Nearly two centuries ago, northeast Arkansas and southern Missouri were rocked by four earthquakes of magnitude 7 or greater. Buildings were leveled, the Mississippi experienced strong seiches that made it seem to be flowing upstream, and church bells clanged as far away as Boston. These four "New Madrid" quakes occurred within a period of less than two months, the first two just six hours apart.
The 1960 magnitude 9.5 Chile earthquake -- the largest ever recorded - -was followed a scant four years later by the second largest, the 1964 magnitude 9.3 Alaska earthquake. Both triggered tsunamis that spread across the Pacific.
Earthquakes are quick, their devastation enormous, the loss of life all too often overwhelming. When they strike back to back it is natural to wonder whether they are related. Did one cause the other? Has Earth changed, moving into an era of increased seismicity? Some people reach further for meaning in the destruction: Are the earthquakes a sign, a warning, a rebuke? Seismology, the science of earthquakes, has answers to these questions.
Large earthquakes are more common than you might think. In an average year, Earth is rolled by 20 quakes of magnitude 7 and greater. Many occur deep enough in the Earth that seismic waves reaching the surface do no damage. Others originate far from population centers and manage to pass largely unnoticed. But they happen. Frequently.
The numbers of earthquakes of different sizes follow an empirical law developed by two famous 20th century seismologists: Beno Gutenberg and Charles Richter (of Richter Scale fame). The law states that for every 200 magnitude 6 earthquakes -- about the number that occur worldwide in any given year -- there will be 20 magnitude 7s and two magnitude 8s. In light of this, it is entirely unremarkable that the Haiti and Chile earthquakes occurred so close together in time. In fact, there was another magnitude 7 earthquake south of Japan that took place just hours before Saturday's earthquake in Chile. With one or two earthquakes of magnitude 7 and above each month, we expect large earthquakes to occur within weeks or days of one another.
So to the question of whether the timing implies a causal relation between the earthquakes in Haiti and Chile, seismology answers no. But to the larger question of whether earthquakes can ever be related, there is more to consider.
Earthquakes occur when sudden motion on a fault releases stresses built up over tens to hundreds to even thousands of years. Aftershocks invariably follow, dotting the stretch of fault ruptured in the main earthquake. Their frequency dies down with time, and after a few months to a few years things are quiet again. But the clock continues ticking, the plates that cover Earth's surface keep moving, and stresses build anew.
While the main shock reduces stress on the fault locally, it can both raise and lower it nearby. Where it is increased, the earthquake clock is advanced, the crust is moved closer to failure and, on occasion, an earthquake is triggered. Seismologists believe that happened in the New Madrid earthquake sequence of 1811 and 1812. Damaging quakes along the Anatoly fault zone in Turkey, including the 1999 Izmit quake that claimed over 14,000 lives, also appear to be related in this way.
So yes, earthquakes can be related, but the influence is local, limited to nearby fault lines -- not the immense distances separating Haiti and Chile. And they can only hasten subsequent shocks. They can't create earthquakes where none would happen otherwise.
By constantly monitoring seismic activity and measuring the motion of the Earth's surface, scientists have developed a good understanding of where big earthquakes are most likely in the future. While specific predictions of time elude us, we can meaningfully assess earthquake hazards around which human infrastructure can be planned. Both recent large quakes were known hazards: GPS measurements of plate motions identified the fault zone in Haiti as capable of a magnitude 7 earthquake and a gap in historic seismicity testified to the threat in Chile.
Earthquakes are an unstoppable consequence of an active Earth. We well understand where and why they happen. When they do, their message for us should be one of humility in the face of Earth's enormous power, and of humanity in compassion and aid for those suffering their aftermath.
Justin Revenaugh is professor of seismology at the University of Minnesota.