Weird 2012 Quake Signals Tectonic Plate Birth

On the afternoon of April 11, 2012, one of the most powerful earthquakes ever recorded — and now revealed to be among the weirdest — struck in the Indian Ocean, off the coast of Sumatra. It’s a region all too familiar with geological catastrophe.

Eight years earlier, in December 2004, the third largest earthquake on record had ripped through a nearby region of the ocean floor. The magnitude-9.1 earthquake and the monstrous tsunami that soon followed killed more than 227,000 people in 14 countries,

So when a magnitude-8.7 earthquake (some put the magnitude at 8.6) shook the Indonesian island on that Wednesday afternoon earlier this year, many expected the worst. Yet, no monster wave appeared. A wave did come ashore, but it was a miniature tsunami, just 12 inches (31 centimeters) high.All told, the earthquake did very little damage — yet only five higher earthquake magnitudes have ever been recorded. So what was the deal? [Video: What Earthquake 'Magnitude' Means?]

New research published today (Sept. 26) in the journal Nature delves into the intimate details of this earthquake, along with the powerful, magnitude-8.2 quake that followed two hours later. The new studies add to an existing body of research that shows this was a remarkable event — one of the most surprising earthquakes ever recorded — and one that offers an unlikely snapshot of a geological process millions of years in the making.

Turning a corner

Data captured by a global network of seismometers on April 11 revealed almost immediately that this quake was a strike-slip earthquake — the sort that races along the San Andreas Fault. Strike-slip earthquakes occur when two sides of a fault jolt horizontally, displacing the ground sideways. Since these earthquakes don’t shove the ocean floor upward — a required move for tsunami generation — no deadly wave appeared. [April 2012 Sumatra Quake (Infographic)]

Tsunamis are typically the devastating handiwork of quakes known as subduction earthquakes. They’re the most powerful earthquakes on the planet, and they occur at plate boundaries, where one tectonic plate is grinding inexorably beneath another. When the bottom plate suddenly lurches deeper, a colossal amount of energy is released, unleashing the sorts of massive earthquakes and calamitous tsunamis that hit the Indian Ocean in 2004 and the coast of Japan in March 2011. [7 Craziest Ways Japan's Earthquake Affected Earth]

It quickly became apparent that the April 11 earthquake was the most powerful strike-slip quake ever recorded. Which was strange.

Not only was the quake of unparalleled power, it hit in the middle of a tectonic plate, not at a plate boundary, like the San Andreas Fault. “So already it has two unusual attributes,” said Thorne Lay, a professor of earth and planetary sciences at the University of California, Santa Cruz, and an author on one of the papers published today.

Lay and his team set out to construct a blow-by-blow account of how the earthquake progressed, and what they found added to the quake’s mystique. This earthquake was able to turn corners.

“It’s totally unusual,” Lay told OurAmazingPlanet. It turns out that the quake began on one fault, streaking along at more than a mile per second, and, when it reached an intersecting fault, it ruptured that one, too. In all, it ruptured four different faults over the course of 150 seconds, unleashing an amount of energy equivalent to about four magnitude-8.0 earthquakes.

Lay said that, typically, when earthquakes spread to connecting faults, the rupture rips along faults that branch away from the initial fault like the branches of a river. These earthquakes raced along in a grid-like pattern, making 90-degree turns along faults that resemble a lattice.

“Here, they really do seem to go along perpendicular faults, and we haven’t seen anything like that with a big earthquake,” he said.

However, he said, the weird rupture pattern reflects the weird geological circumstances at play in the neighborhood where the earthquake hit. The April 11 earthquake occurred in a region that is giving birth to the Earth’s newest tectonic plate.

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