Earthquake in Chittagong |
Dr. Aftab Alam Khan
If an earthquake occurs in Chittagong what will happen? This is a pertinent question since we all are concerned about our vulnerability to earthquake damage.
Earthquake occurring anywhere is felt at long distances but certainly the potential damage is done at the place of occurrence characterised by active fault movements. Intensity of fault movement is markedly reduced with increasing distances of the felt region from the epicentre. As long as earthquake wave is propagated quickly through the geologic materials, the level of damage is less but if the same is attenuated in the passage of propagation the damage level is relatively high. Nevertheless, it is of prime importance to identify whether an area characterises an active fault, what attenuation character does it have. Then the next question is whether an area is vulnerable to earthquake occurrence or not. If so, what probability does it have pertaining to maximum magnitude, recurrence period and ground shaking scenario.
Recently, a field investigation has been conducted in Chittagong region. Certainly a bit of concern, if not panic, cropped-up about the disastrous episode if an earthquake is epicentered right at Chittagong! My deep concern is focused especially on two areas, one within Chittagong metropolitan city and the other in and around Chittagong University. Before highlighting the earthquake scenario and vulnerability status of Chittagong, I would humbly express that some human activities, out of ignorance, especially the hills cutting and leveling have already increased the vulnerability level of damage in Chittagong. The main concern is with isostatic balancing. The removal of materials from the hills has imbalanced the area significantly from isostatic point of view in addition to its naturally occurring fragile subsurface geologic status.
The information on earthquakes in Chittagong region is available from historical records. It suggests that in December, 1830 Chittagong was affected by severe shocks followed by loud noise from south. Most of the houses were severely cracked with an estimated ground acceleration of 0.1g. Earthquake was also felt in October, 1842 having ground motion in east-west direction and the vibratory motion in north-south direction. It is reported that clocks were stopped oscillating during earthquake. There were also two earthquakes in the years 1851 and 1865. December, 1865 earthquake was reported to have been the severe one that badly cracked most of the buildings and was accompanied by a noise like that of coming of a northwester. There were five shocks between 07:30 and 10:30 pm. The first one was severe and lasted about 2.5 minutes.
However the past earthquake record does not imply much severity but certainly indicates area vulnerability. In the recent times, last thirty years, improvement of earthquake recording and monitoring facilities globally, however, does not contradict with the earthquake vulnerability status of Chittagong region. The map generated with the earthquake data since 1973 from USGS record suggests that Chittagong region certainly is characterised by earthquake source zones. These source zones clearly follow a definite trend along well defined faults.
The earthquake of November, 1997 with a magnitude of 6.1 could create much concern because of the sinking of two floors underground of a five-storey building burying and killing 32 people at Hamzarbagh, Chittagong. The irony is that this earthquake was located (epicentre) far in the east of Bandarban outside the international boundary. Although there was considerable damage and trauma in Bandarban area, it failed to draw much attention. Similarly, July, 1999 Moheshkhali earthquake (Magnitude 5.2) has not been given due attention in spite of widespread damage and loss of property and also lives. The survey in 2000 for earthquake damage estimation revealed that 8000 people in seven unions were severely affected, seven persons died and 24 persons were seriously injured, 1292 houses were fully damaged with 5662 partially, 10 cyclone centres, hospitals, and other structures were damaged. The estimated financial loss was about 14 million Taka. However, the July 27, 2003 Kolabunia earthquake in Rangamati district with a magnitude of 5.6 could draw the attention due to its widespread damage in terms of collapse of brick and mud houses, occurrence of extensive zones of fractures, large surface craters and series of sand boiling/liquefaction. Although the loss of life and property was much less in comparison to Moheshkhali one, nevertheless it was a wake-up call.
When we look for earthquake damage assessment in and around Chittagong in an impending earthquake, the scenario is quite alarming. There are no scopes of being reluctant as we definitely could locate 7 to 8 epicenters close by Chittagong between 1989 and 2003. Although the magnitudes were only between 4.1 and 4.5, it certainly tells us the existence of source and active fault zones right across Chittagong. The very recent field investigation has revealed that the subsurface condition in some parts of Chittagong city and Chittagong University campus are quite vulnerable and alarming from earthquake damage point of view. These vulnerable sites are distinctly characterised by faults and lubricating material like clay, mud and shale at very shallow depths. The two subsurface imaging, one from Chittagong city and other from Chittagong university campus reveal very vulnerable sites. However, there is one finding a bit relaxing. The present accumulated strain can only precipitate in an earthquake of maximum magnitude of 6 if it is released in a single event. On the other hand, another finding supports a maximum ground shaking scenario of about 0.4g in some locations of the vulnerable sites on the fault plane. This finding is alarming. The next question is, when the 6 magnitude earthquake would occur? The recurrence time calculation suggests a mean value of 17 years for an earthquake of magnitude between 6 and 6.9 while it is six years for magnitude between 5 and 5.9
Last but not least, earthquake cannot be prevented but we must be aware of the facts and should plan and execute accordingly in order to minimise the loss of life and property.
Dr. Aftab Alam Khan is Professor, Department of Geology, University of Dhaka.