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Name: Ms. Kometer's 4th grade class
Status: N/A
Age: 4th Grade
Location: IL
Country: N/A
Date: 1999


Question:
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 help.

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?

Thanks again.
Ms. Kometer's 4th grade class


Replies:
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.

Good question!

wildman jackson


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 and Europe.

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.

grant l



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