Workshop on Earthquake Disaster Preparedness and Mitigation Organized by Identification Mission Consultants to the EU Delegation-Bangladesh February 18, 2004, Dhaka, Bangladesh Assessment of Earthquake Risk in Bangladesh Keynote Speech by MIR FAZLUL KARIM Director Geological Survey Of Bangladesh Ministry of Energy and Mineral Resources Government of the Peoples' Republic of Bangladesh •There are some valid questions: •Is Bangladesh vulnerable to earthquake? •Should we be concerned about an earthquake when occurrences of earthquake damages are not so significant? •The country faces so many day-to-day problems related to environment, industrial pollution, traffic, water and power shortage, and annual calamities like flood, drought, cyclone and tidal bore etc., and then can we afford to ignore earthquake hazards? •Question is raised in the last years World Summit on Sustainable Development (WSSD) organized by United Nations for International Strategy for Disaster Reduction Can sustainable development along with the international instruments aiming at poverty reduction and environmental protection, be successful without taking into account the risk of natural hazards and their impacts? Can we afford the increasing costs and losses due to so-called natural disasters? •The short answer is, no. We need reduction of losses. •Earthquakes are the detectable shaking of the earth's surface resulting from seismic waves generated by a sudden release of energy from inside the earth. •Any landmass, which has experienced natural ground shaking in the past, is vulnerable to earthquake risk and thus liable to earthquake hazard and a severe earthquake can bring devastation to the economy of the country and we cannot ignore the earthquake aspects. •Let us try to assess the risk posed by earthquake hazard. • Seismic risk assessment rather a rationale assessment of an earthquake prone region involves a wide range analysis of seismological and geological data and engineering characterization of geological materials. • basic factors are related to geological processes, tectonic environment and geotechnical conditions WHY BANGLADESH IS AT RISK? The regional geological setup of the landmass is complex due to the juncture of Himalayan Arc of northeast India and the India-Myanmar plate subduction to the east. Seismicity of Bangladesh 1800 -2003 Four great earthquakes of magnitude exceeding 8 during 1897, 1905, 1934, 1950 and another 10 earthquakes exceeding magnitude 7.5 have occurred in the Himalayan belt during the last 100 years. The earthquake history of Bangladesh and surrounding region indicates that the country is seismically active. Affects of historical earthquakes in Bangladesh The Great Indian Earthquake,Shillong June 12, 1897. Magnitude 8.7 Jolted the whole Bangladesh even Delhi was swung for a while Mymensingh or Srimangal Earthquake, July 8, 1918, Magnitude 7.4 A good example of rapid energy fall-off due to soft sediments in Meghna valley The earthquake confirms presence of Active Fault in the transitional crust beneath Bengal basin Important - further study of this earthquake required to decide on recurrence Dhubri Earthqauke, July 3, 1930, Magnitude 7.1 The earthquake confirms presence of Active Fault in the eastern margin of Half Graben of Stable Shelf of Bengal basin Important - further study of this earthquake required to decide on recurrence, will provide essential data in understanding behaviour of the Jamuna River A potential source structure that could be capable to produce earthquake of equal magnitude January 15, 1934, Bihar-Nepal, Magnitude 8.3 The earthquakes of Main Himalayan Boundary Thrust will cause sympathetic jolts in seismic structures in the Bengal basin January 2001, 2001, Bhuj Earthquake, Magnitude 7.6 The Bengal basin is a Mirror Image of Kutchh basin in west India, the intensity of Bhuj Earthquake was V MM scale in different parts of Bangladesh Account of physical damages in Bangladesh The Great Indian Earthquake,Shillong June 12, 1897, Magnitude 8.7. Many buildings damaged in Bangladesh. Rangpur Mymensingh Dhaka City Muktagacha Khoawai Railway Track, Sylhet, 1897 Khoawai Rail Way Bridge, Sylhet, 1897 Affects of modern earthquakes in Bangladesh The recent earthquakes of Dhaka, Moheshkhali, Chittagong and Chittagong Hill Tracts including the Rangamati earthquake of July-August 2003, justify that the country is seismically active. July 22, 1999, Moheshkhali Island, Magnitude 5.2 Mb Produced Ground Rupture of few kilometers length and caused massive landslides A BIG QUESTION ON MAGNITUDE VALUE Sitakundu Rangamati LANDSLIDES and GROUND RUPTURES, Chittagong,1997 Magnitude 6 Mb Fault Line FAULT LINE Plate 1: Fault plain displacement along Barkal thrust. Abandoned Kalabonia BDR Camp, Barkal (southern top of hill). Plate 2: Fault plain displacement along Barkal thrust. Abandoned Kalabonia BDR Camp, Barkal (northern slope of hill). Rangamati Earthquake 2003 LANDSLIDES and GROUND RUPTURE Produced Ground Rupture of > 10 kilometers length and caused massive landslides S Submerged Point Bar Ground Fissure Plate 3: Submerged 200m x 50m agriculture land and