Earth is now in a seismically active period according to quake cluster theory

Following the second major earthquake that hit Nepal on 12 May, some scientists believe we could be seeing a seismically active period that began in 2004 and could end around 2019.

However, predicting the quakes more accurately in time and space still remains a task for the future.

The magnitude 9.1 Sumatra-Andaman earthquake of 26 December 2004, was one of the largest earthquakes ever recorded, triggering a tsunami that killed over 200,000 people.

Experts like Sun Shihong, research fellow at the China Earthquake Networks Centre have in the past spoken of a 100-year cycle for earthquakes.

In the first 60 years of the 20th century there were seven earthquakes above 8.5 on the Richter scale. In the following 40 years there were no major quakes. Sun believes a new cycle began at the end of 2004 with a massive 9.1 magnitude quake in Indonesia.

More recently, Zhang Zhang Xiaodong, deputy director of an earthquake forecast research institute under the China Earthquake Administration told Xinhua that since 2004, the world has suffered a frequency of quakes above 8 magnitude not seen since the first half of the 20th century.

Rajender Kumar Chadha, chief scientist at the National Geophysical Research Institute (NGRI) in India also believes that we could be in a seismically active period.

In fact, he had said two years ago that the planet is into a very seismically active period that could see earthquakes of magnitude greater than 8.

The twin earthquakes that hit Nepal in April and May this year have killed over 8,500 and many are still missing, making the disaster the deadliest to hit the Himalayan country on record.

Quake clusters

Earthquakes of magnitude 8 and above can occur along any of the collision or subduction zones where two plates collide, whether continent-continent collision as in the Himalayas or ocean-continent subduction zones like the Pacific Ocean plate with different continent plates along the 'Ring of Fire.'

Great earthquakes of magnitude 8.5 or higher occur infrequently but are found to cluster in time, says Chadha, based on the analysis of 115 years of global data. Such "clustering" is not found for magnitudes lesser than 8.0 which show an annual average of about 15 to 16 every year, globally.

Data analysed from 1900 onwards based on the catalogue available with United States Geological Survey (USGS) indicate there have been three possible periods of clustering of magnitude 8.5 or greater, lasting 15 years, which can also be referred to as "periods of seismic activation" of great earthquakes, he told IBTimes UK.

The first was in the period beginning with 1905 till 1920 when there were six earthquakes exceeding magnitude 8.5 and several closer to 8.5.

Next followed the period between 1950 to 1965, when seven earthquakes exceeding magnitude 8.5 occurred which included three of magnitude 9.0 or greater. During this period, the greatest known earthquake of M9.5 occurred in Chile in 1960.

"The third clustering in time of great earthquakes seems to have begun with Sumatra earthquake of magnitude 9.3 in 2004 and till 2015, already six earthquakes exceeding magnitude 8.5 have occurred globally, after Sumatra, including one of magnitude 9.0 in Japan in 2011," points Chadha.

Going by this pattern indicating a "period of seismic activation" lasting for a 15-year cycle thrice in the last 115 years, we could now be in the third cluster, he says.

"This can continue till 2019 and earthquakes with magnitude 8.5 or greater can occur along any of the collision or subduction zones of the world."

Himalayan quakes overestimated

Regarding the Himalayan belt and North India which are in the high seismicity zone, since we already have two earthquakes of magnitude 9.0 or greater in the present cluster, do we expect another magnitude 9.0 earthquake either in the Himalayan region or any other collision zones?

At 20mm a year, the India Plate is moving rapidly compared to plate movement at the San Andreas Fault in California, which is just 2mm a year. Preliminary data suggests the 25 April quake created a slip of 4.5m along a 150km fault.

"As [the India Plate] is moving 4.5 metres, it's not the entire fault surface that's moving…the surface hasn't moved yet," according to Laurent Godin, a structural geologist and professor at Queen's University. "There hasn't been a fault rupture there yet. Meaning the fault hasn't released all its stress."

"There's certainly room there for another 7. Will it go eight? We know the fault system is capable of going to an 8.4 because it has in the past," says Don Blakeman, a geophysicist with the US Geological Survey.

While ruling out a definite reply, Chadha however is inclined to think a 'big one' in the region in the future may be an overestimate.

He refers to a simulation done by Robert McCaffrey, a New Zealand seismologist, to check for recurrence of magnitude 9.0 earthquakes based on the length of trenches and convergence rates.

The simulation, he says, suggests that the global occurrence of M9.0 earthquakes is one to three per century and the five that occurred in the past 100 years is higher than long-term averages.

"This implies the occurrence of a 9.0 magnitude earthquake in the Himalaya in the near future may be an overestimate on the count that i) no such event has ever occurred along a continent-continent collision zone and ii) if the base year is considered from 1950 onwards then we have already experienced 5 earthquakes of magnitude 9.0 or more in the last 65 years which is higher than the long term average," Chadha says.

He is quick to add that such estimates cannot be answered with conviction since our records are not long enough to observe the processes which generate these super great earthquakes.

Indo-Gangetic plain vulnerable

Chadha believes there is need for preparedness in the adjoining regions of the Himalayas like the Indo-Gangetic plains where major cities with large populations exist.

"Due to the presence of thick column of sediments, the Indo-Gangetic plains is prone to amplified shaking because of amplification of seismic waves from the Himalayan earthquakes.

"Earthquakes exceeding magnitude 8 in the Himalaya will pose a threat to the structures in the region, if they are not designed as per seismic codes available in the country," he says and calls for awareness amongst the people, government officials and decision makers to the earthquake hazard in the region.