Why so many earthquakes lately

On January 12th 2010, the small Caribbean nation the Republic of Haiti was rocked by a 7.0 magnitude earthquake. It resulted in an estimated 250 000 fatalities and focused much of the world’s media on the small island nation. A little over a month later, Chile was struck by a huge 8.8 magnitude earthquake, at first giving rise to fears of a tsunami in the Pacific, but later downplayed. The quake resulted in a reported 250 or more deaths. Indeed, every month this year has seen at least one earthquake in the region of magnitude 7.0. The increased media attention has prompted people to ponder the question: why have there been so many earthquakes recently?

One reason for the apparent increase is in our increased use of technology to detect and record earthquakes. Both the United States and Europe now have sophisticated global stations in place to record even the faintest of earth tremors. The European-Mediterranean Seismological Centre detected 52 earthquakes in the vicinity of the Mediterranean Sea on 17th June 2010. Likewise, the National Earthquake Information Centre, a part of the US Geological Survey, now detects between 12 000 and 14 000 earthquakes annually. So clearly, our ability to detect earthquakes has increased many fold.

Similarly, the sheer number of detecting stations has increased dramatically. In 1931, there were around 350 station worldwide; today, there are in excess of 4000 stations. The US Geological Survey indicates that there have been, in fact, fewer high magnitude earthquakes than expected in recent years. In any given year, long-term measurements suggest an average of 18 major (magnitude 7.0 to 7.9) earthquakes and one great (magnitude 8.0 or higher) quake. Few years in the last 25 have exceeded this expected figure, and in no year in the 21st century has this average figure been met; only in 1992, 1995, 1996 and 1997 was this figure reached or exceeded.

Another reason for the apparent increase in quakes may be that increasing global populations put more people in known quake regions. Greater populations in quake zones leads to greater human cost, which, in turn, assures greater media coverage. At the time of writing, the devastating Haiti quake is only the equal sixth largest quake by magnitude for the year 2010. The other five, of respective magnitudes of 8.8, 7.8, 7.7, 7.2 and 7.1, resulted in far fewer, and in most cases no, reported fatalities and consequently did not make the front pages of the newspapers or new web sites – most probably went unnoticed by all bar those directly involved.

Statistical analysis indicates that earthquakes generally occur in clusters. Again, when clusters occur they are often reported in the media and gain widespread attention. Similarly, sequences of small events, often causally linked, are common in some places, particularly geothermal regions; also, larger quakes can often trigger secondary quakes. When the opposite occurs, and we experience a period of reduced global seismic activity, are we all to sit back and ponder: hey, why no catastrophic earthquakes lately?