houses Plate 4: Segment of a ground fissure, developed along 10-km visible crack The fault line goes through on the Point Bar and river bank in Kalabania Village shown in both picture. Lower picture shows Shattered surface of bank the bazar. area, mud cracks helped dismembering. LANDSUBSIDENCE and GROUND RUPTURES 2003 Submerged Point Bar GSB team investigated the landslide and slump failure along RangamatiChittagong Road due to earthquake at Manik Chari, Rangamati. Debris removed for traffic. This volume of slump material removed Due to earthquake large fractures (right) are produced in the shale and siltstone beds to generate sudden movement and landslides (below). Fractured shale 1997 earthquake Sli pp ed su rfa ce THE GROUND DEFORMATIONS DURING PAST EARTHQUAKES INDICATE THAT THE MAGNITUDES DETERMINED BY USGS, IMD AND OTHER GLOBAL SEISMIC NETWORK ARE RECORDED LOW WE NEED OUR OWN STATION FOR ACCURACY AND A REALISTIC EARTHQUAKE RISK ASSESSMENT THE WORK SO FAR WE HAVE DONE FOR SEISMIC RISK ASSESSMENT GEOLOGICAL MAP a basic tool for earthquake risk assessment NEED LARGE SCALE MAPS Lithological Units 1 and 2 5 Folded hills of Tertiary sedimentary rock 7 3 2 6 Pleistocene Terraces Barind and Madhupur 4 1 3 3 Old Alluvial Deposit (Chandina Alluvium) 5 8 4 9 1 Alluvial Fan Deposit6 Paludal Deposit Marshy clay & peat 7 Young Alluvial Deposit (Inter-stream deposit) 8 and 9 Deltaic and Coastal Deposit. Including Beach, Estuarine and Mangrove swamp deposits. A popular and widely used earthquake catalogue “Seismic Zoning Map of Bangladesh and Outline of a Code for Earthquake Resistant Design of Structures” THE FIRST OFFICIAL SEISMIC HZARD MAP OF BANGLADESH PUBLISHED BY GSB, 1979 BASED ON HISTORICAL DATA Revised Seismic Zoning Map of Bangladesh Bangladesh National Building Code 1993 YET BASED ON HISTORICAL DATA Bangladesh in Global Seismic Hazard Assessment Map STEP TOWARDS METHODICAL EARTHQUAKE RISK ASSESSMENT BOUGER GRAVITY ANOMALY MAP 1:1000,000 NEEDS CLOSE DISTANCE COVERAGE OF GRAVITY SURVEY AEROMAGNATIC ANOMALY MAP 1:1000,000 NEEDS DETAIL FOR UNDERSTANDING OF CRUSTAL CONFIGURATION CHARACTERISTS INVESTGATION OF EARTHQUAKE SOURCE STRUCTURE Understand the relationships among tectonic setting, structural and crustal configuration of Bengal basin to classify the earthquake source areas. Three types of earthquake sources areas are identified: a. Intra-plate source areas, b. Transitional source area and c. Inter-plate source area. Present study indicates presence of active faults capable to produce shallow focus earthquakes. The physical attributes and exact locations are not known. Epicenter locations of recent earthquakes in Bangladesh indicate that there are number of active faults in these source areas. The surface geology is simple for being a plain land but the subsurface geology is very complex due to basin’s deep undulated basement complex. The general seismic hazard boundaries will not be as simple as shown in the maps. So far no detailed maps have been prepared as modern facilities for earthquake research and monitoring are yet to be developed in the country. Geology of Bangladesh Understanding of source mechanism of a particular earthquake is an integral part of seismic hazard assessment. The existing facilities on earthquake research and monitoring do not help in determination of accurate location of epicenters. Study indicate that the tectonic behaviour of Bengal basin is very complex due to continuous active segmentation processes of shallow depth crust in the eastern margin of Indian peninsula. The deformation pattern are not distinguishable through surface geological studies and needs geophysical survey. The Bouger Gravity Anomaly and Aeromagnetic Anomaly surveys indicate a wide range of variation in the basement complex. Depending on the vertical continuity of crustal deformation three types of pattern are identified. •Intense deformation pattern with lateral crustal discontinuity exposed at the surface •intermediate deformation pattern with indistinct surface expression and •the deep and vertical crustal deformation with no surface signature. Geotechnical Engineering Consideration Seismic hazard assessment may turn wrong if local geotechnical properties are not accurately followed before design and construction of engineering structures. The earthquake effect at different site level geological condition (soil condition) can be predicted through geotechnical investigations. The basic considerations in determination of geo-technical behaviour of material are type of deposit, geological constraints and ground response at different depths. The young floodplain coastal deposits are very soft and saturated, low shear strength values up to the depth of 10m. Prone to liquefaction. The ground motion is amplified. Intensity trends of earthquake are subsurface structure controlled Tangail * * Intensity Map of Dhaka Earthquake 2001 showing the MM intensity contour lines. DHAKA CITY Arrow line shows the main Terrace Boundary Fault Zone (Banshi Fault), the possible source of 19 December 2001 earthquake. 