In Sumatra: Notes From a Geologist in the Field--UPDATE 1/20/05

January 20, 2005

For a map of Sumatra, please click here. For a map of the Sumatran subduction zone, click here.

Since the first of January, Caltech geologist Kerry Sieh has been in Indonesia, focusing on Sumatra and its accompanying islands, to assess the geological changes caused by the Aceh earthquake. This is familiar terrain to Sieh; for more than a decade he has been studying the active Sumatran subduction zone. Last summer he and his colleagues distributed brochures to people living on local islands to explain the area's geology, and suggest ways to limit future damage. This is the final e-mail update from Sieh, in which he shares his personal observations and his preliminary science findings. To read part one please click here. To read part two please click here. To read part three please click here. Part four is here, part five here.

10:30 a.m., Sunday, January 16, Sinabang, capital of Simeulue Island

Midday yesterday we flew across the Equator to Telukdalam, on the southern coast of Nias Island (itself off the west coast of Sumatra), and near Lagundri bay, a world-famous spot for surfing. My friends Chris and Christina Fowler had sent their boat from Padang, Sumatra, to drop off five barrels of helicopter fuel for us, but we could not find it at the wharf. Anticipating disaster, we called Chris in Padang, who assured us that it had arrived the previous night. He gave me directions to his agent's, Ama Pipir, house, where it would have been stored. By chance I happened to meet Johny, one of Pipir's nephews, just as I was wondering how to get there. He took me on his motorcycle to the house, where it became clear that I had nothing to worry about. Pipir's wife brought me five very delicious mangoes--I ate one, and had a taste of what it must be like in heaven. Saved the rest for the next few days.

Johny and Ama Pipir's family told me they had felt the earthquake Sunday at about 8:30 a.m. and said it lasted about five minutes. The first regression of the water began at 10 a.m. The water did not come back up until 11:30 a.m. Then there were many oscillations, which didn't stop until nighttime. The biggest surge was at 2 o'clock in the afternoon.

We made it to our final rendezvous with the Australian 60-Minutes film crew at Sirombu, halfway up the west coast of Nias. The tsunami damage there was horrific. I felt a bit silly landing in a helicopter, saying a few hellos to the villagers who greeted us, and then immediately starting to work with the film crew. Seemed a bit crass, actually, in the face of all the suffering going on there. I was thankful that we only spent a half hour interviewing and filming, before interviewing villagers and making observations of the tsunami height--about 4.5 meters (almost 15 feet). Only eight villagers died here though, because the big wave hit hours after the first ones arrived, giving all the residents hours of advance notice.

Yamo, a resident of Sirombu, was in Gunungsitoli, the capital city of Nias on the central eastern coast of the island, but told me that his family member Fauzi rode out the tsunami surge in the second floor of his home and is a reliable eyewitness. The big wave came at 4 p.m., and it came from the south, not the north. This must mean that the tsunami reflected off of eastern India and Sri Lanka and came back to the Indonesian islands.

We left Sirombu flying north, reconnoitering the coast all the way to the northern edge of the island. One particularly impressive sight was an entire grove of dead coconut palms sitting out on the reef, seaward of the beach. These obviously had died some time ago, not as a result of the tsunami. They show that the west coast of Nias, like the islands we have been studying farther south, have been sinking during the past several decades. The islands are like a springboard, storing strain for the day when the megathrust below gives way and they spring suddenly back up, producing a great earthquake and tsunami.

We spent the night in Gunungsitoli. Our modest rooms cost us just $5 each! In the evening, I heard amazing stories from the three guys from Telukdalam, a village at the southern end of Nias Island, who had driven our fuel by truck to Gunungsitoli. Alimin is a 52-year-old man who lives on the waterfront in Telukdalam, in a silver-roofed house. He was there when the water came into it, and said the water rose 1.3 meters (four feet, up to his chest) above the floor in his house. The floor of his house is 2.5 meters (eight feet) above low tide. This suggests that the amplitude of the highest surge was about 3.8 meters, or about 12 feet. During the recessions of the sea, the floor of the bay was completely high and dry out to 50 meters (55 yards) from the shore. He and Ama Pipir's son, Handy, estimate the drop was six to seven meters (20 to 22 feet) below sea level! At the time of the tsunami, the sea was near low tide. Handy and his friend Herman were on Ama Pipir's squid boat when the tsunami happened. The boat sank down with the water and came to rest on rocks. They scrambled out and made it to shore before the next surge.

