Effects of Earthquakes
Measuring the Severity of Quakes
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The Richter Scale
is used to measure the amount of energy released in a given earthquake.
The Richter reading won't be affected by the observer's distance from the
earthquake, although the perceived strength will be greater near to the
epicenter and generally decrease as the distance increases. Richter readings
are useful in comparing earthquakes that occur in different times and places.
There are many other factors that contribute to the damage, such as the underlying
rocks, building construction and poulation density. The Richter reading by
itself does not give enough information to tell what the effects will be
in any particular place. That said, however, in general, the larger the Richter
reading, the greater the damage will be close to the epicenter. In recent
years, scientists have used a variety of magnitude scales to measure different
aspects of the waves produced by an earthquake. These different magnitude
scales reflect a greater complexity than can be represented by Richter's
original scale. These different scales sometimes lead to confusion when different
magnitude readings are reported for the same quake. These different readings
reflect different aspects of the quake. Especialy in large quakes, these
differences can be substantial. For instance, the 1964 Alaska quake was originally
recorded as 8.6 Magnitude. Now scientists think that a 9.2 Magnitude more
accurately reflects that quake's intensity. The important thing to remember
is that these are not necessarily contradictory, but are on different scales
and measure different things.
An observer near the epicenter of an earthquake will generally experience
a magnitude 3 or 4 on the Richter Scale as a mild shaking, 5 or 6 will be
enough to cause some damage, 7 will cause a lot of damage and 8 or higher
(classified as a Great Earthquake) will cause large scale destruction.
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The Modified Mercalli Scale
of earthquake damage measures the intensity of an earthquake at a particular
place. It uses the type and amount of damage. Unlike the Richter Scale,
it does not measure the absolute strength of the earthquake, but how strongly
it is felt at a particular place. This makes the Mercalli Scale useful in
comparing damage from different quakes and in plotting damage patterns from
a given quake. This type of comparison has helped scientists figure out
the factors that contribute to earthquake damage. A map showing the Mercalli
intensity at different locations for the same quake can be quite revealing.
These maps can be related to geological maps to see what effect the underlying
rocks have on the intensity of the quake. They show that softer porous soils
shake very violently, while bedrock is less affected.
Factors that Affect Damage
iCivil Engineer website on the effects of several large earthquakes
Earthquakes cause many different kinds of damage depending
on the strength of the quake, distance, type of underlying rock or soil
and the building construction. A given Richter reading will produce vastly
different amounts of damage in different parts of the world. Even the same
quake can have very different effects in neighboring areas. For instance,
in the
1989 Loma Prieta earthquake
, some of the worst damage was in the Marina neighborhood of San Francisco,
seventy miles from the epicenter. Many areas much closer to the quake suffered
only minimal damage. The difference in this case was that the Marina was
built on loose soil that had been used to fill in the edge of San Francisco
Bay. This combination of uncompacted soil with a lot of water in it led to
a phenomenon called liquefaction.
Liquefaction
occurs when the ground loses its cohesion and behaves like a liquid. When
this happens during an earthquake it can result in increased intensity of
the shaking, or landslides. It can also cause rapid settling and collapse
of buildings. Buildings on solider ground do much better.
These earthquake
hazard maps
show the expected severity of ground shaking in the San Francisco Bay
Area based on the underlying soils and distance from major faults.
Another factor that has a major effect on the damage is the building method
and materials used. Unreinforced masonry has the worst record since it has
little ability to flex or move without collapsing. Wood frame buildings,
or reinforced buildings, on the other hand, can hold together under quite
severe shaking. Third world countries with masonry buildings often have huge
numbers of casualties from quakes that, had they occurred in California,
with its stringent building codes and predominantly wooden buildings, would
have been considered relatively minor.
Types of Damage
Building Collapse
People can be trapped in collapsed buildings or under rubble that collapses
into the street. This is the type of damage that leads to the worst casulaties.
The worst thing to do in a quake is to rush out into the street during the
quake. The danger from being hit by falling glass and debris is many times
greater in front of the building than inside. In the 1989 Loma Prieta quake
the streets of San Francisco's financial district were covered by broken
glass and people were buried under the facade of a brick building that fell
forward into the street. Likewise in the 1964 Alaska quake, a huge concrete
facade fell off of a department store onto pedestrians passing by.
Buildings knocked off their foundation
Buildings that can otherwise withstand the quake can be knocked off their
foundations and severly damaged. This type of damage can be largely prevented
by bolting the frame securely to the foundation, so it will remain in place.
Landslides
Buildings can be damaged when the ground gives way beneath them. This
can be in the form of a landslide down a hill, or liquifaction of soils
that can cause severe settling of the ground. Ground movement can change
the whole landscape, as in the New Madrid Quake that changed the course
of the Mississppi River. A landslide into a lake or resevoir can cause flooding
downstream. This kind of damage is not unique to earthquakes, but can be
triggered by a quake.
Fire
Fires often break out following earthquakes. They can be caused by flammable
materials being thrown into a cooking or heating fire or broken gas lines.
Fires can easily get out of control since the earthquake may have broken
water mains or blocked roads firefighters need to use. There are many demands
made on the emergency response systems that slow down response to fires.
In the 1906 San Francisco Earthquake
, for example, the fire that followed the quake caused more damage than
the earthquake itself.
Tsunami
Underwater earthquakes, volcanos, or landslides can produce a tsunami
or tidal wave. This wave can travel very rapidly thousands of miles across
the ocean. In deep water the tsunami may only raise the ocean level by a
few inches, hardly enough to notice. But as it approaches land, the shallower
water causes the wave to build in height to as much as 50 feet or more and
suddenly flood coastal areas. Tsunamis carry a lot of energy and when they
hit the coast strong currents can cause massive erosion of the coastline
as well as tearing apart buildings it encounters. Typically a tsunami will
last for a period of hours with successive waves drastically lowering and
raising the sea level. Although scientists now understand the causes of tsunamis,
there are many local factors including the slope of the seafloor at a given
location, the distance and direction of travel from the earthquake that
will determine the severity of the resulting wave.
A tsunami is a large wave caused by an abrupt displacement of water, very
similar to the ripples from a rock thrown into the water. Water at any given
point does not travel with the wave, but moves up and down as the wave passes.
Unlike normal ocean waves, tsunamis have a very long period between the
waves. While normal waves on the beach might come in every half minute or
so, a tsunami's waves will be separated by many minutes or hours. Not only
crests of the waves very high but the troughs between the waves will be very
low. Usually as the tsunami comes onshore, the wave trough arrives first,
causing the sea level to drop, exposing the seabed. Soon however, the crest
arrives, flooding coastal areas. Anybody living near the coast should be
aware of tsunamis and ready to move to higher ground immediately after an
earthquake or when tsunami warnings are in effect.
Most tsunamis are too small to be noticed but the Tsunami Warning Center
in Hawaii tracks them and sends out warnings when there is a chance of a
large one. Unfortunately, some of the worst tsunamis occur near the epicenter
of the originating earthquake and therefore give little warning. This was
true of the devastating 1998 New Guinea tsunami, which was caused by an earthquake
just off the coast.
Tsunamis contributed to significant damage in the
2004 Sumatra Earthquake and Tsunami
, which devastated coastal areas in the Indian Ocean, as well as the
1755 Lisbon quake
and the 1964 Alaska quake
, among others.
More Tsunami Information
The Tsunami!
Page
is a good source of more information.
