Update: January 3rd, 2011 – A final update on 2010 numbers posted right here.
Damage in Santiago, Chile. Photo by Reuters/Marco Fredes
After the massive earthquake this past weekend in Chile, MSNBC published a sensationalistic piece entitled, “Is nature out of control?” The Wall Street Journal asked if three massive earthquakes around the world in two months are related and a cause for alarm. The mainstream media, always searching for sensationalistic or fear mongering news, has latched onto the question; are we seeing more earthquakes than normal?
Well, not really.
To better understand why, let’s take a look at how many earthquakes occur each year on average. The USGS has a fascinating page of earthquake facts and statistics, with the following table:
|8 and higher||1 ¹|
|7 – 7.9||17 ²|
|6 – 6.9||134 ²|
|5 – 5.9||1319 ²|
|4 – 4.9|| 13,000
|3 – 3.9|| 130,000
|2 – 2.9|| 1,300,000
¹ Based on observations since 1900.
² Based on observations since 1990.
For our analysis, let’s take earthquakes based in the magnitude 6.0 – 6.9 range. Why am I picking earthquakes in the M6 range? It’s arbitrary. You can repeat this process for earthquakes of any range. Based on data recorded since 1990, we’d expect to see an earthquake within this magnitude range occur every 2.7 days or so.
So here we are, on March 1st, 2010, the 60th day of the year. How many earthquakes in the M6.0 – M6.9 range have we had this year? According to this handy search tool from the USGS, there have been 25 earthquakes of M6.0 – M6.9 in 2010.
That works out to roughly one earthquake in the magnitude 6 range every 2.4 days. That doesn’t seem totally unreasonable or a reason for alarm, but we should do some further work to put it in context.
Total results: 21
Mean (average): 2.67143
Standard deviation: 0.41732
So, the number of magnitude 6 earthquakes that we’ve had in 2010 falls within one standard deviation of the mean. If we were to plot up a graph, it’d look like this. The error bars represent one standard deviation.
Awesome! Well, what about those ranges of values that fall outside of one standard deviation from the mean? For those that don’t understand how statistics works, check out the following bell curve from Wikipedia.
This shows roughly the percentage of values that you’d expect to fall within a specific standard deviation away from the mean value.
Dark blue is less than one standard deviation from the mean. For the normal distribution, this accounts for about 68% of the set (dark blue), while two standard deviations from the mean (medium and dark blue) account for about 95%, and three standard deviations (light, medium, and dark blue) account for about 99.7%.
So, if we modify our graph to show an error bar of 2 standard deviations, you’ll notice that every result since 1990 fits inside this model! Statistically speaking, you would expect to find 95% of all results falling within two standard deviations of your average. Simply put, there is absolutely nothing strange happening.
In fact, thanks to this normal curve you can basically predict, with a 99.7% chance of success (three standard deviations), that an earthquake of equal to or greater than M6.0 will occur somewhere around the world within the next 3.5 days. Update: Proven correct! A M6.4 earthquake occurred in Taiwan on March 3rd.
Alright, so what’s with all the coverage on earthquakes? It sure seems like a lot is happening, right? We can attribute this to observer bias. The massive devastation in Haiti warranted a large amount of news coverage. Because this is so fresh in everyone’s mind, people are more likely to notice any news or information related to earthquakes anywhere in the world. An earthquake of M6.0 or greater, usually garners international attention.
It’s the same principle that happens whenever you acquire some new toy, gadget, or piece of clothing. Suddenly, you notice that particular item around all the time. It’s like everyone has it.
So, bottom line, the Earth isn’t becoming more active, more dangerous, or even “out of control.” Despite the fear mongering and what esteemed mainstream media networks would have you believe, the simple reality is that the numbers prove things are happening at an expected rate. Keep that in mind the next time a large earthquake happens and everyone is wondering why the Earth seems so active!
Update (April 21, 2010): Chris Rowan at Highly Allochthonous has a great post on yearly earthquake averages and variability with larger magnitudes.
In the last 28 years, there have been on average around 13 such ‘significant’ earthquakes a year, with a magnitude 8 occuring about every year and a half. This average rate is marked by the grey line on the plot: if we extraplolate the six major earthquakes recorded in the first four months, 2010 is on course to experience 18 major earthquakes, a little above average but well within the variability shown by the whole dataset (and it’s actually closer to the centennial average of 16 major quakes a year reported by the USGS above).