SEISMO-TECTONIC EVALUATION AND SOME ENGINEERING GEOLOGICAL SIGNIFICANCE OF 2003 BARKAL-RANGAMATI EARTHQUAKE

Proceedings, First Bangladesh Earthquake Symposium, Dhaka, Dec.14-15, 2005 SEISMO-TECTONIC EVALUATION AND SOME ENGINEERING GEOLOGICAL SIGNIFICANCE OF 2003 BARKAL-RANGAMATI EARTHQUAKE Mir Fazlul Karim1 ABSTRACT Tectonically Barkal-Rangamati and Chittagong region occupies a crucial part of IndiaMyanmar Mobile Belt. The regional tectonics indicates that at the edge of Indian Plate subduction the Barkal-Rangamati folds suffer optimum stress from the westward push of Myanmar plate. This tectonic movement caused generation of a moderate earthquake of Magnitude 5.1 that struck the districts of Chittagong Hill Tracts and Chittagong in the early morning of July 27, 2003. The earthquakes generated numerable large landslides, ground subsidence and even small-localized jolts or micro-earthquakes in the regions. Presence of active faults like the Subhalang-Kasalang and Barkal Thrust System causes the region to be prone to moderate earthquakes. This paper is based on the geological field investigation, carried out immediately after the earthquake which jolted the entire south east Bangladesh. The locations of fault scarps, landslides, ground fissures are explored in the field, measured their attitudes and mapped. The affected or partially damaged buildings are investigated for understanding of causes of failures. The Chittagong city and Rangamati Town are developed on folded and faulted terrain of Tertiary sedimentary. Due to the complex geological structures the grounds are susceptible to landslide and slope failure and needs consideration in engineering designs and construction. The young deposits of the valleys and floodplains are prone to liquefaction and many buildings in these regions are at the risk of earthquake damages. The affected engineering structures need immediate attention for repair and retrofitting as next earthquake of similar magnitude will cause severe damage to these buildings. The repetitive low magnitude earthquakes pose slow damage process (cyclic) to the building or other engineering structures. An individual geo-engineering investigation was carried out for an immediate understanding of seismic vulnerability of Kaptai Hydroelectric Power Dam site. The Kaptai dam occupies a greater part of a syncline and forms stable basin where a firm ground condition exists and no sign of damage could be found in the dam. The dam design is consistent to the geological structure but needs engineering maintenance and geological monitoring. Introduction A moderate earthquake of Magnitude 5.1 (BMD) in the Richter Scale struck the districts of Chittagong Hill Tracts and Chittagong in the early morning on Sunday at 05:18:17 AM of July 27, 2003. The earthquake was followed by about 60 aftershocks including two light and moderate shocks on Friday, August 01, 2003 at 08:49:38 AM and on Tuesday, August 12, 2003 at 21:56:00 PM. The magnitudes of these earthquakes were 4.8 and 5.3 respectively. A booming sound with strong ground shake made the people panicky and the aftershocks caused psychological stress to them as for many years they have not seen such ground movement. The epicenter is about 28 km NNE of Rangamati Town. Initial reports from nearby stations implied that the earthquakes were felt all over Chittagong Hill Tracts and Chittagong including the nearby Indian and Myanmar bordering region. 1 Director, Geological Survey of Bangladesh, Dhaka. azadkk@gmail.com or mfk@agni.com Mir Fazlul Karim The event caused severe ground shaking and created endless fear and uncertainty among the people of Barkal and Rangamati. Almost similar shock was also felt in Chittagong, Bandarban, Cox’s Bazar and Kutubdia, about 150km away from the epicenter. There was less visible infrastructure destruction due to the quake as not a single building had totally or partially collapsed but severely cracked. These cracked buildings would remain in safety dilemma. The socioeconomic state constraints the decision of abandonment of many cracked buildings. The jolt triggered several landslides and produced ground fissures (Plate 6) in the region and caused death of few people in Cox’s Bazar. The after-shock activity continued for long time and understanding of causes of such movement is essential to delineate the seismic behavior of region. Bangladesh lacks facilities for monitoring micro-seismic activity and the system is essentially required to observe the activity of local faults, evaluate the pattern of ground movement and the state of potential sections of landslides. Figure 1. Seismicity map (1990 – 2003) of Bangladesh showing the epicenter of Rangamati-Barkal Earthquake (in star). Figure 2. Map showing the structural geology of Bangladesh and eastern neighboring countries and major active earthquake generating source areas. Bangladesh is located beside the global earthquake risk belt. The history of earthquakes of the country and surrounding areas (GSB, 1979; Hossain, 1988; Hossain and Choudhury, 1998 and Karim et, al., 1990) indicates