Seismic Hazard & Its Vulnerability Reduction in Guwahati City

Seismic Hazard & its Vulnerability Reduction in Guwahati City

                                                                                                            Moon Ch Deka 

   One of the natural phenomenon, the earthquake give rise to seismic hazard to the part of the globe is very specific from the identification of seismic belt located in the earth crust. It is known by everybody that the Northeast India is seismically one of the six most active earthquake prone zones in the world the other five being are the Mexico, Taiwan, California, Japan and Turkey.   As concern to the possibility of tremor in the Northeast India, it is always responsible for the surrounding seismic belt located between the Indian and Eurasian Plate which are known as Himalayan colliding zone in the north and Burmese arc’s sliding zone in the east. Besides it, many intra_plate seismic boundaries (away from edge of the main plate margin) are also very active due to behavior of neo-tectonic activities in the Meghalaya, Mikkir Hills, foothills of Assam _ Bhutan region and its adjacent Region. From the USGS earthquake’s catalogue, all total 25 numbers of significant earthquakes occurred in this part during the last hundred and thirty nine years. It has also been found that the seismic frequencies are very irregular with the low gap which ranges approximately from 0 to 28 years. There are no major earthquake has been occurred in this region since the last 20 years. At the same time, diving margin in the Sumatra trench and colliding zone in Sichuan Province have been struck by innumerable numbers of tremors in the recent past. Out of 25 numbers of past earthquakes there were seven great tremors originated in intra-plate margins in Meghalaya and its adjacent region. The some great earthquakes occurred in this part in the year of 1869,1897,1930,1947,1949,1950,1988 which had caused of seismic hazard and flash floods in the past. The intraplate’s originated earthquakes in the Meghalaya Plateau are generally big in size and ground shaking duration is also longer along with medium focus depth of 60 to 80 kms. On the other hand, most of the earthquakes originated in the Arunachal Pradesh are the type of very shallow focus which ranges from 0 to 20 kms. These causes reveal that the whole Northeast part is a highly vulnerable and prone to earthquakes of intensity in Magnitude 8 or more. In general earthquakes at the Himalayan Colliding zones are very shallow focus and recent earthquake of Siuchuan Province of China was originated at a depth of 29 kms with the 7.9 magnitudes for which about 80,000 people had been killed. This epicenter was located around 200 kms away from east of international border of Arunachal Pradesh .The 1950 earthquake occurred in border of Assam –Arunachal Pradesh was also a shallow focused which caused disastrous in upper Assam.

             Moreover, the longer the seismic gap has the possibility for occurrence of greater earthquake size which may caused more seismic intensity in terms of risk buffered hazards. There is a seismic gap in the Meghalaya plateau after the 1897 tremor in the Chidrang of Meghalaya where Dhudhnoi fault and many fault had crated in this area. It has known that the impact of earthquakes in the year of 1897,1930,1950,1988 were very significant as concern to the ground acceleration and its velocity. There is no doubt, that Guwahati is not a major concern to the possibility of significant earthquake in this part. The seismic hazard at mega city is controlled by active fault and other lineaments located within a distance of around 40 to 130 kilometers. The recent light size of earthquake having magnitude of 4.2 in the Richter scale had not a significant one but it’s a real concern for the short epicentral distance of around  38 kms from the heart of the city. The city is controlled basically by Shillong plateau, eastern foot hills of Himalayas,   some tectonic lineaments and faults in and around the city.

                The country’s 12% of area falls in the zone V where NE India is itself 8% of the country’s spatial dimension. The other city falls in the zone –V is Srinagar and Bhuj. The cities such as Delhi, Dehradun, Jamnagar, Patna, Meerut, Amritsar, Jammu, Jalandhar are accrued in the Zone-IV. But theses cities are different from the Guwahati city from the point of surface, subsurface geology, position of subduction zone and occurrence of earthquake types.  The earthquake is always posing threat to the Dehradun city where it had occurred in the year of 1905 and 1991. From the views of tectonic earthquake and socio-economic position the Dehradun city is similar to the Guwahati city but some are not. These are population (around 5.5 lakhs), area of city (only 65 sq km), lithological uniform formation i.e large Doon Valley, topographic conditions (moderate slope and flat), safe physical infrastructures and its mass awareness etc. The Guwahati city is on the valley fill deposits having irregular thicknesses overlies the hard rock terrains along with the remnant isolated Precambrian Hills whenever the Dehradun city is on river terraces deposits mostly. The centre of Doon Valley, the Dehradun City has not been fabricated with abrupt high rise apartment. But, it is not necessary that in an earthquake prone area would not be fabricated by mega infrastructures in this millennium era if there are strong management plan and  preparedness s to mitigate the earthquake vulnerability.

