Measuring Earthquake Shaking - MEASURING EARTHQUAKE...

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MEASURING EARTHQUAKE SHAKING " The first earthquake was felt about a quarter of an hour after two in the morning of the 16th. It roused persons from their sleep, by the clatter of windows, doors, and furniture, in tremulous motion. There was a rumbling distant noise, resembling a number of carriages passing over a pavement. In a few seconds the motions and noises had considerably increased… At forty-seven minutes after two, a second shock was felt. At thirty-four minutes after three, a third came; which was as tremulous as the first, but not followed by so much noise. A little after daylight, there was a fourth; at eight, a fifth; and at half past eleven, a sixth; several persons felt, or imagined, others. They were of different lengths, from two minutes to a few seconds. No lives were lost; some chimneys were thrown down; and a few stone houses split.” - Congressman Samuel L. Mitchell talking about the New Madrid Earthquakes in December of 1811
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Animations available from ~45 minutes after a M3.5+ earthquake
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Measuring the Intensity of Earthquake Shaking Various ways of measuring the strength of an earthquake and shaking at a particular location Seismographs give objective measurements of intensity at their location Before the invention of the seismograph, subjective descriptions were used to define the intensity of an earthquake at a location.
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Earthquake Intensity Scales Various intensity scales were designed to semi-quantify the strength of earthquake shaking at any given location by assigning a numerical value to various verbal descriptions of the shaking and its results. Modified Mercalli Intensity Scale Most commonly used intensity scale in the United States Used in this class See Brumbaugh, Table 2.1, p. 23 Others (e.g., Rossi-Forel Intensity Scale, used in Europe but not used in this class)
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Intensity Shaking Description/Damage I Not felt Not felt except by a very few under especially favorable conditions. II Weak Felt only by a few persons at rest, especially on upper floors of buildings. III Weak 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 Light 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 Moderate Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. VI Strong Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
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