that many severe earthquakes have occurred in and around the country (Figure 1). Though the frequency of damaging earthquakes is low, one should not wait for prediction, but prepared for next. In any case, public safety concern is of prime importance and there should be adequate thrust towards understanding the earthquake mechanism, its impact and potential threat to the infrastructures, geo-engineering characterization of ground condition and assessment of possible natural and manmade disasters for effective management plan . This paper is prepared on the basis of a week long fieldwork in Barkal-Rangamati-Kaptai region and Chittagong City, available seismic data from different organizations (NIEC-USGS, GSI, IMD and GSB) and tectonic characterization of southeast Bangladesh ( Khan et al., 1998; Hoque and Khan, 2001 and Nandy, 2001) and data on engineering properties of Tertiary sedimentary rocks Proceedings, First Bangladesh Earthquake Symposium, Dhaka, Dec.14-15, 2005 (Karim et al., 1990). The strike slip displacements of the folded sediments are well correlated with the known tectonic structures along the Indo-Myanmar plate subduction process and transform faulting system located in the east. These active fault systems along the Kaladan Active Fault have generated innumerable earthquakes around the plate boundary zone where the frequency of events gradually reduces towards Bangladesh-India border (Figure 2). A preliminary Intensity Survey was also carried out on the basis of Felt Level and visible damage assessment (Karim and Khan, 2003). The type and pattern of ground ruptures and infrastructure fracture pattern indicate that M 5.1 earthquake at early morning of 27 July 2003 was felt at Intensity VI and VII within the epicenter area. Plate 1 Plate 2 Figure 3. Tectonic and structural map of Chittagong Hill Tracts showing the elements responsible for earthquake generation in the region (transverse and strike slip faults are in thick lines). Plate 1 and 2. Fault plain displacement along Barkal thrust at BDR Camp, Barkal (Top – Southern part and Bottom northern part of hill). Engineering geological significance Chittagong and Chittagong Hill Tracts district are located on the folded sedimentary rocks of Tertiary age. The sedimentary rocks are intensely folded and fractured. Most of the urban growth centers are developed in the synclinal axial zones of the folds, as these present valleys and stretches of plain land. Mostly the synclinal axial parts are composed of younger formation and the dip of beds is at low angle. The Rangamati Town is developed on Dihing and Dupitila Formations. The homogenous loam and sandstone beds present stable small hillocks having high bearing capacity of the ground. Away from the axial part, the shale and siltstone beds are exposed which are highly fractured and landslide, slope failure and slump failures are common in the sections. Generally the earthquakes of hilly regions generate numerable large landslides, ground subsidence and even small-localized jolts or micro-earthquakes. After the event, even many slides occur without earthquakes or aftershocks. The frequent cyclic earthquakes of moderate magnitude are capable to bring failure to both natural and man made structures even after a long time of the main event. There are good examples of Mir Fazlul Karim government building in Rangamati which are well engineered and maintained the slope configuration or valley profile to avoid slope failure, though many of these buildings ignored hazardous geological factors. It is clear that most of the government buildings considered the slope phenomena during the design of foundation. But the private constructors are more vulnerable to the geological constraints. Plate 3. a: Shear cracks in the walls of Kalabonia Health Center and b: Inside the wall. Plate 4. A collapsed earthen wall in Kalabonia village. Plate 5. Shear cracks in a School buildings. Structural integrity of many solid structures was tested through the 5.1 Magnitude earthquake of 27 July, 2003. The well-engineered structures survived intact and poor engineered structures developed innumerable cracks near epicenter (Plate 3, 4 and 5). Most of the buildings in Kalabonia Village (the epicenter area) are constructed with poor material and possibly not supervised by qualified engineers and for this all of them developed shear cracks and declared unusable. Interestingly not a single building collapsed near the epicenter though the construction was absolutely sub-standard. This gives a clearance to the constructors that if buildings are constructed at required standard these are going to tolerate strong jolts in the region. An estimate indicates that the g value at Intensity VII is nearly 0.15 while the velocity is maximum 20 cm/s (depending on ground composition). The brick walls of many building in Barkal Upa Zilla are significantly cracked. The buildings in Rangamati Town did not suffer much damage due presence of firm ground condition. The identified problem is related to slope-failure and creep failure along the shale bed plain. It