Earthquakes 101

There seems to be some confusion about earthquakes, and our associated risks with them. It is easy to watch the news and see the devistation that takes place somewhere else in the world, and worry that it will happen in our own backyard. If you live on the west coast, namely southern california, this may be very true, but for those of us elsewhere this may or may not be the case.

How an earthquake happens:

Simplistically, an earthquake happens when two large rocks slide past each other. This suture between the two rocks is called a fault. Faults range in size from a meter, to hundereds even thousands of kilometers in legnth. What causes an earthquake? It is when pressure builds up between the rocks and something gives suddenly. The point at where this happens is called the hypocenter. Often, earthquakes happen deep underground. Today's earthquake in Chile took place 60 km under the earth. The term epicenter refers to the location on the surface abouve the hypocenter. Kinda like "x marks the spot", and the earthquake itself is the treasure.

Am I at risk for an earthquake?


Well, here is a earthquake hazard map for the US. You be the judge.

With regards to todays earthquake in Chile, and any potential hazard that me and my family has by living in Phoenix:

When an earthquake happens, there are 2 types of seismic waves generated. The first is called a P wave. it is compressional much like sound is going through air. This however travels through the earth and speeds much faster than sound though. the 2nd type and the dangerous type is called a S wave. It is like the kind of wave where you make a wave in a streched out rope or wire. It is dangerous because of it's shear. it moves from side to side, and that what causes the damage. S waves dissapate quickly (see the USGS shake map), the p waves traveled from chile to both AZ an NH in about 20 minutes, then reverberated inside the earth for about 4 hours. The seismogram reading is essentially like putting a microphone in the earth and listening to it grumble. This earthquake, because of its magnitude, was a loud grumble. There are quiet grumbles too, like mining explosions, or large trucks driving near the seismogram.

Hopefully this all helps.