Scientists worried by possibility of another huge Sumatra quake

Syofiardi Bachyul Jb, The Jakarta Post, Padang

The Indonesian Institute of Sciences (LIPI) is worried about the possibility of another massive earthquake that could trigger a tsunami of the west coast of Sumatra, thus endangering the lives of the 800,000 inhabitants of Padang, West Sumatra.

A seismologist at LIPI's Geotechnology Research Center, Danny Hilman Natawidjaja, told The Jakarta Post in Padang that the 200- year cycle of huge quakes -- centered around Siberut island and Sipora-Pagai islands in the Mentawai island chain -- had almost come full circle.

"The quakes in Nias and Aceh have definitely increased the possibility of a huge quake in Mentawai. We are afraid that the 6.9 magnitude quake, with its epicenter in Siberut island, last April 10, could trigger a truly massive quake," he said.

Danny said the April 10 quake was not actually massive, unlike those that took place on Dec. 26 in the Aceh-Andaman segments and on March 28 in the Nias-Batu islands segments.

In addition, it had not taken place in the Siberut or Sipora- Pagai segments, but between the two segments. It had also not affected the rising and sinking of the islands.

Based on Danny's report, the massive quake on Dec. 26 in Aceh caused the northern coast of Simeuleu island to rise by 1.3 meters while the south coast sank by around 50 centimeters.

A similar event occurred on Nias island during the March 28 quake. During the 8.7 magnitude quake, the western part of the island was lifted by about three meters above the surface of the sea.

Moreover, coastal areas in Laweha district were lifted by an average of 3.5 meters. By contrast, Banyak and Singkil islands on the western side of Nias sank by about 1.5 meters.

However, the quake in Nias failed to produce a tsunami as the epicenter was located precisely underneath the island.

Danny explained that the rising and sinking of the islands along the west coast of Sumatra was due to natural processes that had been happening since time immemorial.

The same thing would also occur in the two segments of the Mentawai Islands. The Sipora-Pagai segment was last hit by a magnitude 8.5 earthquake in 1833, while the Siberut and Sipora- Pagai segments were last hit simultaneously by an 8.3 magnitude quake in 1797.

"Based on our analysis using the Global Positioning System, the Mentawai islands are sinking by about a centimeter each year. During the 200 years since the last massive quake, the islands have sunk by two meters. Then a massive quake occurred again and raised the islands back up two meters. They are now sinking again gradually at 1 centimeter per year," said Danny.

He said the quake that struck the Mentawai islands in 1797 triggered a tsunami in Mentawai and Padang. Based on historical records, Padang town was hit by a five-meter-high tsunami.

"A similar occurrence is a possibility, with the worst-case scenario being simultaneous earthquakes in two segments, the Siberut and the Sipora-Pagai segments, which would be capable of producing a 9 magnitude quake and a tsunami," said Danny.

Padang is the most susceptible area in the event of a tsunami as 80 percent of its 800,000 inhabitants live only a few hundred meters from the coastline at an elevation of just five meters above sea level.

"I'm not worried about Mentawai due to its sparse population, most of whom generally live in villages behind which are hills to which they can flee. There are almost no multistory buildings there. It's different in Padang, however," said Danny.