>>Today we are going to discuss earthquakes.
When rocks are brittle and stress is applied,
they can break, and that can produce an earthquake.
The focus of the earthquake is the point of
energy of release within the earth. The epicenter
of the earthquake is the point on the surface
directly above the focus. What happens when
rocks break and an earthquake occurs? Well,
look at this rubber band. If I apply some
stress to this rubber band, it's elastic.
If I take the stress away, it goes back. But
if I continue to pull this apart, which I'm
not going to do, it's going to break and it's
going to snap back and hurt my fingers. When
it snaps back, that is the energy that is
released from an earthquake. Or it's an analogy
for the energy that's released from an earthquake.
What kind of energy is released? There are
seismic waves that are released, and there
are three basic types of waves. There's the
P, or the primary, waves. This is the fast
wave, and it's a push-pull kind of wave. The
second kind of wave is the S, or shear, wave.
It's like a sidewinder snake moving across
sand. And the third type of wave is the surface
wave, which is generally the damaging wave.
So how do we detect earthquakes? You use a
seismograph, which measures ground shaking.
How do you locate the epicenter of an earthquake?
What do you need? Well, you need to determine
how far you are from a quake based on the
time difference between the P and the S wave.
Then you need three seismograph stations to
triangulate and locate the earthquake epicenter.
I would like you to learn about earthquake
waves and how they can provide information
on the structure of the earth by examining
your class notes and the textbook that we
have online.
We'll now talk about earthquake intensity
and magnitude scales. There are two scales
that we commonly see when we talk about earthquakes
and the severity of those earthquakes. The
first is the Richter scale, and it measures
the energy released. It's a scale from 1 to
10, and 8 is a very big earthquake. And what's
important is that an 8 on the Richter scale
is 10 times as strong as a 7 on the Richter
scale. Another scale is the Modified Mercalli
Scale, and it measures how people and buildings
are affected by earthquakes. For example,
a 4 on this scale is that most people will
feel it indoors, and dishes, windows, and
doors will rattle. A 10 on this scale is most
buildings and their foundations are destroyed.
Some bridges are destroyed. So it's based
on how it affects us and buildings.
Another important topic with earthquakes is
prediction of quakes. And I'll start off by
saying there is no foolproof way to predict
when an earthquake is going to occur. There
are various methods that have been used in
the past to predict earthquakes. For example,
in China, they use the behavior of animals.
In some cases they've been able to predict
large earthquakes. In other cases they were
not, and thousands to hundreds of thousands
of people died. Other kinds of things you
can look at to predict earthquakes or use
to predict earthquakes include elevation changes,
magnetic field changes, and seismic gap ideas.
The idea of a seismic gap is the following.
If there's a recent quake along one segment,
then stress has been relieved along that segment.
So it is not likely there will be another
quake in the near future. If there has not
been a recent quake and stress is building
up, then perhaps an earthquake is likely.
In California they rank the land by earthquake
hazard maps or risk maps, and there are certain
types of rocks that are better to live on
than other types of rocks. For example, granite
is a good rock to build a house on. Loose
sand or mud is not a good place to build a
house or a building.
If we think about distribution of quakes,
where do they occur on the earth, well, they're
very common at subduction zones where one
plate is going underneath another. And you
can get very deep earthquakes there, up to
700 kilometers deep. You also get earthquakes
along transform fault margins, like the San
Andreas, where two plates are slipping by
each other. We also have earthquakes in Oklahoma.
Have you felt one of those recently? There
are several reasons for these earthquakes.
One hypothesis is that they're natural. Another
is they're caused by hydro-fracking in the
oil business, and yes, hydro-fracturing can
cause generally small earthquakes. Injection
is another way to cause earthquakes. When
oil and gas are produced, you also produce
water, and that water then is generally put
back into the ground. If that water comes
in contact with a fault, it can lubricate
the fault. And if there's stress on that fault,
what could happen? You reduce the friction.
You have a fault. You could get movement,
and you could get an earthquake. So it is
not a good idea to put injection wells around
faults.
