Earthquakes using Engineering Parameters
Intensity is a subjective, qualitative description of observed damage and the reaction of people and animals to the earthquake. The modified Mercalli intensity (MMI) scale is used to measure intensity in the U.S and other English-speaking countries. Other areas such as Japan (JMA scale) and eastern Europe (MSK scale) use other intensity scales. MMI intensities range from I-XII:
Noticed by people
A number of intensities from different sites can be plotted on an isoseismal contour map, giving a helpful picture of the distribution of damage. Also, interpretive MMI data is available for many past earthquake which do not have instrumental records. However, intensity is not a good index for design because of its qualitative nature.
Since instruments such the seismograph and accelerometer have made it possible to accurately measure ground motions, a quantitative measure of the size of an earthquake is possible. There are several magnitude scales in use, which often leads to confusion in the reporting of magnitudes. The best scale for scientific and engineering purposes is the moment magnitude (Mw) scale since it is related to the rupture parameters. The most commonly used magnitude scales are:
Estimating Magnitude Based on Fault Length
For a specific fault, the Mw of a potential earthquake can be estimated by relating it to the potential rupture length of the fault using the Wells and Coppersmith (1994) relation:
and Coppersmith have also developed relationships between magnitudeand
average displacement, subsurface rupture length, downdip rupture width,
and rupture area .
The total amount of seismic energy released by the earthquake is related to the magnitude, and can be estimated using the relationship by Gutenberg and Richter (1956):
logE = 11.8 +1.5MS (ergs)
Each unit increase in magnitude produces roughly a 32-fold increase in energy release. Also, moment magnitude can be used instead of surface wave magnitude.