Silent surprise: unveiling the secrets of a breaking-record earthquake
On April 11, 2012 a massive submarine earthquake ruptured the Indian Ocean floor. Despite public unawareness, it shook seismologists. This silent creature made it to the top ten largest events ever measured and broke three long-standing world records: fastest oceanic rupture, largest strike-slip earthquake –where blocks at both sides of the fault move horizontally, as in the San Andreas fault in California–, and largest intraplate event –those occurring inside the rigid plates that tile the outer shell of the Earth, instead of at the plate boundaries, where most earthquakes happen–. Seven recent articles in Science, Nature, and Geophysical Research Letters unveil some of its secrets.
Seismologists link this ocean rupture, which ripped the Indoaustralian Plate, with the infamous Sumatra 2004 earthquake. While Australia is moving North underneath Indonesia, as the 2004 megaquake demonstrated, India is jammed against Eurasia at the Himalayas. The April earthquake revealed that because India resists the traction of Australia, the area between them is tearing, creating a new plate boundary (1, 2).
The forces originated by the 2004 event boosted the rupture of preexisting faults favouredly oriented: the earthquake on April broke two fossil fractures aligned with those forces (3, 4). This seems reasonable, because reactivating weakened areas, such as old fractures, is easier than creating new faults. But the earthquake also involved two more faults perpendicular to the others, which means the rupture branched twice into the less favorable direction. Earthquakes rarely display this complex behavior; intriguingly, one of the few previously documented exceptions occurred in this region. To complicate matters, the earthquake ruptured at a fast speed never observed before under the sea, which contradicts the supposed simplicity of oceanic ruptures (5).
And surprises continue: worldwide seismicity rocketed after the mainshock (the global earthquake rate increased five times) and aftershocks took place as far as 20,000 km away from the Indian Ocean (6). This remotely triggered activity lasted for one week and generated significant aftershocks: a longer and larger effect than any observed for a comparable mainshock. The particular fault geometry radiated energy more widely than other configurations, which may explain the unusual peak. But the low global seismicity preceding the earthquake –implying many faults elsewhere were close to failure– could also account for this effect: the energy radiated by the earthquake made all those faults burst.
This rare earthquake happened in a remote oceanic area, causing no fatalities. Experts now wonder whether a similar huge earthquake may strike within a continental plate, such as in the central US, posing a high hazard to populations therein (7). Examples like the unexpected 2008 China earthquake –which killed 80,000 people in a region previously labeled as «moderately quiet» for earthquakes– urge seismologists to untangle the poorly known intraplate ruptures.
Publicado el diciembre 17, 2012 en Geociencias, Geofísica, Geología, Peligros naturales, Sismología y etiquetado en earthquake, natural hazards, terremoto. Guarda el enlace permanente. Deja un comentario.