Why No Tsunami After Giant Indonesia Quake, Ask Surprised Scientists

While the world has heaved a sigh of relief that the giant Indonesian quakedid not generate any tsunami, scientists are puzzled. Given the large magnitude of the quake, there should have been a tsunami, especially since the epicenter wasn’t very deep (just 33 km below the surface) and it lies directly beneath the sea. The question is really this then: Why was there such a huge tsunami during the 2004 quake, measuring 9.3 on the Richter Scale, while this one, just slightly weaker at 8.6 magnitude, didn’t generate anything even remotely similar?

The location of the quake that struck Indonesia. (Courtesy: BBC)

Two Important Factors

Preliminary investigations by geologists point to two factors which together contributed to the diminished effect. One is the strike-slip fault line and the other is that the earthquake occurred within a plate, rather than at a subduction zone. Let me explain this one sentence and the orgy of technical terms used.

Subduction Zone

Subduction zone refers to the zone at which two tectonic plates join. In this zone, plates may get submerged or raised. Subduction zones are regions where the major volcanoes of the world lie in, and thus is given the name “Ring of Fire”. Typically, these occur in the oceans. The entire of the Hawaiian region, for example, lies in the subduction zone. The really major earthquakes have all happened in this zone.

Subduction zone

The present earthquake didn’t happen at a subduction zone. The epicenter was about 150 km from the nearest subduction zone. No plate was submerged or raised. It was a really big intra-plate quake. What has surprised scientists is how big this quake actually was. More of this after a bit.

Strike Slip Fault

The earthquake occurred on a strike-slip fault, which means that the two adjacent plates slid past each other, releasing the built up tension, rather than submerge and rise. This is THE crucial difference between this quake and the 2004 one. The 2004 quake occurred on a normal fault. Land (the sea bed) was violently raised, while another part was submerged. The released tension was directed upwards, rather than sideways, causing the huge waves.

Strike-slip fault. Note how the landmasses slide past each other.
Normal Faulting. One can see the displaced striations and that makes it clear that the rock has been uplifted and submerged along the fault-line.

I want to ram home this crucial point: the difference was the presence of shear in the present earthquake. The fault line sheared (or pulled apart horizontally, along the ground), rather than jut upwards, like it had done during the devastating 2004 earthquake.

Formation of a tsunami.

Scientists are still learning new things though. They are surprised as to how a strike-slip earthquake can be so strong in magnitude. The record was held by the giant 1906 San Francisco earthquake, along the strike-slip San Andreas fault, which measured 7.8 on the Richter Scale.

A Tsunami did form!

Strictly speaking, a tsunami was generated. The Indonesian coast saw the increase of water level by about 1 m or 3.3 ft. However this is nothing compared to the about 30 m or 100 ft waves generated by the 2004 earthquake. The land however has shifted by nearly 70 ft in the northeast-southwest direction.

Residents left their homes in search of higher ground in Banda Aceh, once the tsunami alert was sounded. (Photo Courtesy: National Geographic)

All tsunami watches, issued for all countries across the Indian Ocean, had been lifted by yesterday night itself. Lucky definitely, but humanity needs all the luck it can get right now.

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.