“We knew they were going to be there. But we didn’t know how many we could find, their actual length or exactly where they would be located. We just didn’t know how beautiful they would be!” says Dr. Hélène Carton. For two days, R/V Falkor has traced past earthquakes in the depths of the Indian Ocean west of Sumatra. The trail is not subtle.
The scientific team designed the imaginary lines that guide the way based on previous surveys conducted in the area, so when normal faults began to emerge, it was not their presence what surprised them, but the quality of the data that appeared on their computer screens.
“You have to understand what a challenge this is. To see these scarps at such depths!” Dr. Carton continues, “Particularly with this level of resolution. Yes, some dramatic scarps had been found before at subduction zones, but at depths of about 2000 meters. We are working here at about 6km and the information we are getting is extremely detailed.”
Normal?
It would be a logical guess that the faults are named “normal” because the experts onboard expected to find them in this area. But the fact that the team refers to them as “normal faults” has nothing to do with how predictable their presence may (or may not) be.
There are different kinds of faults. The direction in which the blocks of seafloor move along the fault is the reason the faults are termed “normal.” In this case, fault is not a vertical line; it instead is a diagonal break, creating two parts: the plates are pulling apart, and the block with an overlapping layer is moving downwards. The two walls on the flat are named the “hanging wall” which is located above the fault plane and the “footwall,” which is under the fault.
So why are they here? As the Indo-Australian plate moves under the Eurasian plate, it bends. Not far from the trench itself, the bending reaches a tipping point in which some fractures are formed, just as if you took a chopstick and started to bend it. At some point, where the curve is most pronounced – called the outer-rise by scientists – the chopstick will break.
Which is convenient enough, because as Dr. Carton puts it, the underlying plate is preparing to be swallowed by the overlaying plate, being chopped into smaller pieces.
Mean personalities
“Earthquakes are just like people,” says Dr. Shengji Wei, a seismologist from the Earth Observatory of Singapore, “They have their own personality, and intra-plate earthquakes tend to be the meanest”.
When the bending reaches a tipping point and the ocean floor fractures, it causes an earthquake. But as Dr. Wei points out, intra-plates ruptures lack the lubricant that sediments between moving plates can provide. Here, at the interior of the plate, the floor simply cracks, creating violent shakings.
“Even if the magnitude of these kinds of earthquakes tends to be smaller,” explains Dr. Wei, referring to the difference between the 2010 7.8 intra-plate earthquake and the 2004 9.3 megathrust earthquake, “Their rupture happens very fast and hence the seismic waves can be much stronger.”
Considering that the vibrations would be powerful, and that the amount of movement on the seafloor could be substantial, the possibility of big tsunamis is present, just like in 2010. So as the sun sets again, the team has decided to extend some of the lines over the normal faults, following the fresh trace of angry earthquakes.
