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* a values provided for reference only. MMI > X are due more to geologic effects.

* a values provided for reference only. MMI > X are due more to geologic effects.

Other intensity scales are defined analogously, Table 17.3, which also contains an approximate conversion from MMI to acceleration a (PGA, in cm/s2 or gals). The conversion is due to Richter (1935) (other conversions are also available: Trifunac and Brady 1975; Murphy and O'Brien 1977)

Intensity maps are produced as a result of detailed investigation of the type of effects tabulated in Table 17.2, as shown in Figure 17.5 for the 1994 MW 6.7 Northridge Earthquake. Correlations have been developed between the area of various MMI intensities and earthquake magnitude, which are of value for seismological and planning purposes).

Figure 17.6, for example, correlates Afelt versus MW. For preinstrumental historical earthquakes, Afelt can be estimated from newspapers and other reports, which then can be used to estimate the event magnitude, thus supplementing the seismicity catalog. This technique has been especially useful in regions with a long historical record (Ambrayses and Melville 1982; Woo and Muirwood 1984).

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