OCT 5 — Last week, I was jolted awake soon after dawn one day by an earthquake that gave our newly-built Bali house a couple of savage shakes — not at all like the rolling motion I have become almost used to, living in Indonesia.
It was a 5.8 on the Richter Scale, perhaps a little less on the east coast of Bali where we are located. No cracks, but still a morning surprise I could have done without.
Just as it is difficult to prepare for how quakes will behave, so is it in predicting when and where they will strike in this decidedly shaky archipelago located at the southern end of the so-called Pacific Ring of Fire.
But Wednesday's 7.6-magnitude temblor — which hit the bustling West Sumatran coastal city of Padang, with a population of 900,000 — was as predictable as earthquakes will ever get, even without it triggering a devastating tsunami.
Running down Sumatra's west coast, along the south coast of Java and through the Nusa Tenggara island chain, the boundary of the Indo-Australian and Eurasian tectonic plates is one of the world's most volatile subduction zones.
Only two years ago, international scientists were warning that Padang was especially vulnerable because previous ruptures had imposed severe strains on the Mentawai section of the undersea fault line as it passes west of the city.
Those strains had been caused by the December 2004 Aceh disaster which claimed 168,000 lives, and an 8.2-magnitude quake 100km off the coast of Bengkulu to the south in September 2007 that caused little serious damage.
Scientists said back in 2007 that the two plates moved together at a rate of 5cm to 6cm a year along the 1,700km length of Sumatra, whose oil and gas fields are fortuitously on the opposite coast.
The 2004 earthquake broke 1,200km of fault line. A second tremor near the island of Nias the following year tore up an additional 400km to 500km. Two years later came the Bengkulu event, which left a 300km long rift.
That all added up to a rupture extending from six degrees south off the South Sumatra coast to 12 degrees to 14 degrees north in the Andaman Sea, with the middle 100km to 200km Mentawai section remaining precariously intact.
The United States Geological Survey (USGS) has recorded at least 22 quakes measuring magnitude 5.5 or more along the Sumatran fault since the beginning of this year, including two of magnitudes 6.6 and 5.9 which rippled through the Mentawai section five hours apart on Aug 18.
Both epicentres were in virtually the same place, 110km west-south-west of Padang at a depth of 46km and 35km respectively. An Indonesian scientist warned that the twin events suggested something ominous was brewing.
There were three other quakes in the same area before the end of August, then a period of relative calm until Wednesday's tremor which occurred about 225km to the south-west of Padang.
The only thing Indonesian experts got wrong was their prediction that the impending quake would be of a magnitude of 8.5 to 8.9. In a worst case scenario, they said, a 5.5m tidal wave would penetrate 2km into Padang, destroying half the city.
That never happened, possibly because the quake occurred at a depth of 85km. But in terms of damage and lives lost, it is still the most devastating event since the Jogjakarta tremor which killed 5,000 people in 2006.
Other stretches of the fault line have been active as well. Early last month, a 7.4-magnitude quake occurred 40km off the south coast of West Java, killing 100 people and causing extensive damage.
Further to the east, about 30 significant tremors have been felt in the Nusa Tenggara chain and along the north coast of Papua in the past nine months, according to USGS data.
By far the most active area this year has been off the north coast of Sulawesi, which has experienced 28 quakes of magnitude 5.5-plus so far this year, all in the region where the Molucca Sea, Banda Sea and Sunda tectonic plates converge.
Even if it had been long predicted, it is hard to know what the West Sumatra provincial government could have done to minimise the destruction — apart from improving infrastructure, devising escape routes and ensuring that Padang's building codes are adhered to.
Since 2004, about US$60 million (RM210 million) has been spent on building a tsunami warning system for Indonesia and the other countries bordering the Indian Ocean. But even a 15-minute alert would not have been quick enough to save much of Padang's population.
“Padang is a very difficult place because of its population density,” a United Nations official said. “How can you evacuate so many people in such a short time? It's not possible given the traffic congestion.”
Wednesday's quake occurred at the height of the rush hour, soon after 5pm. Amateur videos showed people milling around on the streets, terrified of the threat of a tsunami, but clearly feeling helpless about what to do.
Compare that with the citizens of Auckland, New Zealand, who, after a tsunami alert was sounded in the wake of Samoa's calamitous 8.2-magnitude quake, flocked to the harbour to see what was happening.
Next time, it might be more than they bargained for. — The Straits Times
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