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This week’s jolts off the west coast of Sumatra raise a critical question: was this the “big one” we’ve been waiting for?

Geology experts on Thursday said no, but it could nudge such an event a little closer.

Wednesday’s powerful quake that shook Padang came from a spot near — but not inside — the Sunda Trench, a long undersea crescent of seismic energy stretching from north of Aceh to the east of East Timor.

Tectonic plates from India and Australia are grinding slowly under plates that support Indonesia and Burma at a rate of up to six centimeters per year, causing explosive releases of force.

Quake expert Sri Widiyantoro, from the Bandung Institute of Technology, said the precise cause of the jolt in Padang did not come from the seam between plates, but was triggered by a break in the middle, a type known as an intraplate quake.

That means the colossal one some geologists say is a near certainty — is still to come.

He warned that if the subduction fault slipped in several places at once, it could set off a massive shock “with a magnitude of more than eight.”

Danny Hilman Natawidjaya, a geologist from the Indonesian Institute of Sciences (LIPI), said the 7.6-magnitude temblor disturbed the subduction zone, which runs directly under the Mentawai island chain off the west coast of Sumatra.

“The quake has affected the tectonic plates, increasing tension between them,” Danny said.

Geologists have long been watching the movement of those plates and measuring massive forces mounting between them. Some have predicted a huge jolt may be due over the next few decades, with the threat of an even greater humanitarian disaster hanging over densely populated Padang or Bengkuklu to the city’s south.

Predictions have been issued “since early 2004, before the great 2004 tsunami and earthquake,” California Institute of Technology geologist Kerry Sieh said. A series of big shakes — including a magnitude 8.2 that struck Bengkulu province in September 2007, and one off Aceh that triggered the 2004 tsunami — showed that all the segments had released their energy, except that of Padang.

But pressure on the fault means a vastly larger 8.8-magnitude quake, coupled with a five meter high wall of water, is sure to hit in the coming decades, Sieh said.

“This earthquake today is a flea compared to this tiger of a quake that is coming,” he said. “It’s 100 percent likely. The question is when is the date. The strain has been building off Padang for this 8.8 for 175 years.”

Experts have called on the government to invest in quake-resistant buildings and widen Padang’s roads — predicting an exodus of about 500,000 people in the event of a major quake. But little has been done. Geological Disaster Mitigation and Volcanology Center head Surono said there is a “perception that building such expensive infrastructure is not economically viable” because of the uncertainty of earthquake prediction.

“West Sumatra is like a supermarket for geological disasters. There are active volcanoes, landslides, land quakes caused by faults,” he said. “Being close to the faults means Padang is always prone to earthquakes. Every day, there is a tectonic quake there, but they may be too small for any effect to be felt.”

Danny said people in Padang, a city of about 900,000, live precariously close to the moving plates, and must always be vigilant for tremors or tsunamis.

Fauzi, a geophysicist from the Meteorological, Climatology and Geophysics Agency (BMKG), said the Padang quake could actually have released some of the energy built up between the plates. But he down played speculation that the quake could trigger an even more destructive calamity.

“No scientist can predict when an earthquake will happen,” Fauzi said, “so let’s not make the public more worried than they already are.”

Fauzi said the subduction zone could give birth to “the big one” as early as tomorrow, in the next 50 years or even over the next century.

Tectonic plates covering the earth, also known as the lithosphere, are constantly on the move. The shifting generates huge amounts of heat and force along the boundaries, which releases at breaking points in the form of quakes or volcanoes.

Fauzi said the earthquake that hit Jambi on Thursday morning was not related to the tremor near Padang.

The 7.0-magnitude quake had an inland epicenter, not under the ocean.

“Therefore, they are two totally different kinds of earthquakes,” Fauzi said, “they just happened to occur within hours” of each other. Some 18 hours before the Padang quake struck, another 8.0-magnitude tremor hit near Samoa in the Western Pacific, triggering a deadly tsunami that killed at least 150 people.

Danny said that the Samoa earthquake was not at all related to the Padang quake, and are located some 7,600 kilometers apart.

Danny said Wednesday’s tremor might be related to the one that hit Tasikmalaya in September, which could be felt in Jakarta.

“We don’t know for sure” if they are directly related, Danny said, “but the two epicenters are located in the same subduction zone.”

Additional reporting by AFP