Geologic records show Sumatran tsunami threat

SAN FRANCISCO — A great offshore earthquake, like the one that killed hundreds of thousands when it struck off Indonesia’s Sumatra coast in December 2004, would seem to offer solace to survivors: The offshore tectonic fault that caused the temblor should require many centuries to recharge.

Now, it appears such optimism is unwarranted.

Three speakers here at the fall meeting of the American Geophysical Union in mid-December warned that the Indian Ocean coast of northern Sumatra could suffer another tsunami disaster in as few as 60 years.

That sobering news came in three talks by paleoseismologists — researchers who literally dig up records of past earthquakes and tsunamis — associated with Nanyang Technological University’s Earth Observatory of Singapore. Charles Rubin, Kerry Sieh, Jessica Pilarczyk and their colleagues had been reading the millennia-long histories of past tsunamis in three kinds of geologic records and determining the age of each tsunami recorded there using radioactive carbon-14 dating.

The most novel record was found in a cave located 200 meters (about the length of two football fields) from the present-day coastline. Only the far-reaching inland surge of a tsunami can carry sand into this cave, where it can then be deposited layer by layer, tsunami by tsunami. Conveniently enough for the researchers, tsunami deposits in this cave are demarcated by dark layers of guano deposited between tsunamis by the cave’s resident bats. Other records were retrieved from tsunami deposits in coastal wetlands and exposed in eroding sea cliffs.

Taken together, the new records paint a disconcerting picture of highly erratic tsunami recurrence. Two tsunamis struck the northern Sumatra coast in quick succession about 600 years before the 2004 tsunami. Previous, more precise dating of coral uplifted by offshore quakes at about that time show the interval between the two tsunamis to have been just 60 years or so. Yet the cave record lacks tsunamis in a 1,900-year interval between about 5,400 years ago — when a thick tsunami deposit was laid down — and about 3,300 years ago. Then, about five lesser tsunamis, to judge by the thickness of their deposits, sloshed into the cave in the next 500 years. “The basic conclusion, then, appears to be that big tsunamis vary in recurrence (time), from only 60 years to about 2,000 years,” Sieh writes in an email.

“It seems Mother Nature has this capricious behavior,” says seismologist Emile Okal, of Northwestern University in Evanston, Ill., who was not involved in the work. “We could be in for something in the next 50 to 100 years. That’s both intriguing and somewhat frightening.”

Figuring out how a single offshore fault can rupture — perhaps in a sequence of different segments — to produce such a varied history of tsunamis will take many more records like the one in the guano-laced cave.