3 1 Central High Land Low risk if structures are well-engineered 2 2 Complex of domes and valley type depression Moderate Risk 1 2 3 3 1 3 Low floodplain and depressions High Seismic Risk SEISMIC RISK ZONE OF DHAKA CITY CONSIDERING GROUND CONDITION CONCLUSION Occurrences of recent frequent earthquakes have increased the need of earthquake safety factors as the rapid growth of population and urban centers, communication and infrastructure has magnified the earthquake vulnerability in the region. Conclusion It has become essential to have a detailed and comprehensive seismic hazard map of Bangladesh as an accurate seismic hazard assessment of the country would reduce losses of lives and properties. Seismic hazard assessment of an earthquake region involves a wide range analysis of seismological and geological data and engineering characterization of geological materials. Conclusion Epicenter locations of recent earthquakes in Bangladesh indicate that there could be number of potential faults to produce unpredictable large earthquakes in the region. Conclusion Epicenter locations of recent earthquakes in Bangladesh indicate that there could be number of potential faults to produce unpredictable large earthquakes in the region. Conclusion Bangladesh is in need of standard seismic hazard map with detail information on the geometry and mechanics of these active faults. It is essential to have clear understanding on the actual relationship among the basic factors like - geotechnical behaviour of materials, local geological constraints (stratigraphic and structural) and ground response. Instrumental monitoring of recurring seismic events including the microseism would help to achieve reliable prediction of geological movements. RECOMMENDATIONS AND PROJECT CONCEPT Considering the present risk assessment it is recommended to focus the following work for minimizing earthquake hazard in the country and reduce vulnerability of cities and other infrastructures. This will also reduce the possible economic losses in one hand and the enhanced geological data will widen up the scope of finding new mineral deposits in the country and would provide economic gain. Recommendations and Project concept 1. Updating of earthquake risk map: Modernization of Geological Survey of Bangladesh for preparation of detail Geological Maps of the country for determination of active faults and seismo-tectonic evaluation of earthquake source structures. Including study on crustal segmentation of Bengal basin for realistic understanding of fault mechanism and updating of earthquake risk map. Recommendations and Project concept 2. Monitoring system for micro-seismic activity: Development of monitoring system for micro-seismic activity to determine the possible precursors (recording foreshocks), including possession of mobile seismic stations to approach site response during the long aftershock period. GSB needs international cooperation for such development including expert services and advance training and education. Recommendations and Project concept 3. Earthquake Recurrence Analysis: Earthquake Recurrence Analysis for the most active faults identified by mapping, detailed mapping work is required in order to come up with realistic recurrence intervals for larger earthquakes. Geological dating to pin down the dates of earlier fault offsets to determine which faults are truly the most dangerous from a ground motion standpoint. 4. Ground Response Analysis: Engineering geological mapping (1:5,000 to 1:10,000) of six large cities of Bangladesh for characterization of ground response by determining spatial distribution of lithological units in different geological environment including geotechnical behaviour of the geological deposits. The engineering geological maps will provide the relationship among acceleration, attenuation, ground motion, moment due to slip in fault andslope failures in the hilly cities/regions in terms landslide and mass movement or liquefaction. 5. Geotechnical Engineering Consideration Seismic hazard assessment may turn wrong if local geotechnical properties are not accurately followed before design and construction of engineering structures. The earthquake effect at different site level geological condition (soil condition) can be predicted if geotechnical investigations are executed involving standard methodology and professional attitude. For geotechnical engineering purpose large-scale seismic hazard maps are necessary to provide the relationship among the physical and shear-strength properties of subsurface geological material (soil type) and their possible ground response due to any particular earthquake. The young floodplain coastal deposits are formed of very soft unconsolidated saturated sediments (mainly silt, fine sand and organic clay) having low shear strength values as determined by Standard Penetration Test bow-counts or ‘N’ value (<15) up to the depth of 10m. The soft sediments are highly vulnerable to liquefaction and the earthquake energy fall-off is more rapid because of multiplication of acceleration than that of dense sediments. The ground motion is also amplified through soft sediments even for a distant event.