Name: Ms. Kometer's 4th grade class
Age: 4th Grade
We are a a 4th grade class at Hunting Ridge School in
Pallatine, Illinois. We are currently studying earthquakes
and have these questions. Can you give us an answer.
You are a very nice scientist. We really appreciate your
What causes earthquakes in the first place?
Why is the epicenter of an earthquake always over the focus?
What was the latest earthquake in California that was greater
than 6 on the Richter scale?
Ms. Kometer's 4th grade class
Earthquakes are caused by the movement of the Continental Plates as
they shift from place to place. They are totally natural, in that
they have been occuring since the beginning of time on Earth. To
expand on this: as two Plates come into contact, one slides up over
the other (the other dives down underneath the first), and enormous
pressures and forces are created. These forces must be relieved in
some way, so the easiest is through the Earths crust. I'm not
sure how far these plates extend down in to the Earth, but it's not
far compared to the radius of the planet.
The Epicenter is usually over the focus, because that is where the two
plates initially impact. The two words are basically synonimous.
I'm not sure what the last Earthquake in California rated. You may have to
check some climatological data source for that.
Expanding a bit on the previous response:
The upper layer of the Earth is broken up into
a number of plates. Imagine cracking an hard-boiled
egg, for example. The outer layer of the Earth is
much like the broken shell of the egg. Unlike the
egg shell, the Earth's plates are in constant motion
with respect to one another. Sometimes the plates
crash together. As described in the previous response,
when this happens one plate often is forced under
another plate. This is happening, for example,
along the coast of Alaska and Japan.
Sometimes the plates slide past each other. This is
happening in California where the North American plate
and the Pacific Ocean plate slide past each other,
causing the earthquakes that plague the state. Sometimes
the plates pull away from each other. This usually
occurs in the middle of the Ocean and is happening
in the middle of the Atlantic Ocean where the North
and South American plates are pulling away from Africa
The last earthquake greater than magnitude 6 to occur
in California was the January 17, Northridge earthquake
(magnitude 6.7) that you probably saw on the news.
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Update: June 2012