                In India, earthquake disaster management plan has geared up after the seismic hazards at Latur and Bhuj-Ahmadebad. In earlier, India’s disaster management plan was concentrated in the field of rescue operation and distribution of relief and rehabilitation. But, recent trend of thematic disaster management plan has been changed and it has given more importance in the technical evaluation of   quantitative pre disasters scenario in details. These assessments of scenario would definitely help to make favorable preparedness and response plan to mitigate the risk seismic hazards in any part .But this preparedness and response plan is different respective to the places or like cities. The awareness and capacity buildings are another two components in the seismic vulnerability reduction and its disaster management.

                Under the GOI-UNDP risk management programmed, the seismic hazard microzonation of  38 numbers of cities over half a million population in seismic zone III, IV and V have mapped .The Guwahati city was included in this earthquake vulnerability reduction project and thereafter, seismic hazard microzonation of Guwahati city had also been completed at the 1:25,000 with the help of various relevant Government Organization. It is a tremendous milestone for the Government of Assam to achieve such type of seismic hazard mapping at Guwahati city because of first step and right direction to reach the goal of favorable preparedness and response plan in the probable seismic hazard at Guwahati city. The second milestones for Government of Assam to have a Finalized Guwahati Master Plan 2025 which will help not only to planning of Guwahti city but it will help also to develop a  Municipal GIS   in the forthcoming years. As a result it will be more positive action in carrying out the preparedness plan for mitigating earthquake disasters in the Guwahati city. The third achievement made by the GOA is to go for adoption of methodology for Rapid Visual Screening of the Buildings in Guwahati city which will help to carry out the type of buildings which may susceptible different grade of damages. The state capacity buildings at all levels are going on through the different project and schemes. The Incident Command Systems, a USAID-INDIA collaborative project running under the Ministry of Home Affairs, Government of India , which   is one of the methodology for growing capacity buildings through the training by the Assam Administrative Staff College to   grow professionalism and skilled human resources to give excellent response in the strategic time. Preparedness and responses through the practices of mock drills at different institutions is also an important component in the disaster management. It will help to create awareness amongst the peoples as well as to think favorable options to give better response in the probable disaster. These above components are the part of to harness the success of preparedness and response plan under the disaster management system. There is a need to do more work to harness the goal of success in this field of management of seismic vulnerability.

                Preparedness and response plan is different subject to the different geographical location of the cities. A preparedness and response plans of Guwahati city is not suited to the plan of Shillong city. Even also response plan of hilly stations between Shillong city and Mussoorie city will not be fitted. Because a preparedness and response plan to the probable seismic hazard is depend upon various factors such as geographical location, geomorphologic pattern, geological setup, probable size and intensity of earthquakes, soil conditions, population distributions, Municipal GIS including road networking system , availability of subsurface information, grade of social & Physical infrastructures, network communication including earthquake warnings, proportionate resources and equipments , efficient management and peoples participation etc. etc.   

            The probabilistic analysis of seismic hazard in the database management system is a valuable tool in the recent trend of earthquake disaster management. The database format has emerged as a powerful simulation for the efficient disaster management due to its quick reliable decision and its accuracy assessment with the help of proper software including other technological facilities. The latest development in Remote Sensing, GIS and GPS are the only tools that it can help in application to carry out the disaster’s mapping and management with the help of integration of different thematic mappings. Development of a techno-legal regime is one of the components of vulnerability reduction project of GOI_UN; therefore, there is a need to strengthen infrastructure and updated facilities in the relevant Government departments in the field of Geology, Hydro_geology, Environmetal Engineering, Structural Engineering etc. As concern to the preparedness plan for seismic hazard  in the Guwahati city , it also need for detail Seismic hazards mapping after considering different thematic options such as  present seismic microzonation of city , land used , present population, physical infrastructures, more subsurface data including geo-hydrology etc. etc.. It may also require more strong seismographs network for monitoring and analysis of the seismic behaviors to provide availability of regular earthquake bulletin systems. Rapid Visual Screening of Buildings is also a methodology adopted by the Government of Assam to find out the building types and possibility of grade of damages in any earthquakes. These together will act as a seismic database of the city for effective mitigation in probable seismic hazard. The evaluation of quantitative seismic hazard is possible with the help of these thematic integrations. Once assessment has been made out it will help to find out the plan of calculative seismic disaster preparedness. Disaster preparedness involves many branches of Scientific Departments, Civil Defense, Police & Fires, Administration, Indian Metrological Departments, NGO,s , Medical etc. A disaster preparedness and techno legal component needs many thematic parameters of demographic, geomorphic, social and physical infrastructure including road networking, and infrastructures & facilities.

