Caltech geologist Kerry Sieh used to describe the Sumatran subduction zone that sits off the Indonesian island’s west coast as a place that is tucked away in a corner of the world that just doesn’t have a lot of scientific traffic.

Not anymore. The subduction zone was the location of the December 26, 2004 earthquake that, with the tsunamis it generated, resulted in 300,000 dead or missing Indonesians. And now scientists from around the world are paying it plenty of attention.

The December quake, estimated to have had a magnitude as high as 9.3, originated along the boundary between the Indian/Australian and Eurasian tectonic plates, which arcs 5,500 kilometers (3,400 miles) from Myanmar past Sumatra and on toward Australia. Near Sumatra, the plates meet 5 kilometers (3 miles) beneath the sea at the Sumatran Trench, on the floor of the Indian Ocean. At the trench, the Indian/Australian plate is diving into the earth’s interior and being overridden by Southeast Asia. For more than a decade, Sieh has studied this fault zone, studying coral growth that measures the rise and fall of sea level changes, and installing an array of Global Positioning Satellite (GPS) stations that measure the deformation of the earth.


Now until June 10, Sieh and field technician John Galetza and their Caltech associates are back in Sumatra. Along with Danny Natawidjaja of the Indonesian Institute of Science (Sieh’s former graduate student and the coleader of the expedition), Natawidjaja’s associate Bambang Suwargadi, and other Indonesian colleagues, the team will be taking measurements of the uplift and submergence caused by the earthquakes, and educating locals to the danger roiling under their feet. Here Sieh shares, via e-mail, his personal observations and preliminary scientific findings.

Thursday, May 26, 2005

Today we began to explore the Banyak Islands, close to the Sumatran mainland between Nias and Simeulue Islands. We chose to meet in what we thought was Balai, the capital of the region, in order to check in with the local

authorities. We had never been to any of these islands before but were aware of their political sensitivity. But the town we landed in, on the northern coast of Tuangku, the largest of the islands, is Haloban, not Balai. Halfway through our inspection of Haloban, we were ordered to immediately fly east to Balai to report in. Both there and at Haloban, though, we had a warm reception from leaders and residents.

  

Throughout the region, evidence for submergence is clear. Mangrove forests fringing the coasts show massive mortality. Haloban and Balai have both sunk so much that waterfront roads are canals during high tides, and buildings stand partially submerged (photo 1). At Haloban a waterfront coconut grove is now flooded and, in fact, a new beach is forming within the grove. At both Haloban and Balai, refugees from flooded waterfront homes are building shelters in public areas around grassy playing fields (photo 2). The great community spirit in these places is clear; everyone is helping the less fortunate. Despite the immense challenges of reconstructing their livelihoods with little support from the outside world, most are in good spirits. The young men commonly come out to watch us survey the amount of submergence. We went into the town tonight, during low tide, and enjoyed hanging out with some of our boat crew and new friends at a pleasant outdoor shop and café.

  

At Balai, Caltech postdoc Rich Briggs and I measured watermarks from today’s high tide that were as high as 74 cm above the water level in the street. We saw one home in which people were still living, where they had rebuilt their beds higher off the floor, so they wouldn’t have to move during high tides. The woman of the house was sweeping the floor of debris that had washed in during the morning’s high tide.

Before returning to Haloban for the night, we finished off the day’s work by flying to the westernmost of the islands, Bangkaru, where we measured uplift ranging from about 70 cm in the west to about 20 cm in the east. The western site has beautiful old snags on the reef, which indicate that in the decades prior to the earthquake the island was slowly submerging. The raised outer rims of the coral microatolls tell the same story—slow sinking of the island before its sudden uplift during the earthquake (Photo 3).

Friday, May 27, 2005

On Simeulue Island, the harbor at Sinabang (the island’s capital) is a sight to behold; what remains of the concrete dock sags enormously and the pilings are covered with dead encrusted marine organisms that no longer are submerged enough to have survived (photo 4). Boats, laden with scrap metal from the destroyed buildings here, are delivering their cargo to the mainland; relief ships are still arriving. At the airport, the modest terminal building that  collapsed has already been replaced by a smaller, lighter structure. In January I was amazed to see so little damage from the first big earthquake (in December, magnitude 9.3) to structures along the road from the town to the airport. Now I’m amazed at how much damage has been wrought by the second, smaller (but still big) 8.7 M (magnitude) earthquake in March. Structures are damaged or down everywhere—the hotel where we stayed in January burned to the ground after the earthquake, along with most of the buildings in the neighborhood.

We have three goals for this month-long journey in the parts of Sumatra affected by the two recent big earthquakes:

First and foremost, we want to continue and to complete measurements of the crustal deformation that occurred during those giant ruptures of the Sunda megathrust. These measurements will be important for understanding how much and where the megathrust moved during both of the earthquakes. We started to do this in mid-January and found that the northern half of Simeulue Island had risen up to 1.5 meters and had tilted toward the Sumatran mainland and toward the southeast.

Now, with the March 28th earthquake, we have about 10 times as much coastline to measure uplift along! And we have places that are submerged as well. The extent of the March 28, 2005 rupture is based on what we have gleaned from eyewitness accounts of uplift and sinking as well as on initial interpretations of seismograms and our GPS data. Let’s see how good a job we’ve done figuring it out before we see it for ourselves!

Second, we want to install a few seismic instruments in the region of the 1797 and 1833 giant earthquakes, because we want to be ready to collect important seismic data if that section should break in the next few months or years. We have six seismometers from Professor Clayton and four accelerometers from professors Heaton and Clayton and Ken Hudnut of the USGS, Pasadena office.

