|Comic art by Zeo|
But there is still that nagging statement that you hear all the time in California. It's today's true-false question:
In California, we are waiting for the BIG ONE! True or False??
When I was on stage at the lecture, I made sure to say "the BIG ONE" in capital letters. How do you say something in capital letters? You do it in a deep baritone voice with your hand cupping the microphone; it's very dramatic.
So what is the answer? Well, actually the question is wrong and requires a bit of modification. Let's try it again, and in that deep baritone voice:
In California, we are waiting for the BIG ONES!
There, that's better. If we can't predict earthquakes, then how can this statement be possible? Doesn't it require the ability to predict earthquakes? Yes, it does. So the statement must be false. But it is not. The answer is TRUE!
Predicting earthquakes is not possible in the short term, as in hours or days, but a great deal can be said about the probability of earthquakes in a particular area over a period of decades. We can do this on the basis of the historical and prehistorical record of earthquakes along a particular fault zone, and by analysis of building stresses and tilting over a large region, as revealed by GPS stations and other related technology Check out the methodology of the 2007 Working Group on California Earthquake Probabilities in this USGS PDF file.
The most recently published studies show that earthquakes of 6.7 magnitude or greater are a virtual certainty over the next 30 years. The chance for an even greater quake is ominous, exceeding 70% for a magnitude 7.0, and a lesser, but still significant chance of even larger events. In a way, one of these quakes has already occurred, even though it was just south of the state: the El Mayor-Cucapah quake this year in northern Baja California was a magnitude 7.2 event.
Another wild card is the possibility of a large quake along the Cascadia subduction zone in the northernmost part of the state. It was not included in the probabilities, but numerous magnitude 7 quakes have taken place in that region in recent decades.
The lesson to be drawn from these predictions is that damaging earthquakes have happened all over California, and they will continue to do so in the future. Some of these quakes will be major events, causing many deaths and massive damage over large areas. The probability forecasts have great value in the sense that they are a warning to residents across the the state to be prepared at all times for earthquakes. We cannot know the precise moment that a large earthquake will strike, but we do know where they will happen, and how big they can be.