Living with Earthquakes - Earthquake Engineering for Structural Design - page 34

Table2.1 Frequency of Occurrence of Earthquakes Based on Observations since
1900 (Mazzolani, 2002, USGS, nd)

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Descriptor Magnitude Average annually
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Very great

> 9.0 1 at 20 years

Great 8.0 - 8.9 1

Major 7.0 - 7.9 18

Strong 6.0 - 6.9 120

Moderate 5.0 - 5.9 800

Light 4.0 - 4.9 6200

Minor 3.0 - 3.9 49000

Very minor 2.0 - 2.9 about 1000 per day

1.0 - 1.9 about 8000 per day

One can see that each year about thousand earthquakes exceed the magnitude
5, which is considered as the lower limit for the damaging earthquakes. But even
earthquakes considered to be strong do not necessarily create critical situations if
the rules for seismic design have been observed. Contrary, even moderate
earthquakes acting on old and degraded buildings can produce significant damage.

(a)

Figure 2.3 (continues)