How are earthquakes measured? Part 2: The Richter ScaleTo advise engineers, emergency services, and

How are earthquakes measured? Part 2: The Richter ScaleTo advise engineers, emergency services, and the general public it is useful to be able to quantify the size of earthquakes.The first scale that sought to do so was the Richter Scale. The Richter scale was created by Charles Richter and Beno Gutenberg in 1935 with the aim of quantifying the size of medium sized earthquakes in California. Their scale was based on the maximum amount of ground motion at a distance 100km from the earthquake’s epicentre as measured by a Wood-Anderson Seismograph. This measurement allowed them to quantify the maximum amplitude of the earthquake, but not the total energy released by it. As a consequence, the scale had an upper limit; all large earthquakes were measured as a magnitude 7, even if they were larger.Due to the limitations of the Richter Scale, Beno Gutenberg went on to expand the work he’d previously done with Richter in a bid to improve the scale. The first version of the Richter scale became unreliable when the seismograph was located over 600km from the epicentre because higher frequency vibrations were reduced in intensity, and the records of surface waves were dominated by 20s long waves. The desire to quantify earthquakes even when measured at variable distances led to the creation of the Surface Wave Magnitude Scale (Ms) which measures Rayleigh surface waves. Gutenberg’s work sought to estimate the energy released in an earthquake in terms of Ms.It should be noted that surface waves are not the only type of seismic waves (as discussed in part 1). Compressional P and transverse S waves are body waves that are measured via the Body-Wave magnitude scale (Mb). As P waves are the fastest of the seismic waves, this scale is the quickest way to determine the size of earthquakes far away from the nearest seismometers.The final issue with these scales was found while measuring Great Earthquakes. These extremely large earthquakes could be measured up to 1000km from their epicentres (including the 1960 Chilean Earthquake). These earthquakes had the 20s long waves that Ms was calibrated to measure, but also had waves of up to 200s that carried huge amounts of energy. The Ms scale was not calibrated including those waves, so measurements of these earthquakes missed large amounts of energy and underestimated the earthquake sizes.~SAImage: http://bit.ly/1KvpON2 Damage from 1960 Chilean Earthquake by Pierre St. AmandPart 1 – Earthquake Magnitude: http://tmblr.co/Zyv2Js1tNbIt4Part 3 – Moment Magnitude Scale: LINK upcoming -- source link

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