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This is part of the messy business of breaking up a plate. … This is a geologic process

will take millions of years to form a new plate boundary

likely, it will take thousands of similar large quakes for that to happen."

All four faults

were strike-slip faults, meaning ground on one side of the fault moves horizontally past ground on the other side

great quake of last April 11 "is possibly the largest strike-slip earthquake ever seismically recorded

although a similar size quake in Tibet in 1950 was of an unknown type

2012 quakes likely were triggered, at least in part, by changes in crustal stresses caused by the magnitude-9.1 Sumatra-Andaman earthquake of Dec. 26, 2004

the 8.7 and 8.2 quakes were generated by horizontal movements

not by vertical motion

explains why they didn't generate major tsunamis

8.7 quake caused small tsunamis, the largest of which measured about 12 inches in height at Meulaboh, Indonesia

Indo-Australian plate is breaking into two or perhaps three pieces

happening because it is colliding with Asia in the northwest, which slows down the western part of the plate, while the eastern part of the plate continues moving more easily by diving or "subducting" under the island of Sumatra to the northeast

subduction zone off Sumatra caused the catastrophic 2004 magnitude-9.1 quake and tsunami

ruptured along a roughly 90-mile length

seafloor on one side of the fault slipped about 100 feet past the seafloor on the fault's other side

second fault, which slipped about 25 feet, began to rupture 40 seconds after

extended an estimated 60 miles to 120 miles north-northeast to south-southwest – perpendicular to the first fault and crossing it.

third fault was parallel to the first fault and about 90 to the miles southwest

started breaking 70 seconds after the quake began

along a length of about 90 miles

slipped about 70 feet

The fourth fault paralleled the first and third faults

began to rupture 145 seconds after the quake began

fault rupture was roughly 30 miles to 60 miles long.

fault slipped about 20 feet past ground on the other side

Most giant quakes don’t trigger temblors so far away

The biggest

was a magnitude 7 in Baja California, about 22 hours afterward

the triggered quakes are well below magnitude 5

The difference, Pollitz says, lay in the strike-slip nature of the April 11 quakes

fault geometry allows the stress of a crustal movement to propagate much farther across the planet’s surface

inferred the presence of these seismic stresses by modeling stress changes from shortly before the 2012 earthquakes

magnitude-9.1 tremor in 2004

and another quake in 2005 — probably triggered the 2012 event by adding to pent-up stresses in the plate’s middle region.