Mexico City provides an example of how soft ground can magnify earthquake damage. In
1985, a magnitude 8.1 earthquake struck about 350 kilometers west of the city. The earth-
quake was caused by subduction of the Cocos Plate beneath the North American Plate.
Mexican government records show that the earthquake killed at least 9,000 people, injured
30,000 more, left 100,000 people homeless, destroyed 416 buildings, and seriously damaged
3,000 other buildings. The reason for so much destruction so far from the earthquake’s epi-
center is that Mexico City is built on a drained lakebed. Beneath the capital city, the ground
is soft silt and clay in a basin made of solid rock. When the earthquake struck, seismic waves
bounced back-and-forth off the sides and bottom of the rock basin amplifying the shaking.
In addition, the wet clay experienced liquefaction (Figure7.45). The buildings were not
anchored to bedrock as they should have been and so they settled into the muck, causing
enormous damage.
Figure 7.45: Liquefaction of sediments in Mexico City caused the collapse of many buildings
in the 1985 earthquake. ( 9 )
Water, sewer and electrical systems were destroyed, resulting in fires. Acapulco, which was
much closer to the epicenter but built on bedrock suffered little damage. To prevent Mexico
City from being taken by surprise again, the government built an alert system. The next
time there is an earthquake in the subduction zone, a signal will be activated and sirens will
sound in the city. This will give residents about one minute to prepare for the inevitable
earthquake. At the least, this is enough time for most people to get in a secure location.
The population density of a region is also important to the number of casualties and the
amount of damage. The 1964 Great Alaska Earthquake, near Anchorage, was the largest
earthquakeeverrecordedinNorthAmericaandthesecondlargestglobally, withamagnitude
of 9.2. The earthquake lasted for several minutes, resulted in slip of up to 11.5 meters (38
feet), and affected an area of 100,000 square miles (250,000 square km). Ground liquefaction
caused landslides (Figure7.46).