IT MAY SO HAPPEN THAT THE SOIL WILL TURN VISCOUS LIQUID…..EXPERTS SAY, DO NOT PANIC, BUT YOU MUST FOLLOW PRESCRIBED BUILDING RULES.
Entire Tripura is susceptible for earthquake as the state is located on highly vulnerable Zone 5. However, a recent seismic susceptibility study carried out by an expert team of the Geological Survey of India recently clearly pointed out that Dharmanagar in North Tripura appeared most susceptible due to various factors like proximity to seismic sources, underlying lithology, geomorphology, geophysical and geotechnical parameters.
The GSI report said, the study on the seismic susceptibility of Dharmanagar was prepared by considering seven seismic contributing factors- liquefaction susceptibility, geology, geomorphology, predominant frequency, peak ground acceleration, peak amplification, shear wave velocity by index by Analytical Hierarchy Process (AHP).
The severity that Dharmanagar might face could very well be guessed from the fact that it had a serious threat of ‘liquefaction’ which meant in case of massive earthquake hit the solid ground soil would just melt down—become ‘viscous liquid’. And if it did so, the magnitude of the catastrophe would be simply apocalyptic.
There was one more thing about Dharmanagar. The study team found driftwood deposits at 3-7m depth with a thickness ~2 m, which act as a cushion during seismic wave propagation. That was why Dharmanagar always felt less tremor during recent earthquakes. The driftwood deposits also indicates that there had been massive earthquake devastation in Dharmanagar area once in the past —- several thousands years ago , may be.
The study said, “20% of the area ( of Dharmanagar) falls in very high seismic susceptible zones, which covers the entire synclinal valley covering areas like Chandrapur, Dewanapasa, Hurua, and major part of Dharmanagar city. As much as 40% of the area falls in the high susceptible zones, which covers the south of Ganganagar, south of Kameshwar and east of Haphlong. Moderate seismic susceptible zone covers 28% of the study area, which include east of Latugaon, southeast of Hura, and west of Chandrapur. Low seismic susceptible zones cover an area of 12%, which covers the anticlinal/hilly areas in the east and patches south of Kalikapur”.
The report added: The seismic susceptibility map of Dharmanagar supports of present geology, geomorphology, geophysical, and liquefaction potential of Dharmanagar. The younger alluvial sections have very high to high seismic susceptibility levels, where the predominant frequency is very low and -liquefaction potential is critical to very critical.
Moderate and low seismic susceptible areas are mainly covered with Tipam formation, part of highly dissected hills and valleys.
The study pointed out, “Dharmanagar, the second largest city in Tripura, falls in the seismically active Tripura Mizoram fold belt (outer wedge of IBR). Dharmanagar falls in Deo- Juri –Dharamanagar syncline. This syncline is bounded in west by Machhlithum- Sakhan Anticline and east by Langai – Jumpui Anticline. Further Khubai and Patharia anticlines marks the southern and northern boundary respectively. The entire area falls in the strongly deformed domain (SDD) of outer wedge. The stretches of these hill ranges from north to south consist of numerous faults both across and parallel to the hills and plains play a significant role in controlling river flow on many occasions”.
“Earthquake catalogue studies shows, the main earthquake which occurred near Dharmanagar is the 1918 Srimangal Earthquake (Mw=7.6). However, the destruction during that time in Dharmanagar was meager due to less population and urbanization”.
But, recently the population in Dharmanagar increased manifold and unscientific urbanization made this area more vulnerable to seismic activity.
The GSI report warned: …Shallow groundwater condition makes this area vulnerable to seismic amplification. Geotechnical studies and liquefaction potential studies imply that the present Dharmanagar city and nearby areas are susceptible to both seismic and liquefaction events. In addition to the above, another interesting thing observed in Dharmanagar is the occurrence of driftwood deposits at 3-7m depth with a thickness ~2 m, which act as a cushion during seismic wave propagation. However, recent unscientific urbanization along with over-exploitation of driftwood may make this area more susceptible to seismic events.
The GSI also came out with specific recommendations for Dharmanagar :
Dharmanagar falls in the strongly deformed domain of the outer wedge of IBR and part of the Deo-Juri- Dharmanagar syncline. Hence tectonic activity is prominent due to the ongoing subduction of Indian plate. Detailed studies on neo-tectonics faults, morphotectonics of drainages in this area, and DGPS studies will give an insight to the rate of tectonic activity in this area. Furthermore, continuous observation of spatial and temporal characteristics of total electron content (TEC) in tectonically active areas using GPS will help in understanding ionospheric perturbation and accordingly earthquake occurrences.
Overexploitation of groundwater and driftwood for domestic work may be avoided.
For any engineering construction liquefaction study needs to be carried out in detail.
Moreover, the earthquake and liquefaction resistant engineering parameters; preferably, IS 4326:1993 Earthquake Resistant Design and Construction of Buildings– code of practice and ISS 6922:1973 criteria for safety and Design of structures Subject to Underground Blast (www.bis.gov.in/other/quake.html; BIS 2002) is recommended for any future development in Dharmanagar.
The new bypass constructing over the seismically very high to high susceptible area, part of active TMFB. Road construction without proper slope management may cause subsidence or liquefaction-related problems. Proper InSAR studies and slope stability measurements are paramount.
Furthermore, the recent 14th August 2023 Bangladesh earthquake (5.4 M) has affected the hydrogeological condition implying the importance of continuous hydrological monitoring and InSAR studies are important in this tectonically active area.
When contacted the State Project officer of Disaster Management of Tripura and an expert Dr Sarat K Das said, despite the highly vulnerable position of Dharmanagar there was no need for panic.
He said, “ Dharmanagar people should not panic to the event rather they should adhere to Tripura Building Rules and National Building Codes and safe construction practices for their safety of their lives and properties”.
“Any development activities should strictly follow the seismic microzonation report to consider what kind of development activity is suitable, steps should be taken up for vulnerability assessment of built environment for both government and private structure and take up of retrofitting of vulnerable buildings, North Tripura district should create more trained masons, artisans for ensuring safe construction practices in the ground, Regular earthquake awareness programmes should be conducted at all levels”.
(Also published in the Tripura Times)