There is a frequently asked question that in spite of having an earthquake prone area, there are many metro cities built in the Japan then why not in Guwahati? To meet the query, firstly geographically and geologically it is not similar to Northeast Region. It is true that Japan is very experiencing country to face with an earthquake that had occurred frequently due to position of trench zone in the Pacific Oceanic region and received moderate to large magnitude of earthquakes. Secondly, they are the top as concern to the earthquake resistance technology in the world and they have strong seismic observation facilities. It is very strange, that they have all total 5904 numbers of seismographs including high sensitive seismographs, broadband seismographs and strong motion seismograph apart from 1500 of GPS, SLR monitoring the crustal deformations. The seismographs have been placed in surface, subsurface and also under the oceanic water. Recently they have developed the technology related with earthquake warning bulletins within two minutes of source of earthquakes. But it could not be successful in the Kawasakimachi prefecture due to arrival of S waves within 22 seconds. Yet, this bulletin will help in near future for forecasting of earthquakes. Thirdly, Government is very strict in promotion of earthquake resistance design along with standard quality of construction materials apart from availability of skilled lab ours and peoples are also well preparedness to mitigate the earthquake hazards. Fourthly, all the parts of Disaster management (Pre, post and during) plan are very advanced in the world. The 64000 elevators in Tokyo stopped running as soon as earthquake begins and equipments of control operation works automatically.

Forecasting of earthquake in terms of size, location and real time has not been fulfilled till now. But long term probability of earthquake prediction has been given on the basis of crustal deformation, interval occurrence etc. But there is a maximum probability for true prediction of earthquakes keeping view with worldwide advance researches. Till the true prediction we must have a pre-disaster plan in this city as well as to the region. The assessment of damages and destruction of buildings along with estimated danger of life respective to the size, depth, source of probable earthquake in different seismic belt of location will be fruitful measures with the help of integration of different thematic mappings in the field of seismic hazards. The identification of primary and secondary risk zone has come more prioritized to evaluate the assessment of damages in seismic hazard mapping. The vulnerable and risk zones are to be identified in the Guwahati city where there is a maximum possibility of damages at the earthquake disasters. To find out the vulnerable and risk zones it will have to consider different thematic options seismic, geology, socio-economic conditions, physical & social infrastructure, technology etc apart from mainly of seismic microzonation of city. The secondary risk and vulnerable zone is also a major criterion because earthquake proof buildings may collapse in the vulnerable zones due to liquefaction or submergence. This has occurred in the recent earthquakes in the Japan and China. Even the safe Assam Type house may come under high risk zone below the high rise apartment having necked short column in the ground floor situated on earth filling deposits. The secondary risks hazards may be broke out from the sources of  fire, landslides, road blocks by probable debris, electrical networking, telephone networking etc. The different themetic action plans may have been  prepared in the digital database format to mitigate the probable risk hazards in the city. These   preparedness plans are required urgently as concern to the  mitigation of the  probable seismic hazards in the Guwahati City .                                  

Conclusion: Till the Pre disaster management plan is completed the citizens must have follow the suggestions during and after an earthquake:

§  Locate the safe open spaces near your house where no eclectic line passes.

§  Try to stay in the plain level in the hilly areas and come away from abrupt elevation spot and go away from undercutting of soil and boulders.

§  Don't run, and move as little as possible to avoid falling debris.

§  No need to panic if you are inside a building.

§  Try to come out from the building where it is situated just near the wet land or on the beels.

§  Avoid ground floors as these tend to be crushed first if a building collapses. If you can't escape, go under a table or desk to protect yourself from falling objects.

§  Big earthquakes usually have 3 phases of tremor. The first tremor is almost perceptible to humans, but it can be understood from the abnormal behavior of animals like dog, birds, snakes etc. A few seconds or minutes later it may strike the second tremor, which also humans can feel. Wait till it has stopped, then rush outside if you are near the exit, or go away from tall buildings.

§  Go away from the riverbanks of Brahmaputra, Bharalu and also from wet lands of the city.

§  In general, second or third tremor is usually the most powerful. So, after the first tremor try to come out from the houses and stay for at least half an hour in open spaces.

§  Individual Public Buildings i.e. Schools, colleges, hospitals, markets, Offices should make the strategic plan for their better preparedness.

§  LPG plants at Amingaon, Storage at Bamunimaidam, Uzanbazar etc Noonmati Refinery and others should be made proper plan that fire could not break out at any instant.

§  As soon as you feel tremor immediately switch of the regulator of LPG if it is on.

§  Proportionate Machineries and equipments: Excavators, Bulldozers, Tipper Trucks, Hydraulic Jacks (Platform type), Hydraulic Cranes, Detectors, Pneumatic Cutters etc which are essentials in an earthquake prone city like Guwahati.

§  Before going to purchase a plot, to know the subsurface conditions of the area please consult with a Geologist first and see the Master Plan of Guwahati city and finally take the latest earthquake resistance design from a structural Engineer apart from concrete Government Norms and policy.

§  Before construction of high rise buildings first drill for taping ground water and analyzed your underground formation by a geologist apart from soil testing and others Governments Rule and Regulations.

§  Earthquake does not kill directly except liquefaction and sand craters but physical structure kill the peoples, so always keep in mind for earthquake resistance structures and accurate soil testing.