Our new fuel-supply boat arrived at about 4 a.m., and we loaded on the extra fuel barrel that had been delivered by truck from Telukdalam the night before. They took off again for Sinabang ( Simeulue Island's capital) around 11 a.m. It will be a 20-hour trip for them at about seven miles per hour from Gunungsitoli to Sinabang. The same trip took us just 1.5 hours by helicopter late this morning.

We were warmly welcomed by Durmili, the local Bupati (government official) of Simeulue. His assistant, Riswan, was waiting for us at the airport when we landed. We were spirited away to the Bupati's home in the city of Sinabang, where we met a crowd of foreigners who were there for other reasons--medical, media, etc.

We went on a car caravan with the whole crowd, led by the Bupati, to view tsunami damage on the southern part of the southwestern coast. We measured heights of the tsunami of about 2.5 meters (eight feet). All the stories we heard still say that the first indication was a recession of the sea. Also of interest to Danny Natawidjaja, my colleague from the Indonesian Institute of Science (LIPI), and myself was the evidence for small amounts of permanent submergence in these towns. Areas that used to be dry now have up to 30 centimeters (nearly a foot) of standing water in them. Local residents insist that the beaches have eroded ten meters (33 feet) or so since the tsunami.

We heard from the Bupati that word has come from the northern coast that the coral reef there is about a meter (three feet) out of the water. This is almost precisely what Danny and I had been guessing would be the case. We will fly there tomorrow morning to see if the reports are true and, if so, to take some measurements. If the northern part of the island has risen, it means that the southern end of the great megathrust rupture that caused the earthquake is under the island.

At the end of our trip, back at the Bupati's home, we learned from one of the staff that one of the members of our car caravan, Pak Riswan, had lost all four of his children in the tsunami in Banda Aceh, the provincial capital. He was such a stoic all afternoon long. One would never have suspected that the tragedy was also a very personal one for him.

A few minutes ago dozens of people began running up the street past my second-story room, away from the wharf, yelling out to one another. Turns out a rumor of another tsunami had caused them to flee. It didn't even occur to me that was the cause of people running, since I hadn't felt an earthquake. People are clearly on edge here and on Nias Island. So many people ask us if another earthquake and tsunami are coming.

8:46 a.m., Thursday, January 20, en route from Padang, Sumatra, to Jakarta, Indonesia, then on to Singapore

Monday and Tuesday on Simeulue Island were extraordinary, both scientifically and emotionally. On Monday we flew along the southwestern coast, past a score or more of fallen bridges and as many coastal villages, devastated by the tsunami. Near where the island doubles its width we began to see evidence of what looked like an extremely low tide--barren, pale-tan ribbons of coral reef paralleling the coast and extending 100 meters (100-plus yards) and more from the beaches to the waterline.

During our first circling of one of these reef ribbons, we saw striking evidence of emergence--pristine pancake-shaped heads of Porites coral, well above current water level. We landed on the 200-meter wide (700 foot) former shallow reef platform, about halfway between the former sandy beach and the new shoreline. Before we could shut down the engine, 100 or more children and adults swarmed out onto the reef from the trees. We immediately split ourselves into a science team and a relief team; Danny and I began to inspect the corals, while Dayat and Samsir (our pilot and mechanic) began to talk to the villagers and distribute the materials we had brought along as relief aid--clothing, powdered milk, hammers, and fishing equipment.

Even though Danny and I have for the past several years been studying ancient evidence of the slow sinking and fast emergence of the Sumatran coral reefs, we were astonished to find ourselves walking through a pristine marine ecosystem, missing only its multitude of colors, its fish, and its water. Corals of every shape and size rested lifeless on the reef platform--branching corals, massive corals, staghorn corals, fire corals, brain corals, whorls, fans. And here and there a poor crab. Even though the tsunami had raged across the reef, there was scant evidence of any breakage of the delicate whorls and dendritic corals that crunched beneath our feet. But a fishing boat in the trees beyond the shoreline and an overturned, two-ton umbrella-shaped Porites coral heads were testimony to the power of the tsunami. The scene was the marine equivalent of a village on the flank of a volcano after the passage of a nuee ardente (a destructive "glowing cloud")--life quick-frozen in place at the moment of death.

