In 2004 a Magnitude 9.1 subduction earthquake triggered a tsunami that killed over 230,000 people.
Yet a nearby magnitude 8.7 earthquake in 2012, caused little damage and generated
minimal ocean waves. Although the earthquakes appeared similar in magnitude and were close
in proximity, they were caused by different tectonic processes related to the greater
Indo-Australian Plate. Two surprising elements of the latter earthquake include, that it
was both the largest intra-plate earthquake and the largest strike-slip earthquake ever
recorded. This north-northeast migrating plate is pushing
into Eurasia where an initial ocean-continent subduction zone became a continental collision
zone with the Asian continent slowing the northward movement of the plate. Southeast
of the Himalayan collision zone, the plate is diving beneath the Sunda plate. Lets look
at the 2004 earthquake in cartoon cross section. The locked plates are pushing eastward building
strain between the plates. The magnitude 9.1 earthquake occurred at the contact between
the plates as they released. The upward thrust generated the tsunami that swept the region.
The 2012 earthquake was not in the subduction zone but was 430 km west of the trench. So
what was that about? On April 11, 2012 data captured by a global network of seismometers
quickly revealed that this was a powerful strike-slip earthquake. Which was odd. Earthquakes
of this magnitude have only occurred in subduction zones.
To explain it we will simplify the setting. Although the combined motion of the Indo-Australian
Plate is north-northeast, varying deformation of the plate is due to both a distant response
to compression on the north where it is crushing into the Himalayas, and some stretching due
to slab pull where the oceanic plate is diving beneath the Sunda Plate.
Geodetic data and plate modeling have revealed differential plate motions across the plate.
Australia is moving northward at 5.6 cm per year, and India, pushing against the Himalayas
has slowed to 3.7 cm per year. This differential movement is resulting in the compression of
the plate near its center where it is slowly being squeezed. Although we are generalizing
these movements, the curvature of the Earth makes simple plate motion complicated, and
beyond the scope of this animation. The strain across this broad zone may eventually lead
to dividing this plate into two sub-plates, the Indian and Australian plates, along this
boundary.
Let’s look more closely at the actual earthquake. Earthquakes in this intra-plate zone are common.
The magnitude 8.7 event, was not a simple release on a long fault. Let’s watch what
happened. The initial mainshock rupture raced along at more than a kilometer a second expanding
bilaterally on a right=lateral strike-slip fault, That triggered a perpendicular rupture
on a left-lateral strike-slip fault, first to the south, then a second rupture along
the same line to the north. 150 km south of the initial rupture, a similar right lateral
fault was unleased. That triggering yet another westnorthwest-trending fault to rupture. Together
this event unleashed the equivalent of four magnitude 8 earthquakes. Adding them together
yielded a total event magnitude of 8.7, making it also one of the most complex earthquake
ever recorded. What caused this complex configuration at
the heart of the Indo Australian plate? Remember that we suggested that, according to recent
theories this area is being compressed as the northern half is being halted by the Eurasian
plate and the southern half is still pushing northward. This area is in the process of
becoming a future subduction zone where one plate, here selected as the Australian oceanic
part of the plate, will relent in the pushing contest and dive beneath the Indian plate,
eventually creating an island arc, similar to Sumatra and Japan. This process which began
millions of years ago, will take millions more to form the new boundary, releasing countless
earthquakes in the process.