Learning from Earthquakes - Earthquake Engineering for Structural Design - page 79

The geotechnical aspects of earthquakes are involved with the lessons
considering the influence of travel path and site soil on the ground motions.
- The influence of travel path and site soil depends on the earthquake type,
being very different for interplate or intraplate earthquakes.
- For interplate crustal sources, the travel path and local soil conditions
modify the basic characteristics of the source, acting as the band-pass
filters. The bad soil conditions of 1906 and 1989 San Francisco, 1977
Bucharest and 1985 Mexico City have a great influence on site ground
motions, overshadowing the source characteristics (see Chapter 2).
- For intraplate crustal earthquakes, characterized by high-frequency
movements, the influence of site conditions is more reduced. Notable
exceptions have been observed in case of shallow soil deposits and soil
liquefaction, when significant amplification of ground motions can occurs.
The soil liquefaction during the 1811 New-Madrid earthquake produced a
very strong earthquake (with magnitude over 8) in a region without evident
faults. The soil deposits and liquefaction played also a very important role
during the Niigata and Kobe earthquakes (Fig. 3.6).
- Soft soil, valley or relief effects can produce important ground motion
amplifications. The Mexico-City is the most demonstrative case about
valley and soft soil effects (Fig. 3.7).

- The presence of buildings and the high density of built-space can be a very

important factor, producing an important amplification of ground motions,
especially in case of soft soil conditions. It is possible that this effect has

Figure 3.6 Overturning of some buildings during the Niigata earthquake, due
to the soil liquefaction (USGS, nd)