Digging in: How Aceh's past tsunamis cast light on future seismic threats

Investors in stock markets are cautioned that past performance is not a guide to future events. For scientists who study earthquakes and tsunamis, the opposite is true: Where the earth’s crusts are colliding, the only certainty is that the ground will shake again one day.

By sifting through the geological and archeological record, researchers can try to piece together historic patterns for seismic events. Known as paleo-seismology, such research is a form of time-travel into the past, where displaced rock and plants tell a story of tectonic upheaval and churning oceans. In some cases, these centuries-old events can be pinpointed to within a few years.    

What scientists can’t accurately predict is when the next big earthquake will occur. Will the pause between quakes last a decade or a century? Will the next one be bigger or smaller? For the next undersea quake, will it trigger a giant tsunami, like the Dec. 26, 2004 temblor that sent waves rippling across the Indian Ocean, wreaking havoc in 14 countries? And what can policymakers do with the information scientists uncover?

The epicenter of that quake was off the west coast of Aceh, on the northern tip of Sumatra Island. When the tsunami made landfall, a half hour later, nobody was prepared and over 165,000 people died in Aceh alone. By contrast, villagers on Simeulue Island, only 25 miles from the epicenter, ran for higher ground and almost all survived. The difference was that these islanders knew that their ancestors had suffered tsunamis in the past, notably in 1907, and that massive temblors had preceded them, hence the immediate dash for the hills.

A decade on, Aceh has rebuilt its devastated coastline and installed some tsunami safeguards, including warning towers linked to oceanic sensors. It remains highly vulnerable due to its proximity to an active fault known as the Sumba megathrust that runs the length of Sumatra's west coast and whose movements triggered major quakes in 2009 and 2012. Yet, except for the Simeulue islanders, the historic record of tsunamis in Sumatra is spotty, which helps explain why so few communities built defenses against a surging sea.  

Aceh has “no folk memory of tsunamis,” says Kerry Sieh, an American geologist and director of the Earth Observatory of Singapore. “There’s no memory of one. Let’s go looking for one.”

Clues at sea

On an overcast morning, Mr. Sieh sets out on a small motorboat from a fishing port on Aceh’s east coast. The port was rebuilt after the tsunami, and most of the fishing fleet was supplied by the UN and aid agencies. On a choppy sea, the captain steers the boat around a headland in search of a limestone cave that Sieh’s fellow researcher, Edmund McKinnon, spotted several years earlier. Mr. McKinnon, a Scottish expert on Asian pottery who first visited Aceh in 1975, has become Sieh's close collaborator – an expert in tracking changes on the scale of human generations blended with Sieh's expertise in deeper, geologic time.

Tsunamis typically dump large amounts of sand, so protected coastal features like caves can be excavated to reveal these layers between other deposits. One of Sieh’s associates recently found a cave – a former Acehnese rebel hideout – where the 2004 tsunami had deposited a foot of sand between layers of bat guano. Further digs at the bat cave revealed sand deposits in various layers dating back more than 7,000 years.

Soon the boat approaches a cave in a craggy cliff. It looks promising. But the cave’s roof has collapsed, and seawater swirls at the base, making it dangerous to land. “We’re not going in there. No way,” Sieh tells McKinnon.

Instead, they circle back and drop anchor in a deserted bay at the mouth of an estuary. McKinnon steps gingerly onto the black sands. Sieh, a rangy Midwesterner in a brick-red shirt and khaki baseball cap, leads us along the beach to a ruined fort and an exposed ridge of sand where pine trees grow. The 2004 tsunami pounded this bay, as evinced by the fort’s scattered stone foundations. Further down the beach is a Japanese pillbox built in the early 1940s during Japan’s brief spell as ruler of these islands.

But Sieh is far more interested in the exposed ridge, known as a sea cliff, which dates from a millennium ago, when the sea level was higher along this coast. The ridge’s cross-section resembles a giant marble cake. Kerry pulls a pick from his bag and begins to chip away at a layer about two-thirds of the way down, where fragments of white rock poke out of the black-sand crust.

As he plucks out a tiny piece of coral, he looks like a toddler who just discovered his favorite toy in a long-forgotten box. “Great, guys. Great!” he cries.  

Corals are a useful find for paleo-seismologists because they can be dated more accurately than sandy rocks. Sieh knows all about the uses of coral: In 2008 he was the lead author of a paper on ancient corals on the seabed near the Sumba megathrust. By studying coral growth patterns, researchers found evidence of past earthquakes that had raised or lowered the sea level, which is how tsunamis are generated. Like trees, corals lay down annual growth rings that reflect environmental conditions.