has been seen that apparently many buildings in the Reserve Bazar area look very risky (Plate 11) but most of them survived due to presence of thick hard shale bed where the footing of the buildings are placed. The identified problem in the Reserve Bazar area of Rangamat Town is related to sever hill erosion along the Karnaphuly River bank and erosion will expose the foundation to get weaken gradually. A detailed geological mapping is necessary for risk assessment. The risky areas are so called “Chara” or streams in the town. Proceedings, First Bangladesh Earthquake Symposium, Dhaka, Dec.14-15, 2005 Ground Ruptures The Barkal-Rangamat region suffered severe ground ruptures. The ruptures were verified and mapped through field investigations and aerial photographs. A 10 km visible ground rupture was identified in the Kalabunia region (Plate 1, 2 and 6). The total length of the segmented rupture along the Barkal Thrust and Subhalong-Kasalong could be more than 100km transecting several thrust system in the west and expected to continue up to Shitakundu–Chittagong Thrust. The ground rupture was characterized mostly by strike slip faults (distributed in an en echelon array) with a general trend of NNW_SSE and had a maximum opening of 55 cm in the soil forming sandstone exposure. Fractures in the well engineered structure Interestingly many well engineered structures suffered severe fractures due to this earthquake and these buildings are located more 100 km distance from the epicenter. The Public Library Building and Polytechnic Institute Buildings in Chittagong are a well-engineered structures but the walls of these buildings are severely cracked due to 27 July 2003 earthquakes. Most of the cracks are developed in the partition walls of Public Library and Polytechnic Institute Buildings (Plate 7. 8 and 9). Failures of the structures are related to sub-surface movement of the underground strata along the old valley floor. Few horizontal and vertical cracks are also seen in the RCC Columns. All east-west aligned walls are fractured with minor lateral displacement. No fractures were found in the N-S aligned walls. Angle of fractures is almost 45 degrees and dip to the west. All failures are related to sub-surface geological factors. The campus occupies valleys of hills formed of Dupi Tila Sand and Clay. General retrofitting may not sustain another earthquake unless the dip of the bedding planes and their behavior are consistent to the foundation of the buildings. Intensity (MMS) - VII Epicenter Intensity contour Figure 4. Preliminary Intensity map of Barka-Rangamati Earthquake (Karim and Khan, 2003). Cracks are also developed in the walls of different multistoried residential building of Chittagong Polytechnic Institute. Few horizontal and vertical cracks are very distinctly seen in the RCC Columns. Mostly east-west aligned walls are fractured with minor lateral displacement. The gradient of the fracture planes are also towards west the residential buildings. A three-storey Mir Fazlul Karim building is declared abandoned by the investigation team though the buildings are expected to be well engineered. The campus occupies a part of valleys of hills formed of Dupi Tila Sand and Clay and later filled and developed for construction. Apparently the fracture pattern indicates that by general retrofitting the buildings may not sustain during another earthquake. It has been observed that the some of the structures in Chittagong Polytechnic Institute require immediate maintenance (may be retrofitting) as the building materials are heavily weathered due to long time water percolation and chemical disintegration. Plate 6. Ground Fissures in the epicenter area. Plate 8. Fractured Chittagong. wall of library building, Plate 10. The landslide and slump failure along Rangamati-Chittagong Road due to earthquake at Manik Chari, Rangamati. Plate 7. Shear cracks in Polytechnic Institute buildings, Chittagong.. Plate 9. Cracks in library buildings, Chttagong. Plate 11. Poorly engineered structures in Reserve Bazar. Two-storey building constructed on four soft or weak levels. The young deposits in the valleys and strip of floodplain deposits of Karnafuly River are prone to Proceedings, First Bangladesh Earthquake Symposium, Dhaka, Dec.14-15, 2005 liquefaction and many buildings in these regions suffered minor damages during earthquake. The affected structures need immediate attention, as next earthquake of similar magnitude will further increase the crack to make the buildings more vulnerable. Frequency of earthquake of magnitude 6 is relatively less in Chittagong region but occurrence of light to moderate earthquakes is high. The repetitive low magnitude earthquakes pose slow damage to the building or other engineering structures, which shall be given proper attention for engineering solutions (Plate 11). An stable Kaptai Hydroelectric Power Dam An intensive geological and geotechnical reconnaissance investigation was carried out in the vicinity of the Kaptai Hydroelectric Power Dam and reservoir periphery starting from Harina Bazar