The Earth Holds Steady

By Roger Musson, a seismologist with the British Geological Survey (THE NEW YORK TIMES, 11/04/10):

Are earthquakes becoming more frequent? This is a question that every seismologist is used to. I was asked it 30 years ago. Thanks to large quakes in Haiti and Chile — not to mention 7-plus magnitude quakes in Indonesia and Baja California over the past week — I’ve been asked it a lot lately. And the answer is no. You would think this would be good news, but sometimes people seem faintly disappointed when they hear it. It’s as if a dose of disaster makes life more interesting.

It’s true that more earthquakes are recorded than used to be the case, but that’s simply because there are more monitoring stations that are able to pick up minor earthquakes that once went undetected. If we compare the average global rates of large earthquakes, we find that these are stable as far back as we can trace them. On average, we record an earthquake with a magnitude over 6 every three days or so, and over 7 at least once a month.

Why then, does it sometimes seem they are more common occurrences? There are two reasons for this. First, people notice it when earthquakes happen in populated places. A big earthquake in California is news; a big earthquake in the Southern Ocean is noticed only by seismologists. So a run of earthquakes that by chance hit populated places makes it look as though the rate has increased, even if it hasn’t.

The classic case of this was in 1976. That year, there were a number of high-casualty earthquakes — including a 7.5 magnitude quake in Tangshan, China, that killed more than a quarter of a million people — prompting a lot of news media questions about the increasing frequency of earthquakes. But, in the end, 1976 turned out to have a relatively low number of quakes. It was just that an abnormal number hit populated areas.

The second reason is that in any semi-random process, you get clustering. Throw enough dice, and sometimes you’ll get several sixes in a row. People notice the clusters; they don’t notice the gaps in between. No one ever asks me during the quiet periods if earthquakes are becoming less frequent. Also, people tend to have short memories; they notice the current cluster, but don’t remember the previous one.

Basic geology explains why the number of earthquakes remains relatively constant. Quakes release a lot of energy, and that energy has to come from somewhere. Ultimately, the source of it is heat released by the steady decay of radioactive material deep inside the earth. For a real long-term increase in earthquake activity, there would have to be an increase in that energy supply, and it’s hard to see how that could happen.

One problem that we do have to face is that our exposure to earthquakes is increasing. As the world becomes more populated and cities grow ever bigger, the potential for quakes to become disasters rises. Tehran, for instance, has been destroyed by earthquakes several times, but it was still quite small at the time of its last damaging quake, in 1830. Now the city is home to millions, and when the next major quake hits, the results will be catastrophic.

Unless we devote more effort to protecting communities, the number of earthquake disasters will grow, even if the number of earthquakes stays the same.

Fuente: Bitácora Almendrón. Tribuna Libre © Miguel Moliné Escalona