Earthquakes and tsunamis, such as the powerful quake that occurred today in the South Pacific and the resulting wave it generated, can often go hand-in-hand. Why does one earthquake generate huge waves while another does not?

Earthquakes that occur under the seafloor can generate tsunamis when chunks of the planet's crust separate. One slab lifting rapidly acts as a giant paddle — thus the term "the paddle effect." This often occurs around two types of faults in the planetary crust. One is called a subduction zone, such as the area where the India continental plate slides beneath the Burma plate. This area is also called a thrust, or dip-slip, fault.

"A thrust fault, where one side goes up and one side goes down, is the perfect type for a tsunami," Jian Lin of the Woods Hole Oceanographic Institution told LiveScience in a telephone interview.

Another type of fault, called strike-slip, involves two plates sliding horizontally with respect to each other. This kind of motion does not generally cause the kind of water displacement, or paddle effect, necessary to create a tsunami.

Not all quakes are the same, and sometimes the paddle effect is minimal or nonexistent if the slabs move laterally instead of up and down, or if the origin of the quake is too deep beneath the seafloor. "Shallower events will have a huge effect," explains Lin.

Despite this knowledge of undersea quakes, the resulting waves are hard to predict, for several reasons. Nobody knows how a quake has affected the seafloor until hours, days or even months after the event. And a tsunami is almost imperceptible on the open ocean, rising to full ferocity only as it nears the shore. Plus, buoys that measure the tsunamis are sparse, often thousands of miles apart.