Civil Engineering Reference
In-Depth Information
limiting the catalogues used in hazard analysis to a short period of time may grossly overestimate or
underestimate the ensuing hazard. The over- and underestimation is a function of whether the observa-
tion period was an exceptionally quiescent or energetic epoch. Seismograms recorded at different
epicentral distances are employed to determine origin time, epicentre, focal depth and type of faulting
- as discussed in Sections 1.1.2 and 1.1.3 - as well as to estimate the energy released during an earth-
quake. Descriptive methods can also be used to establish earthquake-induced damage and its spatial
distribution. In so doing, intensity, magnitude and relevant scales are utilized; these are outlined
below.
1.2.1 Intensity
Intensity is a non-instrumental perceptibility measure of damage to structures, ground surface effects,
e.g. fractures, cracks and landslides illustrated in Section 1.4.2, and human reactions to earthquake
shaking. It is a descriptive method which has been traditionally used to establish earthquake size,
especially for pre-instrumental events. It is a subjective damage evaluation metric because of its qualita-
tive nature, related to population density, familiarity with earthquake and type of constructions.
Discrete scales are used to quantify seismic intensity; the levels are represented by Roman numerals
and each degree of intensity provides a qualitative description of earthquake effects. Several intensity
scales have been proposed worldwide. Early attempts at classifying earthquake damage by intensity
were carried out in Italy and Switzerland around the late 1700s and early 1900s (Kanai, 1983 ). Some
of these scales are still used in Europe (alongside modern scales), the USA and Japan. Some of the
most common intensity scales are listed below:
(i)
Mercalli - Cancani - Seiberg
(MCS): 12-level scale used in southern Europe;
(ii)
Modifi ed Mercalli
(MM): 12-level scale proposed in 1931 by Wood and Neumann, who adapted
the MCS scale to the California data set. It is used in North America and several other
countries;
(iii)
Medvedev - Sponheuer - Karnik
(MSK): 12 -level scale developed in Central and Eastern Europe
and used in several other countries;
(iv)
European Macroseismic Scale
(EMS): 12-level scale adopted since 1998 in Europe. It is a
development of the MM scale;
(v)
Japanese Meteorological Agency
(JMA): 7 -level scale used in Japan. It has been revised over
the years and has recently been correlated to maximum horizontal acceleration of the ground.
Descriptions of the above intensity scales can be found in several textbooks (Reiter, 1990 ; Kramer,
1996 ; Lee
et al
., 2003, among many others). A comparison between MCS, MM, MSK, EMS and JMA
scales is provided in Figure 1.13. Intensity scales may include description of construction quality for
structures in the exposed region. For example, the MM- scale specifi es different damage levels depend-
ing on whether the structural system was poorly built or badly designed (VII), ordinary substantial
buildings (VIII) or structures built especially to withstand earthquakes (IX). However, intensity scales
do not account for local soil conditions, which may signifi cantly affect the earthquake- induced damage
and its distribution. Correlations between earthquake source and path, on the one hand, and intensity
measures on the other are therefore highly inaccurate.
Intensity scales are used to plot contour lines of equal intensity or ' isoseismals ' . Intensity maps
provide approximate distributions of damage and the extent of ground shaking. Maps of local site
intensity include reports of all observation sites and whether or not the strong motion was felt. For
example, the isoseismal map of the 17 October 1989 Loma Prieta earthquake in California shown in
Figure 1.14 locates the epicentre (marked as a star) and provides MM intensities between isoseismals
(Roman numerals), and MM intensities at specifi c cities (Arabic numerals). The MM intensity of VIII
