Stiff sediment caused killer tsunami

U. TEXAS-AUSTIN (US)/U. SOUTHAMPTON (UK) — A thick plateau of hard, compacted sediment was a major factor in the 2004 undersea earthquake off the coast of Sumatra that spawned the deadliest tsunami in recorded history.

Once the fault snapped, the rupture was able to spread up from tens of kilometers below the seafloor to just a few kilometers below the seafloor, much farther than weak sediments would have permitted—allowing it to move a greater column of seawater above it, unleashing much larger tsunami waves.

“The results suggest we should be concerned about locations with large thicknesses of sediments in the trench, especially those which have built marginal plateaus,” says Sean Gulick, research scientist in geophysics at the University of Texas-Austin. “These may promote more seaward rupture during great earthquakes and a more significant tsunami.”

For the research, published in the journal Nature Geoscience, scientists used seismic instruments, which emit sound waves, to visualize subsurface structures.

Early in the morning of Dec. 26, 2004, a powerful undersea earthquake started off the west coast of Sumatra, Indonesia. The resulting tsunami caused devastation along the coastlines bordering the Indian Ocean with tsunami waves up to 30 meters (100 feet) high inundating coastal communities. More than 230,000 people died and millions were left homeless.

The earthquake struck along a fault where the Indo-Australian plate is being pushed beneath the Sunda plate to the east,  known as a subduction zone—in this case the plates meet at the Sunda Trench, around 300km west of Sumatra.

The Indo-Australian plate normally moves slowly under the Sunda plate, but when the rupture occurred, it violently surged forward.

The Sunda Trench is full of ancient sediment, some of which has washed out of the Ganges over millions of years forming a massive accumulation of sedimentary rock called the Nicobar Fan.

As the Indo-Australian plate is subducted, these sediments are scraped off to form what’s called an accretionary prism. Usually an accretionary prism slopes consistently away from the trench, but in that location, the seabed shallows steeply before flattening out, forming a plateau.

Subduction earthquakes are thought to start tens of kilometers beneath the Earth’s surface. Displacement or “slip” on the fault, as geologists call it, propagates upwards and generally dissipates as it reaches weaker rocks closer to the surface.

If it were an ordinary seismic zone, the sediment in the Sunda Trench should have slowed the upward and westward journey of the 2004 earthquake, generating a tsunami in the shallower water on the landward (east) side of the trench.

But the fault slip seems to have reached close to the trench, lifting large sections of the seabed in deeper water and producing a much larger tsunami.

The new research builds on work published last year in the journal Science that found a number of unusual features at the rupture zone of the 2004 earthquake including the seabed topography, how the sediments are deformed, and the locations of small earthquakes (aftershocks) following the main earthquake.

The fault zone was also at a much lower density zone than surrounding sediments, perhaps reducing friction and allowing a larger slip.

Researchers from the University of Southampton, The Agency for the Assessment and Application of Technology in Indonesia and The Indonesia Institute for Sciences contributed to the study that was funded by the U.S. National Science Foundation and the UK Natural Environment Research Council.

At a typical subduction zone, the fault ruptures primarily along the boundary between the two tectonic plates and dissipates in weak sediments (a), or ruptures along “splay faults” (b); in either case, stopping far short of the trench. In the area of the 2004 Sumatra earthquake, sediments are thicker and stronger, extending the rupture closer to the trench for a larger earthquake and, due to deeper water, a much larger tsunami. (Credit: U. Texas-Austin)

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