"Earthquake Hazards"
| This webpage looks at some of the earthquake hazards that occured during the San Fernando Valley earthquake of 1971. The San Fernando Valley is in California, just north of Los Angeles. Lots of people live there. This earthquake is also known as the Sylmar earthquake. The intent of this page is to introduce you to earthquakes in an easy to understand manner. As you proceed the level of conversation will increase, incorperating the basics onto the common language of earthquake
seismology
.
|
Earthquakes
Earthquakes are best discribed as a shaking or vibration of the ground caused by breaking of rock. Sometimes they are very strong and other times you would hardly notice them. This shaking occures when
stress
that builds up in the crust is suddenly released as the
crust
breaks free and/or slides against the other pieces of crust.
Earthquakes may also be thought of as the breaking of a popcicle stick by applying
pressure
to both ends at the same time. Should you try this
experiment
, you will feel the pressure build up as you apply more
force
until the stick snapps. When the stick snapps you will feel an instant of pain at your fingers as the stress reduces and energy waves move throughout the stick. When the earth's crust is placed under similar types of stress, binding as the stress builds, it will also snapp and release the energy into the surrounding rocks, 'ooch'. Thinking about earthquakes will become more clear if you try another experiment.
|
|
This
picture
is courtesy of Mr. Edgar V. Leyendecker, USGS, 1971.
|
Experiment
Find two regular red bricks, or any color will do. Place them on the table side-by-side touching each other. Now push them together with a little bit of force and slide one past the other while applying the force. You will find that they do not slide past each other very easily while applying a force to push them together. As you manage to force the bricks past each other, you may notice by feeling and hearing a vibration and a jerking type of movement. You have simulated an
earthquake
.
Now set a small toy , a rubber eraser, a coin or other item on each of the bricks and do the experiment again. You can see that the objects bounce, fall and move about as the bricks slide past each other. Taadaa! You have created an earthquake in the toy world! Now place one of the bricks one on top of the other. Put some toys on top of the upper brick and slide the top brick across the bottom brick. This is called a
'thrust'
fault
when one piece of crust slides over top of the other. This is much more like what happened during the
Sylmar
earthquake. Are the toys O.K.? Unfortunately, this is what happens in the real world too.
Real Life
The vibration and jerky movements cause things to fall, bounce around and sometimes break. Nobody knew that the crust under Sylmar was like this; or they probably would not have chosen to live there.
This picture of the mall, taken by Mr. Leyendecker who works for the 'United States Geological Survey' (USGS, a fun job), shows what happens in the real world as blocks of the earth's crust slide accross each other for just a moment or two. Many lessons have been learned from this particular earthquake. Before the '
quake
', the community of Sylmar, located in the San Fernando Valley, had built structures that were intended to resist damage during an earthquake. As you will see, this effort provided marginal relief. They knew of the possible hazards that the near by San Andreas
fault
posed. Their schools, roads, shops and city, state and federal buildings were built to withstand a moderate quake. What they did not know was that the San Fernando Valley is on top of a thrust fault. On Febuary 9, 1971 Sylmar and the surrounding region experienced a
magnitude
6.7 earthquake. This was a very strong quake that caused a lot of damage to many structures. Both new and old building and roads recieved a lot of damage.
|
| This
picture
is courtesy of Mr. Edgar V. Leyendecker, USGS, 1971.
|
History
The whole world has earthquakes, and California has its share of them. 'In January 1994, a magnitude 6.8 earthquake struck Northridge California, 60 people lost their lives and 7,000 were injured. An estimated 20,000 people lost their homes. June 1992 a magnitude 7.6 earthquake hit Landers California. One person was killed and 400 people were injured. October 1989 Loma Prieta California was shocked by a magnitude 7.1 earthquake that also killed 60 people. 3,757 people were injured and 5.6 billion dollars worth of damage occured. And of course, the Febuary 9, 1971, magnitude 6.7 earthquake that hit Sylmar killed 65 people.'
(www.iris.edu
)
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of this page.
This page was updated December 8, 2001.