Chapter 3
Chapter 3
Chapter 3
Earthquakes are serious natural hazards that affect people across the globe,
sometimes at long distances from where the quakes occur. They are
especially dangerous because seismologists, the scientists who study
earthquakes, cannot predict them in time for evacuations or other
precautions. Your goals in this module should be to:
know which global regions are most at risk for earthquakes and shy
they are at risk.
know how human beings interact with and affect the earthquake
hazard.
Image copyright: United Nations Development Programme, licensed as Creative Commons Public
Domain.
Recall that all around the planet, tectonic plates are moving because of
convection in the mantle. Tectonic plates are also composed of two types of
crust, oceanic and continental. The oceanic crust, which is made mostly of
basalt is more dense than continental crust that is made of granite. When
these tectonic plates come in contact, the denser oceanic crust subducts
below the continental crust. Now sometimes when two tectonic plate come in
contact they become stuck. As the rocks begin to bend or strain under
tectonic forces, large amounts of energy - called strain - builds. When the
stress becomes too great for the rocks to hold, segments may suddenly
snap, releasing large amounts of energy. This is called theelastic rebound
theory.
Movement along a fault can occur vertically or horizontally. The greatest
horizontal displacement was 21 feet along the San Andreas Fault in the Great
San Francisco Quake of 1906. Imagine in an instant being moved 21 feet
horizontally! The greatest maximum vertical displacement used to be the
Alaskan earthquake in 1964. The vertical displacement was 33 feet! But on
December 26, 2004 a 9.1 underwater earthquake occurred in Indonesia. It
had a vertical displacement of 60 feet over 800 miles long! Imaging being
thrown 60 feet instantly and that it occurred for 800 miles. The compression
caused by the oceanic-oceanic convergence actually sped up the earth's
rotation 2.676 millionths of a second and shifted the axis 1 inch! It produced
a massive tsunami traveling 500 mph. Over 240,000 died from the
earthquake and tsunami; one in three were children. The image below if of
an earthquake in Pakistan that killed 80,000.
Editors Note/Document: The Indian Institute of Technology did an interesting study on
the social aspects of Earthquake disasters. It is short but it talks of how the social and
natural disasters go hand in hand. The article is found here:
http://www.iitk.ac.in/nicee/wcee/article/7_vol9_251.pdf
Image copyright: United States Geologic Survey, licensed as Creative Commons Public Domain.
There are two basic ways to measure the strength and destructive power of
an earthquake. The first is called the Richter scale. Based on a range from
1 (weakest) to 10 (strongest), the Richter scale measures the magnitude
(energy released) by an earthquake. The scale is logarithmic meaning that
every whole number increase in magnitude is 10 times more ground shaking
and 30 times more energy released. Example: a magnitude 7.0 earthquake
has ground shaking 10 times more than a 6.0 and 30 times more energy
released. A magnitude 8.0 earthquake has ground shaking 100 times more
than a 6.0 and 900 times more energy released. The December 26, 2004
Indonesian earthquake had a magnitude of 9.1. It should be noted that for
the strongest earthquakes, the Richter Scale is no longer used. Instead,
the Moment Magnitude Scaleis used, though it is very similar to the
Richter Scale. The Richter Scale and Moment Magnitude Scale
provide quantitative information of the energy released from an earthquake.
Editors Note/Video: When people below the poverty line and earthquakes
mix devastation occurs.
One final note, with the recent popularity of smart phones and the iTouch,
there are several apps now available in relation to natural disasters. In terms
of earthquakes, a highly popular app in Apple's iTunes is called QuakeWatch.
There are probably similar apps for other smart phone devises.
Image copyright: United Nations Development Program, licensed as Creative Commons Public
Domain.
Earthquakes do not kill people; falling buildings and highways kill people.
History has taught us the importance of building codes to create safer
buildings. Many of the massive death tolls reported by earthquakes are
caused by poorly built buildings rather than the earthquake itself. In general,
buildings or structures built out of brick, stone, mud, or reinforced concrete
fair poorly in large earthquakes because there is very little flexibility in the
structures as the ground shakes. The best types of buildings to be in are
those built of wood because of there flexibility; the house may not be
Click here to see a map of liquefaction potential and ground shaking within
Salt Lake County. (You can also look at maps for Tooele, Cache, Davis, Weber
and Utah Counties.) For those who live in the Salt Lake valley, notice how the
liquefaction potential is greatest along the Jordan River and near the Great
Salt Lake. Any ideas why? It has to do with the fact the water table is highest
near those areas. So when an earthquake occurs and the ground begins to
shake, the water will rise and destabilize the ground. In these maps, the reds
and purples represent the areas of greatest liquefaction potential from a
magnitude 7.0 earthquake.
Earthquakes can also destabilize steep slopes causing them to slip and fail.
These landslides tend to occur where the ground is mostly loose sandy soil
with a high water table. Fires are also another serious hazard created by
earthquakes as our infrastructure collapses and electrical and gas lines
break. The treat of fires will also increase as emergency crews have a hard
time maneuvering through the debris. There are two simple things
individuals can do to reduce their risk of a fire: first is to have a wrench
attached to their outside gas line. If you smell gas at your house, quickly turn
off the gas. But if you do not smell gas, do not turn it off; doing so may
prevent you from having heating in your house in the winter and it may be
months before it gets turned back on. Second, make sure your water heater
is attached to your house. A simple $20 bracket wrapped around your water
heater and bolted into your house may prevent it from falling over and
breaking its gas line.
Another interesting effect of powerful earthquakes is island creation. In
September 2013, a moderately powerful earthquake off the coast of Pakistan
created a 200 foot wide, 60 foot tall island. It won't last long because ocean
waves will erode it back down, but it's an interesting and rare effect of
earthquakes. Click here to learn more.
Each member in your family should also know where to meet in case of
a disaster.
The number one reason why people end up in the hospital after an
earthquake is glass in their feet. Having a pair of old shoes under your
bed can greatly reduce that probability.
Know how to shut off your gas line if you smell gas in your house. If it
requires a wrench to shut off, always have one next to the line for
quick shutoff. You will know if you have a gas leak because the gas
companies place a chemical in the gas that will smell like rotten eggs.
Also make sure your water heater is attached to your house. If your
water heater falls over and the gas line breaks, your house can catch
fire.
If you and your family are safe, take care of others in need.
Finally for those interested, look into getting CERT certified as a first
responder.
Editors Note/ Checklist: Being prepared is really the biggest thing that you can do for
you and your family to help mitigate the disaster that can be potentially devastating.