Like us, the villagers quickly segregated into two groups. Most of the adults surrounded Dayat and Samsir, but many of the barefoot children came racing on to Danny and me. Smiling, cheering, boisterous young boys and girls, eager to play with us and to watch what we were doing. We had noticed as we circled the reef that their village, Ujung Salang, had been completely washed over by the tsunami. Hardly a building remained. Yet, there was no trace of sadness in their beautiful faces. I have no idea where they are living now--on higher ground in the forest, I imagine. They were eager to be in the pictures I was taking. In fact, I had to coax them to the sides of the images, so that I could see the corals. I have one picture with several kids standing on top of a pancake-shaped coral head. They are standing at what used to be lowest low tide of the year. At the time of our visit, the water level was a meter (three feet) lower.

We estimate the emergence here to be nearly a meter and a half. To produce so much uplift, the block above the gently northeast-dipping megathrust, 25 kilometers (15 miles) or so beneath the reefs, must have slipped about ten meters (11 yards) toward the southwest.

We hopscotched our way farther north for the rest of the afternoon, stopping only occasionally to make an additional measurement and to divert around rainstorms. A systematic, detailed survey will have to wait until we can return, hopefully in a few months. Along the northern coast, newly emerged reef ribbons were everywhere. The emergence had doubled the diameter of some of the smaller islets. And along most of these coastlines were old stands of decayed coconut palms and other trees out on the reef, seaward of the old beach--testaments to the fact that the land had been sinking slowly in the decades before the earthquake. In the complexly embayed coastline of the northern coast, muddy flats surrounded by mangrove forest have also emerged above the water. Some of these have muddy brown rectangular fields on them. I think these are very old rice paddies that slowly submerged into the intertidal zone or below in the decades prior to the earthquake. Now they are back above high tide, ready for cultivation again! Some villagers have, in fact, asked us if the water will return soon and submerge the newly dried reef and mud flats. We tell them with confidence that submergence of these new lands will not occur soon. It will take more than 100 years for the water to return to its levels on the day before the earthquake.

On Tuesday it was more of the same. We filled some of the gaps between our measurement sites and reconnoitered the northeastern coast. The former lowest low tide level is now at least 25 centimeters (10 inches) above water. So the island tilted coseismically both from northwest to southeast and from southwest to northeast. The pattern and magnitude of uplift is strikingly similar to what we know happened to the south in the Mentawai Islands during the magnitude 8.7 earthquake of 1833. The half-hour flight back south to Sinabang took us over vast tracts of virgin forest, full of tall, white-trunked dipterocarp trees towering over the lush understory. Inspiring flight, tempered only by the fact that Danny and I were a bit uncomfortable in our pants, soaked from our wading in shallow water.

Late Tuesday evening, back in Sinabang, we briefed the Bupati, Darmili, on our findings. We presented him with two gifts from his own island--a small, bulbous, bleached-white head of pristine Goniastrea retiformis (a honeycomb-like coral) from one of the dead reefs and a CD with many of our photographs. He is a gracious and thoughtful man, who seems intent on understanding what has happened, so that he can make good decisions about what to do to help his island recover. We mentioned our interest in establishing a couple of continuous GPS stations on the island, to monitor the "healing" of the earthquake wound. He said he would welcome our return.

Yesterday, Wednesday, we all left Simeulue Island. Danny and Imam flew to Medan, the capital of North Sumatra, on a small commercial airplane, while I flew with the helicopter crew back to Padang, Sumatra. Passing low over the virgin forests and rice paddies of southern Simeulue, it occurred to me that this might well be the most beautiful of all the islands in the chain. We landed twice along the five-hour journey to refuel, at the towns of Gunungsitoli and Airbangis, where we had made sure to stash barrels of fuel for the return flight. At Airbangis, a town almost right on the Equator, an official came to see me while I was waiting in an airy little hotel lobby for the refueling to be completed. He asked for advice about what to do to protect the city. His concerns are warranted: Passing along the coast of mainland Sumatra, between the equator and Padang, we saw ample evidence of the possibility of a repetition of the disaster of December 26. As along the west coast of Aceh, North and West Sumatrans have built many of their villages, towns, and cities right on the coast, directly east of the source of giant earthquakes. In some cases, towns and villages sit on barrier beaches, between the sea and long estuaries. Without the construction of bridges across the estuaries in the coming years, there will be no safe place for the townspeople to flee in the 15 minutes or so between the earthquake and the tsunami surges.

Kerry Sieh

For more information, see:
Sumatran Plate Boundary Project: http://www.tectonics.caltech.edu/sumatra/main/public.html

An essay by Sieh that appeared in the January 10 edition of Time Asia can be viewed here.