In Sumatra: Notes From a Geologist in the Field

January 5, 2005

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

Since January 1, 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, and determine future locations for seismic monitoring equipment. 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. Now in these occasional e-mail messages to Caltech, Sieh shares his personal observations and his preliminary science findings.

Dear Colleagues and Friends,

I arrived in Jakarta late in the morning yesterday (January 1, 2005), nearly a day after departing Los Angeles. Just walking around this bustling capital city of Indonesia, one would not know that a major disaster had struck the northwestern corner of the country just a week ago. In the evening I met with Danny Natawidjaja and Bambang Suwargadi, my colleagues from the Indonesian Institute of Science (LIPI), to compare notes on our preparations for field work in western and northern Sumatra.

We knew several days ago from friends in Padang, the large coastal city of West Sumatra, that it had escaped damage from the tsunami. Now there is additional good news from two sites on the Mentawai and Batu islands, off the west coast of Sumatra, where we have GPS stations and many local friends. In Sikakap, the main town of South and North Pagai islands, they report that the tsunami was only about a meter (three feet) high and did not injure anyone, but flooded stores and homes. A similar story comes from Tello, the main town of the Batu islands: Relatives of friends of Danny report that the tsunami washed into the main street but caused no injuries. Both of these towns are on the east coast of their islands. We still have no word from friends in the villages on the more exposed western coasts of the islands, but fear that the tsunami height will have been greater there.

In Padang, there are reports that many people fear that a large tsunami will soon devastate the city from a large aftershock. Large tsunamis did strike the city following the giant earthquakes of 1797 and 1833, so their fears are not baseless. But we have no reason to believe that such an event is imminent. We expect to have discussions with local officials about these concerns when we arrive around January 6. Even if there is no imminent threat to Padang, we know that it will be in a precarious position when the Mentawai section of the megathrust, just offshore, next ruptures. I wonder what urban planners and civic authorities should do in a situation like this?

We are hoping that a subset of our group will begin to download information from the Sumatran GPS Array (SuGAr) stations on the 6th, starting with Jambi in eastern Sumatra, then going on to the three stations on the mainland south of Padang, to measure the crustal movements during and after the earthquake. Simultaneously, another subset of the group would conduct a reconnaissance by fixed-wing aircraft of Simeuleu island, southwest off Aceh's coast and very close to the earthquake's epicenter, and of other, neighboring small islands. There we expect to see evidence of uplift, if these islands were above the rupture. We also hope to reconnoiter the west coast of Aceh, at the northern tip of Sumatra, to survey evidence for submergence due to the earthquake, east of the megathrust rupture. We would like to get a few measurements of vertical deformation in both areas, to help guide a mission to document more thoroughly the deformation pattern. But we are not sure if the dire humanitarian situation on the coast of Aceh will allow us to make measurements on the ground. One media outlet reported yesterday that Simeuleu island has survived the quake and tsunami relatively well. Islanders there have a tradition of running for high ground when they feel a big earthquake, based upon their ancestors' experience in the 19th century. On about the 9th, we hope to begin downloading SuGAr stations on the Mentawai and Batu islands by helicopter.

Today, we hope to meet with representatives of Bakosurtanal, Indonesia's national surveying agency, to synchronize plans to resurvey geodetic monuments in the source region. We also expect to meet with aviation companies to discuss arrangements for our fixed-wing and helicopter flights.

Tuesday evening, January 4. Jakarta

So far, everything is coming up roses with respect to getting our official travel permits from the scientific organization LIPI, the political offices (Sospol), and the national police (PolRI).

Yesterday we had a press conference at the headquarters of Pasifik Satelit Nusantara, a private company that is partnering with us to develop telemetry (satellite-based telecommunication). About 60 print and television journalists attended to listen to Danny and me explain the science of the megathrust that produced the December 26 earthquake. Many stations broadcast the conference. One station caused a panic in Padang by editing Danny's comments in such as way as to make it sound as if we thought a giant tsunami was imminent there. Danny was scheduled to go on national TV tonight, live and uneditable, to attempt to quell concerns. We are going to be in Padang and on the nearby islands in less than a week, so obviously we think the likelihood of another giant earthquake and tsunami is very small.

Yesterday we also met with a prominent former minister and a former ambassador to discuss the trouble in Padang and to discuss the longer-term problem of how to mitigate the effects of the eventual earthquake and tsunami there. We also met with the head of LIPI to discuss our desire to reconnoiter the stricken area by helicopter rather than by boat and on foot. Today, the heads of the office, Pak Jan and Pak Umar, told us that they have arranged with the military to move us around in a helicopter for the time that we are in Aceh. We need to firm this up and discuss the particulars tomorrow.

For our downloading of SuGAr stations south of the equator, we contracted today with a private helicopter company to take us to the island stations between January 9 and 12. It will be strange to see from a fast helicopter the islands that we have visited so many times by slow boat. Today, while calculating our itinerary, I could not get used to the fact that distances that usually take us 12 hours to traverse by sea will only take us an hour or so by air.

We learned that two teams of surveyors from Bakosurtanal were flying today to Nias island, west of Sumatra, to begin their resurvey of the campaign GPS monuments of Yehuda Bock and his colleagues, scientists with the Scripps Institution of Oceanography. I'm sure they will recover spectacular evidence of deformation from the region of the earthquake.

We have pretty much decided to install the four new continuous GPS stations on Nias and Simeuleu islands, and on the mainland Aceh coast, to monitor post-seismic transients. (Because the earth's crust is elastic, it will rise or fall during an earthquake. Then, over a period of years it will rebound, or return to some degree, with respect to its original position.) Originally, these new stations were to go in south of the Equator, to densify the existing network.

The meeting with the vice president of Indonesia today (Tuesday) lasted over an hour. We explained the potential scientific value of the continuous GPS network for monitoring the "heartbeat" of the Sumatran megathrust and suggested an international meeting within a month or two to bring together scientists from around the world who were working on various aspects of the earthquake and its effects. The vice president asked about the value of an early detection and warning system for tsunamis. We suggested that it would be in the best interests of the Indonesian people to make sure that such a system for the Bay of Bengal be able to issue a warning within a minute or so, since the tsunami could arrive on the shores of western Sumatra and its islands within just minutes of the occurrence of the earthquake.

On a personal note, what a joy it was yesterday evening to treat myself to a short jog on the treadmill and a little bit of exercise after a day driving around Jakarta in taxi cabs and being in meetings! No such luck tonight - the meetings weren't over until after 10 p.m.

It appears that we will arrive in Padang on January 7 and fly out to the islands on the 8th.

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.

To read part two of Kerry Sieh's notes, please click here.

To read part three of Kerry Sieh's notes, please click here.

To read part four of Kerry Sieh's notes, please click here.

To read part five of Kerry Sieh's notes, please click here.

To read part six of Kerry Sieh's notes, please click here.

To read part seven of Kerry Sieh's notes, please click here.