Jakarta. The tsunami that devastated the Mentawai Islands off West Sumatra last month featured waves much larger than would normally have been expected from the magnitude 7.7 earthquake that spawned it, according to a tsunami expert.
Jose Borrero, from ASR Ltd., a marine consulting firm, said on Friday that the finding was extraordinary.
“We were extremely surprised by the size of the waves on one of the small islands offshore of Pagai,” he said at a press conference at the office of the presidential advisory council.
“We found evidence that the tsunami waves reached a height of 17 meters, which was much bigger than we had expected to find,” he said.
Borrero, who is also a researcher at the University of Southern California’s Tsunami Research Center, said that while most residents of the islands were aware of the potential for a tsunami, the majority had been caught unawares by the relative weakness of the Oct. 25 quake.
“They knew that earthquakes are associated with tsunami, and that they should be aware of evacuating if they felt a strong earthquake,” he said.
“However, this earthquake didn’t feel that strong to them, especially when compared with previous earthquakes in the area. So some people didn’t evacuate immediately upon feeling the earthquake. It wasn’t until they actually heard the sound of the waves coming through the trees and tearing down the forest that they actually knew that they had to go.”
He said that because Indonesia would always be prone to tsunamis, residents should learn to read the signs.
“Basically, tsunamis and earthquakes are part of natural life,” Borrero said.
“You can’t live in fear of them. It’s best to understand them and work with a national system. By understanding them, we have the chance for survival. This isn’t something to be afraid of, but to be aware of, to know what to look for,” he said.
At least 461 deaths have been confirmed as a result of the tsunami, while 43 others remain missing and are feared dead.
The waves also rendered nearly 8,000 people homeless.
Hermann M. Fritz, a tsunami expert from the Georgia Institute of Technology in the United States, said the real measure of an earthquake’s potential for causing a tsunami in tectonic subduction zones such as the Mentawais, was not the perceived strength of the quake but rather its duration.
“How long the quake lasts will indicate its potential for a tsunami,” he said.
“If you feel the shaking for more than 30 seconds or a minute, it’s important to evacuate immediately.”
Danny Hilman, a researcher at the Indonesian Institute of Sciences (LIPI), said the Mentawai quake lasted more than a minute.
However, he said most local people interviewed by the experts had said that at the time of the quake they only felt a swaying motion, and thus did not feel the need to evacuate.
He said that in the case of Mentawai, while the quake registered at a magnitude of 7.7, its long duration meant it was more like a magnitude 8 temblor.
When this event took place, we were monitoring the situation closely before the tsunami was generated.
Several reports came through of an earthquake but each report had a slightly different magnitude.
At first, we thought that the reports on the size of the earthquake were confused and put this down to a lack of confirmed data or some news angencies “inflating” their headlines.
We checked the USGS data and found that there were three earthquakes that day, all within minutes of each other and all with a magnitude of 6+.