In Thailand, researchers have also peeled back geologic history. Scientists studying marshland on an island near Phuket found 4-inch sheets of sand between alternating layers of peaty soil, just as the Aceh cave held alternating layers of guano and sand. The top sand layer was deposited by the 2004 tsunami. Radiocarbon dating of bark compacted in the soil below revealed the ages of previous tsunamis, including one from 2,500 years ago. 

Brian Atwater, a geologist with the US Geological Survey who took part in the Thai survey, says such excavations offer up a “geological archive” to scientists. The trick is finding a coastal feature that is undisturbed until the sands sweep in and create a timestamp.

His work has taken him to the marshes and mudflats of the Pacific Northwest, which is where he found evidence of an earthquake that drove down the land level and led to intrusions of seawater that he dated to 1700. Natives of the area told stories of such an event. “You can combine history and geology,” Mr. Atwater says.

Japanese researchers took note of Atwater’s study. That same year is recorded in Japanese history as that of the Honshu Island “orphan tsunami”, when giant waves washed ashore but the ground never shook. A Japanese mystery was solved: the quake had originated in the Pacific Northwest, where two ocean plates are colliding along a 600-mile front.

Celadon and destruction 

Back on the beach in Aceh, Sieh picks pottery shards out of the compacted sand layers. He hands a delicate blue piece to McKinnon, who identifies it as Thai celadon – a type of glazed pottery – that’s over 600 years old, a relic of maritime trade between Aceh and Bangkok that began long before European sailors first landed here. “1450, Thai. Give or take,” McKinnon says.

The pottery and other human artifacts discovered on this peninsula offer more clues for McKinnon and Sieh. Aceh’s history is bisected by the mysterious collapse of an early civilization, before the rise of an Islamic sultanate in the 1500s. And Sieh thinks that a tsunami that wiped out the older culture holds the answer.

“Historians and archeologists have known for decades that there was a discontinuity [in the historic record], but no one knew it was caused by the environment,” he says.

Sieh’s working theory, the subject of a forthcoming scientific paper, is that Aceh suffered two major tsunamis circa 1394 and 1450. The second was bigger and more devastating. In addition to his work with cave deposits, he has drawn on studies of the historic uplift on the Indian Ocean seafloor. Thai researchers found sand deposits that suggest that the last major tsunami struck there between 1300 and 1450.

Pinpointing exact dates is tough, and Sieh knows he needs to find more tsunami sand deposits in Aceh to test his theory. “We’re clinging by our fingertips, looking over the edge,” he tells me.

Still, he’s encouraged by archeological digs here that are unearthing glimpses of the settlers who first came to Aceh, possibly as early as the 11th century. Could their descendants have survived the tsunamis and settled elsewhere on Sumatra? Or did they perish, leading to the rise of Malacca as the region’s trading hub in the 1500s?

Uphill from the beach is an exposed plane strewn with 14th-century gravestones that resemble giant tent pegs. McKinnon explains that the settlers were Tamils from southern India, and they brought Islamic beliefs to this island and others along the Malacca Straits between Sumatra and Malaysia, a day’s sailing from here.

Using history to save lives

The tough part for Sieh, and other paleo-geologists, is distilling the historic record into something useful for policymakers today.

The cave digs indicate that a tsunami struck 7,600 years ago and that subsequent tsunamis left deposits at intervals that range between 60 and 1,600 years. Does this mean that Aceh can rest easy for centuries to come, or does the pattern of repeat earthquakes pose an imminent danger that merits building costly mitigation infrastructure?

Japan responded to the 2011 Fukushima disaster by raising its sea walls because scientists had miscalculated the maximum size of tsunami waves. This means residents – and nuclear power stations – should be better protected against future tsunamis. By contrast, Aceh has rebuilt houses along its exposed coast and ignored calls for an exclusion zone.

Geologists recognize that the next big one is usually a surprise. “You can’t tell how much energy is left” in a fault after an earthquake, says Sieh.

Many in Aceh say they already know the answer: It’s in the hands of God. 

Last month Sieh presented his findings on historic tsunamis at an earth sciences conference in Banda Aceh. Saiful Mahmud, the conference organizer, says that an Islamic studies professor rose to challenge Sieh and posit that the 2004 tsunami was the result of sinful behavior by Muslims in Aceh. Such beliefs are widespread in Aceh, says Prof. Mahmud, a Cornell-trained economist who came away with a different view. “Kerry’s presentation was very convincing. It will happen again.”

Atwater, the USGS researcher, says geologists need to keep a skeptical public in mind when they try to piece together the mysteries of the past. His work has included recording oral histories by survivors of an obscure 20th-century tsunami along the southwestern coastline of what was then British India. The idea is that future generations will take more seriously a threat relayed by their forefathers than one from a government official.

“The bottom line of all this work is to try to understand some of the fundamental elements of how the earth works. But the practical side is to try to improve public safety,” he says.