Earthquakes 101: How they happen

The horrifying images of destruction out of Japan make the scope of Friday's disaster all too clear. A magnitude 8.9 earthquake and resulting tsunami left devastating damage.
Why and how do such temblors happen?
It all has to do with plates that make up the Earth's crust moving around, seismologist James Gaherty, a Lamont associate research professor at Columbia University explained to "Early Show on Saturday Morning" co-anchor Rebecca Jarvis.
"Most earthquakes occur on the boundaries of the very large tectonic plates that make up the outer rigid crust of the earth," Gaherty said. "These plates are all shifting around relative to each other, in many places moving fairly rapidly, inches per year relative to each other, and they push against each other, some places going underneath, other places rubbing past each other. So, the western part of the Pacific Ocean, for example, the 'Ring of Fire' (earthquake hotbed along the Pacific Rim) -- that all takes place on these tectonic boundaries. That's where we get these earthquakes.
"In this part of Japan, basically, the Pacific Plate is trying to move underneath the Earth's crust where Japan sits. ... It's moving down underneath, constantly building up pressure as it tries to move underneath and, in this case, it releases that pressure, and these very large earthquakes occur in a very large area along the entire length of the coastline of Japan ... on the order of 200 miles along the length and 100 miles offshore, all sliding on one large fault at the same time."
The pace of mega-quakes seems to be picking up, Gaherty continued. "These kinds of events are very well-understood in Japan. The fact that they have large events on the order of magnitude 8 is something they've had many times over their history. This one is a little bit unusual in that we're not necessarily expecting something quite as large as this. These mega-quakes, more like a magnitude 9, are very rare, even over geologic history looking back. We have a hard time finding evidence of them. We've observed now three, really, in the last six years, since Sumatra. So we seem to be in a period of very active occurrence of these. But how the really big quakes develop is something that we're really trying to understand."
Why such an active period now? "It's probably just random statistics. ... There were events in 1960, 1964, that are about this magnitude. But then, there was kind of a quiet period of about 40 years. We've now entered a period where this kind of activity has (increased) again."
The Ring of Fire is as active as it is, Gaherty said, due to "those tectonic plates and how they're moving relative to each other. The Pacific Plate happens to be a large, coherent plate that's moving at a fairly high velocity relative to the other plates around it. So it's continually interacting with it. On the Western side of that system, in Japan ... it's pushing underneath. Those are the kind of earthquakes that tend to be the largest, and also tend to be the kind of earthquakes that are pushing, moving material up and down that cause tsunamis. That's one of the reasons why it's such a destructive part of the system."
And the Earth's crust isn't finished with Japan this time around, either. "There are going to continue to be large aftershocks of this earthquake," Gaherty said. "We would expect, for typically a rule of thumb, for the largest aftershock after a big earthquake like this, it's about one magnitude unit smaller. A magnitude 8 earthquake can still cause a significant tsunami. Certainly not of the devastating level of this one. But still something to keep an eye on.
"These things propagate out very efficiently from the earthquake. ... The energy travels very efficiently in the water, and so we do tend to see these spread out over the entire basin of the Pacific, and they can affect regions very far from the event."