THE EPICENTER MYTH

There are plenty of misconceptions regarding earthquakes.

I remember a song by a group called 100 Flowers titled “California’s Falling Into the Ocean”.  There were movies in which the ground opened up in an earthquake and people fell in, I assumed all the way to the center of the Earth! 

These days, as more reasonable information comes in, people don’t seriously believe such nonsense.  Do they?

Still, there is plenty of nonsense to go with the new sense that arrives seemingly daily for the world of earthquake research.

For example, here are some common areas where many people may have the facts a bit mixed up:

1. Do not get into a doorway in an earthquake—get under a table and hold on if possible
2. There is no such thing as “The Triangle of Life” area next to your bed.  In an earthquake, stay in bed and cover your head with a pillow
3. In a major earthquake, there is nothing in your home that is too heavy be moved, sometimes to a dangerous degree, by earthquake ground motion.
4. If you want to count on outside help to arrive within 3 days of an earthquake, then a 3 day kit is all you need.  Katrina and the recent Japan earthquake indicate a 3 day kit may not provide enough provisions.

And there is this one:

1. The damage is always the worst at the epicenter.

While this can be the case, it isn’t always true.  The term “epicenter” simply refers to the focus, or origin of the break in an earthquake.  In the giant Chile mega-thrust quake last year, the rupture occurred in fits and starts in multiple areas over a huge surface area, dozens of miles wide by hundreds of miles long.  We think of earthquake breaks as occurring along a fault “line”.  Not necessarily so in these Subduction zone events, where giant masses of land slip all at once.  One USGS scientist indicated that the Chile quake had six epicenters!

The location of the most significant damage in an earthquake is called the “meizoseismal” area.  This is where things are the worst.  In earthquakes such as Loma Prieta and Northridge, the S Waves, or secondary waves, bounce all over the place, off of hills and mountains and back upon themselves in a reverberation that can last much longer than the shaking at the origin of the break.  Liquefaction, or the liquefying of the ground, can occur where the ground lowers and/or shakes to a degree that the water table rises, and buildings sink into the dirt.  This causes tremendous damage in quakes such as the recent Christchurch earthquake, where a 6.3 earthquake shook with enormous force for just a few seconds (9-10) and big parts of the city suddenly became repositioned below the water table.  These factors, along with others such as building construction quality, dictate the degree of damage much more that where the fault break originated.

So just as an earthquake’s magnitude does not necessarily have a relation to the force of shaking, the epicenter does not always have a relation to the degree of damage.