A 7.1 magnitude earthquake struck Western China this morning, killing over 400 people and injuring at least 10,000. The quake is the latest in a string of fatal tremblers this year: Haiti in January, Chile in February, Taiwan in March, and Mexico just last week. It may be easy to suspect some sort of connection among the disasters, or to wonder if we're experiencing more frequent or powerful quakes than we have before.
Seismologists, however, are quick to dispel such presumptions. Since 1900, the U.S. Geological Survey has recorded a yearly average of nearly 1.5 million earthquakes. The large majority are not felt, but the number of quakes that reach a magnitude of 7.0 or greater has stayed relatively steady -- if anything, it's decreased. The USGS expects about 18 major earthquakes (7.0-7.9) and one great earthquake (8.0 or higher) per year. Since 1971, we have only reached or exceeded this expectation four times. Though we've seen four serious earthquakes already this year, only Chile measured 8.0 or above.
In a Q&A with Washington Post readers, USGS geophysicist Michael Blanpied noted that quakes that would previously have gone unnoticed are now picked up by better seismic technology and publicized through internet news reports. Big quakes are also likely to affect larger numbers of people than they used to thanks to population growth and urban development.
Earthquakes may not be happening more often, but scientists are researching whether close timing between quakes might indicate some sort of interconnection. Statistics show that earthquakes frequently occur in clusters. Blanpied cites two related Turkish quakes in 1999 to demonstrate the concept of "earthquake triggering," when the stress of one quake affects nearby faults. But the jury is out on whether more distant earthquakes -- Chile and China, say -- might be related.
Some geologists are also exploring a potential link between climate change and increased seismic activity. In 2006, Sharon Begley explained how melting glaciers could trigger earthquakes and volcanoes:
"One cubic meter of ice weighs just over a ton, and glaciers can be hundreds of meters thick. When they melt and the water runs off, it is literally a weight off Earth's crust. The crust and mantle therefore bounce back, immediately as well as over thousands of years. That 'isostatic rebound,' according to studies of prehistoric and recent earthquakes and volcanoes, can make the planet's seismic plates slip catastrophically."