage great in substantial buildings with partial collapse Buildings shifted off foundations X Some well built wooden structures destroyed most masonry and frame structures destroyed with foundations Rails bent XI Few if any masonry structures remain standing Bridges destroyed Rails bent greatly XII Damage total Lines of sight and level are distorted Objects thrown into the air underlain by soft soils alluvium or water saturated unconsolidated soils and less in areas underlain by bedrock There is plenty of landfill along the banks of the Potomac River in the Washington D C and Vir ginia areas and significant fill up to 30 feet deep was placed for the construction of the western half of the National Mall where the important historical buildings were dam aged Among FM Global insured properties most of the damaged buildings are sitting on relatively soft soil and several buildings sit ting on the artificial fill in the Washington D C area sustained the severest damage What have we learned Significant earthquakes do infrequently occur in the eastern United States and can affect populated areas Fortunately the Vir ginia earthquake struck a lightly populated area Future earthquakes may happen in a densely populated region Moreover seismic building codes did not come into wide use in the eastern United States until the mid 1990s whereas in the western United States seismic codes substantially improved con struction as early as the 1930s The densely populated east coast has weak structures of high monetary value especially in the met ropolitan areas of New York Washington D C Boston Philadelphia and Baltimore Due to the lack of seismic design consider able seismic risk exists This earthquake underscores the fact that soft soil especially artificial fill ampli fies ground motions and exacerbates dam age It proves the importance of accounting for site amplifications in earthquake risk estimation Fortunately we incorporate the site conditions in the FM Global earthquake risk maps Yufang Rong is a senior research specialist at FM Global Figure 3 The Modified Mercalli Intensity MMI scale measures the effect of an earthquake on the earth s surface in 12 increasing levels of intensity felt by people without special instruments or training ISSUE 2 2014 Reason 35 theSCIENCE