BASIC EARTHQUAKE PRINCIPLES - Geotechnical Earthquake Engineering - page 25

Ranking Location Year Moment magnitude M w

1 Chile 1960 9.5

2 Alaska 1964 9.2

3 Russia 1952 9.0

4 Ecuador 1906 8.8

5 Japan 1958 8.7

2.5 INTENSITY OF AN EARTHQUAKE

The intensity of an earthquake is based on the observations of damaged structures and the
presence of secondary effects, such as earthquake-induced landslides, liquefaction, and
ground cracking. The intensity of an earthquake is also based on the degree to which the
earthquake was felt by individuals, which is determined through interviews.
The intensity of the earthquake may be easy to determine in an urban area where there
is a considerable amount of damage, but could be very difficult to evaluate in rural areas.
The most commonly used scale for the determination of earthquake intensity is the modi-
fied Mercalli intensity scale, which is presented in Table 2.3. As indicated in Table 2.3, the
intensity ranges from an earthquake that is not felt (I) up to an earthquake that results in
total destruction (XII). In general, the larger the magnitude of the earthquake, the greater
the area affected by the earthquake and the higher the intensity level. Figures 2.16 to 2.18
present the locations of U.S. earthquakes causing VI to XII levels of damage according to
the modified Mercalli intensity scale. Table 2.2 presents an approximate correlation
between the local magnitude M L and the modified Mercalli intensity scale.
A map can be developed that contains contours of equal intensity (called isoseisms ).
Such a map is titled an intensity map or an isoseismal map, and an example is presented in
Fig. 2.19. The intensity will usually be highest in the general vicinity of the epicenter or at
the location of maximum fault rupture, and the intensity progressively decreases as the dis-
tance from the epicenter or maximum fault rupture increases. There can be numerous
exceptions to this rule. For example, the epicenter of the 1985 Michoacan earthquake was
about 350 km (220 mi) from Mexico City, yet there were buildings that collapsed at the
Lake Zone district. This was due to the underlying thick deposit of soft clay that increased
the peak ground acceleration and the site period, resulting in resonance for the taller build-
ings. This effect of local soil and geologic conditions on the earthquake intensity is further
discussed in Sec. 5.6.
The modified Mercalli intensity scale can also be used to illustrate the anticipated dam-
age at a site due to a future earthquake. For example, Fig. 2.20 shows the estimated inten-
sity map for San Francisco and the surrounding areas, assuming there is a repeat of the 1906
earthquake. It is predicted that there will be extreme damage along the San Andreas fault
as well as in those areas underlain by the San Francisco Bay mud.

2.6 PROBLEMS

The problems have been divided into basic categories as indicated below:

Identification of Faults

2.1 The engineering geologist has determined that a fault plane is oriented 5NW 34W.
The engineering geologist also discovered a fault scarp, and based on a trench excavated