Inland earthquake triggers distant tsunami

by Mary Caperton Morton
Monday, November 3, 2014

On Sept. 24, 2013, a magnitude-7.7 earthquake struck southern Pakistan, killing more than 800 people. The quake made global headlines in part due to the birth of a small island it triggered just off the coast — a mound of mud dubbed Quake Island that has since washed away. A new study has found evidence of another curious event linked to the quake: a small tsunami that appears to have been remotely triggered by a submarine landslide far from the earthquake’s inland epicenter.

Earthquakes and tsunamis are notoriously destructive partners, with the most dangerous waves triggered by large megathrust events along undersea subduction zones, such as the magnitude-9 Tohoku earthquake and tsunami that struck Japan in March 2011. But the 2013 event in Pakistan occurred far inland along a strike-slip fault, an unlikely setup for a tsunami, says Gösta Hoffmann, a geologist at the German University of Technology in Oman and lead author of the new study in Geology.

“The epicenter of this earthquake was 350 kilometers from the coast, so it’s unlikely that the quake itself triggered the tsunami,” he says. “But the timing of the wave, and the way that it moved, strongly suggest the two events are linked.”

To narrow down the potential triggers for the nondestructive wave, which was only a meter high and struck during low tide along a sparsely populated area of Oman’s coastline, Hoffmann and colleagues turned to data from tide gauge stations and also interviewed eyewitnesses. “Fishermen know the sea; we found several who were able to describe how the wave moved in great detail and some even had video of the event,” he says. “These days, even fishermen in Oman have smartphones.”

The accounts led the team to eliminate other triggers such as offshore earthquakes, undersea volcanic eruptions and strong winds. They even considered the possibility that the formation of Quake Island may have triggered the tsunami, Hoffmann says, “but the size of the island and the depth of the water were simply not great enough to have generated even a small wave.”

The team concluded that the tsunami was likely triggered by a submarine landslide along the Makran Subduction Zone, an active trench in the Arabian Sea that is known to sit under a large pile of unconsolidated sediments. “It’s very difficult to say how much mass would need to slump to displace that amount of water, but the wave was not very large, so it didn’t need to be a massive slump,” Hoffmann says. Based on the timing of the wave and the direction in which it propagated through the water, the team concluded that the landslide was likely triggered by the onshore quake.

The event is not the first tsunami to strike the region; shoreline evidence suggests that larger events have struck in the past, likely triggered by earthquakes along the Makran Fault itself, says Hermann Fritz, an engineer at Georgia Tech University who was not involved in the new research.

“It’s clear that this event was not generated by the classic subduction zone mechanism, but whether it was a submarine landslide is hard to say,” Fritz says. “The distance between the epicenter of the quake and the source of the tsunami is unusually large, but it’s not impossible.”

The Makran Subduction Zone last shook in 1945, when a magnitude-8.1 event triggered a deadly tsunami that killed several thousand people on the coast of Pakistan. “This subduction zone has not been very active since the 1945 event, so there may be a higher risk of submarine landslides simply because they’ve been building for decades without much shaking,” Fritz says.

The 2013 tsunami was not a major event, but it’s still an important data point in the patchy history of earthquakes and tsunamis in the Arabian Sea, Hoffmann says. “People in this region have not traditionally kept written records so we have to rely solely on the geologic record to forecast the risk of future events.”