to Kaptai. Apparently the study indicates that the dam site occupies the best possible geological location and no sign of ground displacement was found on the surface (Karim and Khan, 2003). For subsurface investigation available geological cross sections are used and determined that the materials below the dam site is strong enough to withstand the seismic force generated due to Barkal-Rangamati earthquake. Geological analysis of reservoir area by aerial photographs (prior to the water trapping) indicates that the upstream ground up to 10 km to the north is void of any threatening fault or fractures. The structural geological investigation indicates that the dam occupies a greater part of a syncline and forms stable basin, which is floored by impervious hard shale. The dam-site is bounded by three geological faults (tear fault) and a detailed study is needed for engineering geological characterization of these faults. Conclusions The July 27, 2003 earthquake and following moderate shocks were generated along the Subhalang-Kasalang and Barkal Thrust System of Chittagong folded belt. This active earthquake source structure is located near Subduction Zone of India-Myanmar Plates Boundary and Kaladan Active Fault Fault Zone. The generation of earthquakes larger than magnitude 5 indicates the region is tectonically active and such thrust faults are capable generate even larger earthquakes. A significant displacements along the Subhalang-Kasalang and Barkal Thrust System has occurred. The maximum intensity of was VII in MMS as severe damages occurred in the masonry structures along with number of ground rupture. Intensity of damages and the ground shakings were concentrated along the strike direction of the folds. The earthquakes of the region have an unique characteristic of affecting long distance damages along the strike direction of the fault system. Damages occurred in many school buildings, residential buildings, library and educational institutes in Barkal, Rangamati, Chittagong and Cox’s Bazar. No building in the region has been totally or partially collapsed but formed cracks of different sizes. The cracked buildings are not safer to use without further treatment or retrofitting. Landslide, slope-failure and slump failures are common along the Rangamati-Chittagong Road sections. The earthquake has increased possibility of such failures due to generation of new fractures and joints, ground fissures and also widening of pre-existing fractures. Precautions are needed while traversing along the hillside road-sections after any such moderate earthquake in the region. The Chittagong city and Rangamati Town are developed on folded and faulted terrain of Tertiary sedimentary rock. A detailed geological and geotechnical mapping is necessary for risk assessment and remedial strategies for the urban infrastructures. Information dissemination about earthquakes and awareness development among people should be given emphasis. At present, the geo-dynamic activity of Subhalang-Kasalang and Barkal vicinities cannot be Mir Fazlul Karim overlooked and further studies are recommended to ascertain the level of activity and monitor the activity of faults. The complex geological structures in the hilly terrain make the grounds susceptible to landslide and slope failure and needs consideration in engineering designs and construction. The valleys and floodplains are composed saturated sand and silt which are prone to liquefaction. The Kaptai dam site occupies the best possible geological location of the region and no sign of ground displacement or structural failure was found on the surface. The dam has been constructed on a syncline that forms a stable basin where a firm ground condition exists. The dam design is rightly consistent to the geological structure but the dam needs engineering maintenance and geological monitoring. All affected engineering structures in the region need immediate identification for necessary repair and retrofitting as next earthquakes of similar magnitude will cause cumulative impact on the preexisting cracks of the buildings. A continuous monitoring system is needed for observation of geodynamics of the region and temporary micro-seismic stations should installed by all concerned authorities. The people of hill districts shall be given knowledge to understand the behavior of the geological processes, which cause damage to their properties, so that they can decide them selves, what to do for their safety. Acknowledgment The author is grateful to the Director General, Geological Survey of Bangladesh for giving permission to publish this paper. Sincere thanks to the people of the area who rendered cooperation and help to the investigation team. It is a pleasure to acknowledge the Rangamati district administration for providing all logistic support and transport facilities to the investigation team. Special thanks to Dr. Zafar Ahmed Khan, the D. C. of Rangamati District for his keen interest in earthquake hazard management. References Choudhury, J. R. 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