Third, we have some repairs to make to some stations in our GPS network.

-K.S.

 

Saturday, May 28, 2005

Today Aron, a Caltech grad student, Nug, an Indonesian student, and I measured uplift on the small islands west of Simeulue. To my surprise, the uplift does not increase westward; rather, it is only about half the 1.7 meters our GPS station recorded at the Simeulue airport. I wonder if this means that the megathrust did not fail all the way out to the trench. If so, this might help explain the relatively small size of the March 28th tsunami.

In Busong Bay today, on the west coast of Simeulue, and just south of our GPS station, we came across a small island that had emerged during the earthquake. The evidence was clear that prior to the earthquake, high tides had submerged all but the snags of a few dead coconut palms there. Kids and young men came swimming or canoeing over to see us. A local fellow, Arsen, told us that his family had farmed the coconut grove on the island since at least 1950 and that as recently as 1985 some of the trees had still been alive. I told him some good news—they could replant since the island would take another century or so to sink back beneath the waves!

Sunday, May 29, 2005

Unlike the southeastern coast of Nias, which submerged on March 28th, the eastern coast of Simeulue has risen. Today both survey teams measured emergence along this stretch of the coast. In one large, murky, mangrove-lined bay, Nug, Aron, and I found a family of coral microatolls that clearly shows earlier emergence about two years before they emerged again during the March 28th earthquake (photo 5). Most likely this is evidence of the 7.4 M quake in November 2002, which we now recognize was a precursor to both giant earthquakes and appears to have occurred precisely between their rupture planes.

Monday, May 30, 2005

We’re overnighting in Telukdalam, the middle of three large bays along the eastern coast of Simeulue. Quiet here, without the harbor sounds of Sinabang. Just the lights of a home a couple hundred meters away at a small dock.

More evidence for uplift as we proceeded today farther north up the eastern coast of Simeulue Island. The changes in the landscape are spectacular, but they are everywhere and everyday, so we are becoming used to the spectacular. More old, drowned coconut groves have now been uplifted out of the water (photo 6), more islands are now connected across barren new ground, more beautiful dead corals, more reefs now starting to support the growth of grasses and small trees.

We began working today with a film crew that is shooting us for television. They are pretty much just filming us while we are doing our thing. Those times where they need to stage our activities in order to get a good sequence feel a bit awkward to me. It seems a bit stupid to climb in and out of a helicopter three times in front of a crowd of villagers while the film crew gets the footage they think they need. Oh well, no harm done I suppose; people here probably already think we’re a bit strange, spending so much time walking over the reef and making measurements.

Tuesday, May 31, 2005

Today, I particularly enjoyed it when, as the film crew were shooting, four local fishermen paddled over to the helicopter as it sat waiting for us on a little beach. By the time I had swum over from our survey site across the mouth of a little bay, our pilot Machfuld was translating between the TV director, Simon, and the fishermen. Simon was asking them really good questions about their experiences during the earthquakes and tsunamis. They were saying that they were frightened that their world had become so uncertain in the past few months. Things they had thought so reliable, the sea and the landscape, had changed, and they were worried that there might be more changes. Simon asked them if they thought it would help if they knew more about why the earthquakes and tsunamis had happened, and they responded enthusiastically in the affirmative. It was a natural opening to drag out the few copies of the new Nias-Simeulue poster that I’d brought along. So, they filmed us talking about the reasons for all these changes. The fishermen seemed relieved to hear that it takes many decades or longer for the earth to prepare for an earthquake and that without a big earthquake, another tsunami is exceedingly unlikely.

Tonight we are anchored in the small harbor of Lewak, a small village on the northeastern tip of Simeulue Island and the closest habitation to the epicenter of the December 26th earthquake (photo 7). One of the four new GPS sites that we established after the first earthquake is on the hill just above the town. In the photo you can see the cargo boat that is carrying all our helicopter fuel. Next to it the small fishing boat that the TV crew rented—for an arm and a leg—to help them get around. Their “fixer,” the Indonesian guy who made their arrangements, told them it would be fully outfitted for them and that it slept 10. We are wondering if he was referring to people or to chickens! We have loaned them blankets and mats and invited them to dine with us and use our toilet and shower facilities, as the little fishing boat has none of these things.

When we landed at Lewak late this afternoon, we got the usual warm greeting from dozens of men, women, and children. In addition, though, one old man was so grateful for our presence that he kissed both Aron and myself on both cheeks! My colleague Jean-Philippe Avouac and our French crowd in Caltech’s Tectonics Observatory would feel right at home here. These villagers would have been the first in the world to feel the beginnings of the giant earthquake that would in the following minutes and hours forever change so many lives around the Bay of Bengal. Imam Suprihanto, one of our assistants, recorded a video of their interview back in February of a man who saw, from the hill, the tsunami bore (a large, vertical wave) roil into the bay here.

Tomorrow morning, we plan to cut one of the dead coral microatolls on the reef platform on the other side of town. Iman and John Galetzka, my Caltech colleague in charge of technical matters, such as installing seismometers, accelerometers, and the like, found some particularly beautiful ones when they visited here to install a GPS station last February. The television crew was particularly interested in filming this aspect of our research, so Dudi and crew have dusted off the waterproof chain saw, engine, and water pump, and carted it to the boat. Cutting a slab now will give us a chance to see whether or not there were any anomalies in sea-level changes in the decades prior to the earthquake.

 

All images courtesy of Kerry Sieh/Caltech Tectonics Observatory

 

Courtesy of Kerry Sieh and Catharine Stebbins 

Click to enlarge