Thursday, May 23, 2019
  • Announcement 1

    Museum of the City of Skopje

    The Seismological Observatory of the Faculty of Natural Sciences and Mathematics in Skopje contributed in the exhibition,

  • Announcement 2

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  • Announcement 3

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  • 1

What is the difference between the Richter scale and the Mercalli scale?

The most frequently asked question from the citizens:

“What is the difference between the Richter scale and the Mercalli scale?”

         Answer

The two basic types of measurement scales in seismology are connected with the two basic dimensionless measures of the earthquake strength, which are the earthquake magnitude and macroseismic intensity. Thus, there are magnitude and macroseismic intensity scales

The earthquake magnitude is a dimensionless measure of the earthquake strength in its source, which for the tectonic earthquakes can be at a depth down to 700 km. The magnitude is generally defined as being directly proportional to the common logarithm of some earthquake parameter. Thus, there are also different magnitude scales.

The most popular among them are those defined within the Richter’s concept of the magnitude (Charles F. Richter, USA, 1935), which is based on the common logarithm of the parameters of the seismic waves. The Richer (local) magnitude scale (range up to 9) is used for short-distance and local earthquakes, and it is calculated from the maximal ratio of the amplitude and the period of the short-period transverse body seismic waves (periods up to 3 s). For regional and distant earthquakes, magnitude scales defined through the parameters of the various phases of short-period, middle-period and long-period body or surface seismic waves are used. Except the magnitude defined through the maximum spectral amplitude of the surface seismic waves with periods from 100 s to 200 s, all other mentioned magnitudes exhibit saturation: above some specific value, they do not change significantly with increasing of the earthquake strength. (For example, the Richter magnitude saturates at value of about 7.9).

Therefore, concepts of magnitudes which do not exhibit saturation were created. In these concepts, the magnitude is directly proportional to the common logarithm of some of the earthquake parameters which are not connected with а specific type of seismic waves. Such earthquake parameters are the so-called seismic moment, the whole earthquake energy, the maximal observed macroseismic intensity and others.

The earthquake macroseismic intensity is a qualitative evaluation (a description in terms of human perception) of the earthquake strength on the Earth’s surface, i.e. of the earthquake effects on the people, animals, buildings and natural environment. At a specific observational point, the intensity depends on many factors. It decreases with the distance of the observational point from the seismic source. It also decreases with the depth of the earthquake source. It also depends on the orientation of the seismic source, the mechanism of the earthquake generation, the size of the released earthquake energy, the chemical and geological structure of the medium between the seismic source and the observational point. For example, it was always observed that the buildings constructed on ground consisted of sand and different unconsolidated sediments had suffered higher damages during an earthquake in comparison with thе buildings on compact and firm ground. The later is the case with the 26 July 1963 Skopje strong earthquake, since the Skopje valley is composed exactly of sandy and unconsolidated sediments.

The first more accurate macroseismic intensity scale was proposed in 1564, by J. Gastaldi. The most popular macroseismic intensity scales in Europe in the last century were the MCS scale, with 12 degrees (completed for the first time in 1917 by G. Mercalli, A. Cancani and A. Sieberg), the MSK−1964 (or MSK−64) and MSK−1981 scales, both with 12 degrees (constructed by S. V. Medvedev, W. Sponheuer and V. Kárník), the European macroseismic scale from 1998 (EMS−1998), with 12 degrees, recommended by the European Seismological Commission.