Measuring Earthquakes Logarithmic scales and human perception

How intense is an Earthquake?  The severity of an earthquake can be expressed in terms of both intensity and magnitude.  Intensity  based on observed effects of ground shaking on people, buildings, and natural features.  It varies from place to place within the disturbed region depending on the location of the observer with respect to the earthquake epicenter.  Magnitude  related to the amount of seismic energy released at the hypocenter of the earthquake. It is based on the amplitude of the earthquake waves recorded on instruments which have a common calibration. The magnitude of an earthquake is thus represented by a single, instrumentally determined value.

Intensity Scales (qualitative)  Poarid (Italian)– 1627; first attempt to classify earthquakes by intensity in 1627; four levels of intensity use to describe effects of earthquakes experienced at different towns.  More attempts made in 18 th and 19 th centuries.  Early 1900’s Mercalli’s scale; 12 levels of intensity.  Modified version (1931) published by Wood & Neumann. Known as the Modified Mercalli Intensity Scale of 1931, it has become the standard used by the United States engineering seismology community (incl. National Geophysical Data Center and the U.S. Geological Survey.)

Modified Mercalli Scale  The Modified Mercalli intensity scale is used for measuring the intensity of an earthquake. earthquake  Quantifies the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures.  12 levels, with 1 denoting a weak earthquake and 12 one that causes almost complete destruction. (See handout)

USGS Earthquake investigations.  USGS is the Federal agency responsible for collecting earthquake intensity data.  Used "Earthquake Report“ questionnaire and field investigators to analyze earthquake damage in serious earthquakes.  Questionnaires sent to postmasters, National Weather Service offices, military installations, etc. requesting a report of all effects of the earthquake in their area. Newspaper accounts, published scientific reports, and reports provided by seismology collaborators were also used.  Expert observers may investigate and photograph the damage.  In the end, a seismologist analyzes each report and assigns intensities on the basis of the effects at each town;  Intensity (or isoseismal) maps are then constructed for earthquakes felt over large areas.

The Richter Scale  Magnitude of most earthquakes is measured on the Richter scale, invented by Charles F. Richter in The Richter magnitude is calculated from the amplitude of the largest seismic wave recorded for the earthquake, no matter what type of wave was the strongest.