M A S T E R P A G E
Teacher Background Reading
SiteCharacteristics
2.5a
“Earthquakes don’t kill people, buildings do.”
Architects and engineers consider this a fair one-sentence summary of earthquake-related deaths, injuries, and
damage. Yet, underneath every building is the Earth, which can shake and damage or destroy the building. In the
final analysis, the cause of the death and destruction may not be the earthquake or the building, but rather
someone’s lack of knowledge about the soil and subsoil under the building. Much of the scientific study
surrounding earthquakes is focused on the geological characteristics of building sites, the relationship of building
sites to earthquake damage, and how buildings respond to ground shaking induced by earthquakes. Location is
just as important as building design for making sure that a building can survive an earthquake. Geological site
considerations include the location and history of faults, sedimentary deposits, landfill, liquefaction, steep slopes
and landslides, tsunami, and human-made hazards.
Faults: Displacement and Ground Shaking
Earthquakes happen when two sides of a fault are displaced, releasing energy in waves. Buildings can be
damaged either by direct displacement on the fault or by ground shaking.
Geologists have mapped the locations of many of the most dangerous fault zones in the U.S., yet many faults are
not yet recognized. A building within a fault zone can be severely damaged by an earthquake on that fault, but
this kind of damage is rare. Most buildings are not in fault zones, and the recurrence interval for any particular
fault may be hundreds or thousands of years. The most common cause of damage in earthquakes is the ground
shaking caused by the earthquake waves. These attenuate, or die off, with distance, so the two most important
factors controlling the amount of shaking are the magnitude of the earthquake and the distance of the building
from the fault.
The distance from the fault, not from the epicenter, determines the amount of damage. Energy is produced by all
the parts of the fault that move in an earthquake. Because in big earthquakes the fault can be hundreds of miles
long, a structure may be hundreds of miles from the epicenter and still be on top of the quake’s impact zone.
Several other factors can affect the amount of shaking. Waves do not travel evenly in all directions from the fault,
so the orientation of the fault and the way in which displacement on the fault occurs can change the characteristics
of the waves. Even more important are variations in local topography—the lay of the land—including the subsoil
layers, which may trap or amplify seismic energy, and the type of rock and soil that underlie buildings.
Sediments and Landfill
Ground shaking is greatest on soil that has arrived in place fairly recently, whether it was put there by natural
processes (in which case, geologists call it
sediment
) or by artificial ones (in which case, it is called
landfill
).
Unfortunately, most of the world’s urban centers are sited on relatively young, loose, sedimentary deposits.
Sediment age and particle size are important in predicting how soil will respond to shaking during an earthquake.
Areas near the shores of rivers and oceans are especially likely to contain young sediments washed there by the
water.
Structures located on former watercourses (such as old river beds) or on sites that have been artificially filled with
sand dredged up from the bottom of a body of water are among the worst locations for construction in earthquake
country because the soil can shift so easily. In Mexico’s devastating 1985 earthquake, Mexico City, 320 km (200
miles) from the epicenter, suffered far more damage than the shoreline towns closer to the epicenter. The
shoreline is made of solid rock, but Mexico City is built on the sediments of an ancient lakebed.
Old watercourses are usually low and wet, so they are frequently filled when someone wants new land to build on
and sell. Landfill is usually a mixture of soil, rock, and decaying organic material in particles of varying sizes.
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