Dhaka’s earthquake risk rumbled to the surface early yesterday after a M5.1 earthquake jolted the city awake at 12:49am, followed by a M4.6 aftershock nearly 20 hours later at 8:28pm, both with epicentres some 407 kilometres away in Mawlaik, Myanmar.
Wednesday's tremors followed a M5.5 earthquake at 3:09pm on January 3, epicentred just 147 kilometres away in Ambassa, Tripura.
"Research has found that the Indian plate and the Burmese plate are locked, creating immense pressure due to their counter-motion," says Dr Syed Humayun Akhter.
"Tuesday’s quake originated in the locked zone where this geological pressure is building and that carries a very significant meaning for us."
Columbia University’s Michael Steckler, who led the research study published last year, found that a locked and loaded megathrust fault buried under miles of sediment could unleash an earthquake up to magnitude 9.
Geological experts have found that millions of tonnes of sediment from the Ganges and Brahmaputra rivers have been dumped into the megathrust fault, where one of the earth’s plates is being pushed under another, obscuring its exact location.
Humayun explains: "The immense pressure caused by subduction – when one tectonic plate moves beneath another – has not been released by a major earthquake. It has been building."
Magnitude versus intensity
Magnitudes of this level on the Richter Scale may not seem high, but the alarm felt by residents is warranted due to the intensity they create - as measured on the Mercalli scale. The M5.5 tremor on January 3 saw city residents and even the President of the Republic descend from office buildings and into the street.
"The intensity scale of an earthquake normally gets bigger in our country because of its alluvial soil," says Humayun.
According to the United States Geological Survey, intensity and magnitude measure different characteristics of earthquakes.
"Magnitude measures the energy released at the source of the earthquake. Magnitude is determined from measurements on seismographs. Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from the effects on people, human structures and the natural environment," the survey says.
Humayun says the intensity of an earthquake varies from place-to-place, even though the magnitude or energy released will be the same in other parts of the world as at the epicentre.
"The intensity scale depends on local soil conditions. The intensity can be felt less in hard soil than in soft soil conditions. Earthquake damage depends on intensity and soil conditions," says Humayun.
Following the M5.5 event on January 3, eruptions of mud, sand and water were observed in parts of Moulvibazar district.
Eruptions took place at Kamalganj High School playground about 30 minutes after the tremor. Similar incidents of mud being spewed from the earth observed along the river bank in Kamalganj were captured and shared on social media sites like Facebook.
'Major eruption could occur soon'
Humayun says eruptions of liquid and mud after an earthquake are a normal occurrence, especially in alluvial areas. Eruptions of saturated sand are caused by fissuring and cracking of layers of the earth’s surface due to the immense pressure building up beneath it.
"But this is being observed in the country for the first time in a long time," he adds. "We have not seen this in the last thirty years. This carries a significant meaning – a major eruption could occur in the near future."
"In the past, the area had been in a latent position, but now it has begun to be unlocked with very slow motion. Further small or medium quakes might take place with gaps of two to four years.
"This could increase to an event of massive energy that will be severely felt in our alluvial country."
Humayun believes the capital would be severely affected by such an earthquake.
"We must be prepared for this."