Tuesday, October 4, 2011

California has the Biggest and Mostest Quakes in the World! True or False? What Everyone "Knows" About Earthquakes

Art by Zeo
I guess I should point out that although my public lecture last week on earthquakes was done in a somewhat light-hearted manner, befitting an audience that included quite a few children, I also pointed out the deadly serious nature of disastrous earthquakes. So please don't think that the comic art above is belittling the loss of life and property in horrible events like that which struck Japan this year. It is actually a critique of the way that news is reported in American media, especially cable news (I'm talking about you, CNN and FOX), and local news outlets.

The question of the day on our little quiz on what everybody "knows" about earthquakes:

California has the biggest and the mostest earthquakes in the world. True or False?

I know that "mostest" is not really a word, but it somehow seems to fit well in the question of the day (a little like Stephen Colbert's 'truthiness'). So, what is the answer? My California audience was a little divided on this one, but mostly fell into the "false" camp.

So let's check on some data. If you look at the maps in the previous post, you can see that California certainly has a great many earthquakes, perhaps 10,000-15,000 year, and some of them are rather considerable.

There have been three earthquakes in the last 150 years that are thought to have approached magnitude 8 in size: 1857 at Fort Tejon, 1872 at Lone Pine, and 1906 in San Francisco.
San Francisco earthquake of 1906
There have been about two dozen magnitude 7 quakes in the same time period, including last year's El Mayor-Cucapah quake in Baja (7.2), the 1999 Hector quake (7.1), and the 1992 Landers quake (7.3).
Fault scarp from 1992 Landers earthquake. Photo by Garry Hayes
There have been at least 50 quakes greater than magnitude 6.5, meaning that a potentially damaging quake occurs somewhere in California roughly every two or three years. The 1989 Loma Prieta quake and 1994 Northridge quakes caused damage measured in the tens of billions of dollars, and killed dozens of people.

Those are a lot of earthquakes. So let's take the "biggest" issue first. The quake in Japan that destroyed the nuclear power plant and produced the Pacific-wide tsunami was magnitude 9.0. The quake in Sumatra in 2004 that produced the horrific tsunami in the Indian Ocean was magnitude 9.1. So, California doesn't have the biggest earthquakes. Not even close (but check for the wild card issue at the end of this post).

There is a great deal of confusion about the nature of the magnitude scale for measuring earthquakes. It is not a 1-10 scale, for instance, even though literally all recorded earthquakes fall within that range. It is open-ended, and quakes of greater than magnitude 10 are technically possible, but not likely from terrestrial origin. It would take the impact of an asteroid miles across to cause a quake larger than about 9.5 on the magnitude scale.

The biggest confusion concerns the relative size of quakes. Magnitude is a measure of the energy released when an earthquake strikes. A magnitude 9 quake is not just a little bit bigger than a magnitude 8. It is exponentially larger, by a factor of 32.  To put it a different way, a magnitude 9 quake releases an amount of energy that is equivalent to 32 magnitude 8 earthquakes!

It gets worse: Since a 9 is 32 times more than an 8, and an 8 is 32 times bigger than a 7, a magnitude 9 quake is more than 1000 times larger than a 7 (32x32).

A magnitude 9 quake is the energy equivalent of more than 1,000 magnitude 7 earthquakes...

Put another way: the Japan 9.0 quake released more energy than all of California's historical earthquakes combined.

The story is pretty much the same with the total number of earthquakes. California has a great many smaller quakes, but just about any subduction zone around the world has more. Even in the United States, Alaska has more earthquakes than California (as well as the second-largest earthquake ever recorded in the world, the 9.3 magnitude Good Friday quake of 1964).

So why does California get this reputation of having an inordinate number of earthquakes? I feel compelled to blame the way news is reported in this country. Cable and local news coverage of earthquakes is atrocious for the most part, with badly misinformed reporters and news-readers (I don't consider them to be news anchors or journalists anymore). There is a tendency to display blood and gore over actual conditions on the ground. The media will spend weeks talking about an earthquake in Los Angeles or San Francisco that kills a few people and practically ignore monumental tragedies in Pakistan or Iran where tens of thousands of people have died.

There is a beautifully done parody of practically every overblown news story ever seen on television on Onion News. The language is not safe for work, but imagine the reporters saying earthquake instead of b...sh.t and you get the idea of how it all works.

As mentioned above, there is one wild card in the possibility of large earthquakes hitting California. The state does in fact have a subduction zone that is active north of Fortuna and Eureka. It is part of the Cascadia subduction zone that threatens Oregon and Washington. There is good evidence that a magnitude 9 earthquake took place along the system in 1700, and there is no reason to think that the fault system is any less dangerous now.
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