People who live anywhere along the Earth’s tectonic plate boundaries (the so-called “ring of fire” know that at any time  “the big one” – a really, really large, damaging earthquake – could happen. The largest earthquake in recorded history was a 9.5 event along the coast of Chile in 1960. The second largest was a 9.2 along the southern coast of Alaska in 1964. Since January, 1700, only six events have occurred that were 9.0 or larger – one of them as recently as 2004 in Sumatra, Indonesia.

An earthquake measuring 6.9 on the Moment-magnitude scale has struck southern Qinghai,
China, near the city of Yushu in the Tibet Autonomous Region.  Using the Hazard Viewer tool
on this web site, together with the Recent Earthquakes display, it can be determined that the ground
shaking resulting from the earthquake in Yushu was between 8% g and 12% g, equivalent
to Modified Mercalli Intensity V to VI.  Shaking of this intensity is enough to collapse some buildings
constructed of unreinforced masonry, as might be common in the region.

Aftershocks are earthquakes that are triggered by a previous, larger earthquake mainshock.  Today we know that aftershocks are a result of the transfer of stress from the region of the mainshock to surrounding, smaller faults.   About 5%-10% of mainshocks have foreshocks, which can be regarded as instances when the aftershock is larger than the mainshock.

Haiti, January 12, 2010, magnitude 7.0.  Chile, February 27, 2010, magnitude 8.8.  Baja, April 4, 2010, magnitude 7.0.  Sumatra, April 6, 2010, magnitude 7.7.   Are they related?  Although it is rather unlikely, the simple answer is that no one knows for sure, because it is a very difficult proposition to unequivocally prove. 

About 40 years ago a Japanese seismologist by the name of Kiyoo Mogi proposed the idea that before a large earthquake occurs, there are a series of earthquakes of increasing magnitude that occur as a "run-up" to the main event.  These events surround the eventual fault that breaks in the main earthquake in a kind of "donut" pattern, with the main earthquake then occurring in the "hole" of the "donut".  Since that time, seismologists call this type of precursory pattern a "Mogi Donut".