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2.3 Methods of Rating Individual Earthquakes

2.3.1 Earthquake Intensity Scales

A scale for rating individual earthquakes was proposed in 1828 by P.N.G. Egen. It contained six classes of intensity which were more sharply defined than those developed thereafter. The next advance was that of the Irish engineer Robert Mallet, who was one of the principal founders of modern seismology. Mallet combined theoretical and archival scholarship with a rigorous field investigation of southern Italy directly after the earthquake disaster of December 1857, which killed nearly 9,750 inhabitants of the Kingdom of Naples (Guidoboni & Ferrari 1987). His methods of recording earthquake damage were highly systematic and analytical, and new scales could thus be propounded on the basis of information that was much more comprehensive and less subjective than had previously been the case. However, the scales remained inductive, that is, they were derived directly from observed phenomena, with only limited input of explanatory of predictive theory.

The foundations of modern scales were laid in 1879 by De Rossi, an Italian, and were further developed in 1883 by Forel, a Swiss. Their scale of 10 classes was broadened to 12 by Giuseppe Mercalli, who in 1902 proposed what has proved to be the most durable system for classifying perceptible seismic phenomena. In 1931 the Mercalli-Wood –Neumann Scale was introduced in order to take account of the effect of earthquakes on motorized vehicles, tall buildings and other modern inventions that were not significant in Mercalli‘s time (Wood &

Neumann 1931). The principal modern variants of Mercalli‘s original scale are the Modified Mercalli (MM) Scale of 1956 (Table 2.1), which is standard in the Americas, and the Mercalli-Cancani-Sieberg (MCS) Scale, which is used widely in Europe. Other scales include the Medvedev-Sponheuer-Karnik (MSK) Scale (Karnik et al.1984), the Japan Meteorological

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Association (JMA) Scale (which has seven classes, rather than the 12 utilized in Western scales), and the GOST Scale, which is standard in the Commonwealth of Independent States (Bolt et al. 1990). Right from time the development of earthquake scale has been of great concern to seismologists. Domenico Pignataro in the late 18th century made the first attempt to grade the severity of earthquakes. His analysis classified earthquakes as very strong, strong, moderate or slight. In the mid 19th century Mallet came out with a list of earthquakes and plotted their estimated locations and hence produced the first map of the world seismicity. He also used a four–stage intensity to classify earthquakes damage and constructed the first isoseismal maps.

Ross and Forel developed Ross-Forel intensity in the late 19th century that was made up of ten stages describing the effects of increasing damage. The Rossi-Forel scale is one of the first scales designed to describe the effects of an earthquake, at a given place, on natural features, on industrial installations and on human beings. The intensity differs from the magnitude which is a quantity describing the strength of an earthquake. Due to its limitations, particularly in respect to the relation with ground acceleration, it has been replaced by the Modified Mercalli scale.

Wadati (1931) in an attempt to develop an earthquake scale, constructed a chart of maximum ground motion against distance for a number of earthquakes and noted that larger earthquakes produced larger amplitudes. Richter (1935) used Wadati‘s ideas and methods to construct the first earthquake magnitude scale, which is used to measure the size of an earthquake. Medvedev-Sponheuer-Karnik (MSK) scale came into being in Europe in 1964 and had stages just as the MM scale but with the difference in details (Lowrie, 1997).

In 1992 the European macroseismic scale (EMS) was introduced. The twelve-stage EMS was based on MSK scale and further considered how unprotected structures are to earthquake damage and give more explanation on the extent of damage to structures with different building standards.

Molchan et al (1999) used the frequency-magnitude relation to construct a multi-scale seismicity model for the main shock in seismic risk assessment. Their analysis revealed an understanding of seismicity at different space-time scales.

14 Table 2.1: The modified Mercalli scale

I Instrumental Detected only be seismographs II Feeble Noticed only be sensitive people

III Slight Resembling vibrations caused by heavy traffic

IV Moderate Felt by people walking; rocking of free standing objects V Rather strong Sleepers awakened and bells ring

VI Strong Trees sway, some damage from overturning and falling objects.

VII Very strong General alarm, cracking of walls

VIII Destructive Chimneys fall and there is some damage to buildings IX Ruinous Ground begins to crack and many, houses begin to

collapse and pipes break

X Disastrous Ground badly cracked and many buildings are destroyed.

There are some landslides.

XI Very disastrous

Few buildings remain standing; bridges and railways destroyed; water, gas electricity and telephones out of action.

XII Catastrophic Total destruction; objects are thrown into the air, much heaving, shaking and distortion of the ground.

15 2.4 Earthquake Magnitudes

This is the experimentally measured amplitude of ground motion produced by a seismic wave. They are determined directly from seismograms and are related indirectly to the energy released during an earthquake.

Magnitude scales have a general relationship of the form :

M = Log( A/T) + F(h,Δ) + C 2.6

( Dowrick, 1977) Where,

A is the amplitude of the signal, T is the dominant period,

F is a correction for the the variation of amplitude with earthquake depth,h, and distance, Δ, from the seismometer

C is the regional scale factor.

The early estimate of earthquake size was based on non-instrumental measures of the earthquake effects. Values such as the number of fatalities or injuries, the maximum value of shaking intensity, or the intense shaking were used as determinants of earthquake size. The problem with these kinds of measurement is that they don‘t have a good correlation. The damage and destruction produced by earthquake will depend on its location, depth, and nearness to populated regions and its ‗true‘ size.

With the invention of seismometers it is now easy to accurately locate earthquakes and measure the ground motion produced by seismic waves. One of the means of quantifying earthquakes using seismogram is the magnitude.

Also the USGS defined magnitude as a logarithmic measure of the ‗size‘ of an earthquake, which is related to the energy releases as seismic waves at the focus of an earthquake. As measures of earthquake size, magnitudes have two principal advantages. First, they are directly measured from seismogram without sophisticated signal processing.

Secondly they yield units of 1, which are intuitively attractive: magnitude 5 earthquakes are moderate, magnitude 6 is strong, 7 are major, and 8 are great (Stein and Wysession, 2003).

Several factors influence the determination of earthquake magnitude. They include focal depth, distance between earthquake focus and observation station, frequency content of the sampled energy and earthquake radiation pattern.

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Magnitudes are usually measured from the amplitude and period of seismic signals as they arrive and are recorded at a seismic station. For a given earthquake, the amplitude decreases with an increasing distance (due to geometric spreading and attenuation of signals) and a distance dependent correction is applied when computing magnitude to result in one value for each station. Although the magnitude scale has neither ‗top‘ nor ‗bottom‘ values, the highest magnitude ever known is about 9.5 and the lowest about -3.0 (USGS, 2000).