magnitude 9.5. The 1556 Shaanxi earthquake in China measured a magnitude 8.0, but is
estimated to have killed about 830,000 people. The Great Sumatra - Andaman earthquake
of 2004 makes the list of the largest earthquakes and was also one of the deadliest. However,
most of the 230,000 fatalities were due to the tsunami that followed the earthquake, not the
earthquake itself.
Damage during an earthquake depends on the type of ground under the buildings. Solid
bedrock vibrates much less than soft sediments. Buildings on bedrock will only fail if the
earthquake is extremely violent. Soft ground, such as sand, silt, or clay, settles when it is
shaken and so buildings tilt or fall over, and pipelines, roadways, and other structures break.
Sediments that are saturated with water undergoliquefactionduring an earthquake and
become like quicksand. Soil on a hillside that is shaken loose can become a landslide, which
takes houses downhill with it or buries structures at the hill’s base.
In earthquake-prone areas, city planners spend a lot of time understanding which locations
are the most vulnerable and trying to reduce the hazards. For example, in the San Francisco
Bay Area, planners study maps that show how much shaking is expected in various areas
for different magnitudes and locations of earthquakes (Figure7.44). Using this information
can allow them to understand and prepare for the hazards. For example, when faced with
two possible locations for a new hospital, planners must build on bedrock rather than silt
and clay.
Figure 7.44: This map shows the amount of shaking on the Modified Mercalli Intensity Scale
that would be expected for an earthquake of magnitude 7.1 on the northern portion of the
Hayward Fault. Much of the land near the bay, where shaking is predicted to be most violent
is loose mud & soil, called fill. The hills around the bay, which are mostly colored green, are
bedrock. The outline of the Hayward Fault can be seen in black, since it is where the most
violent shaking is predicted to be. ( 8 )