Emmaline Atherton
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1st Class Honours
B.Sc. (Honours) Thesis
Seismic strain and the state of stress in the crust of the Himalaya
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This study analyses how the seismic strain and geometry of stress in the crust of the Himalaya changes along strike of the Orogen. The active collisional setting of the Himalayas has very high rates of seismicity and thus poses a significant threat to the densely populated surrounding areas. A large-magnitude earthquake would have catastrophic effects on both the infrastructure and people of north-eastern India. Therefore, it is essential to have a good understanding of the crustal distribution of stresses in this area.
The Himalayan Orogen is characterized by a series of north-dipping thrust faults and shear zones formed as a result of the ongoing convergence of the Indian and Eurasian tectonic plates. The Main Frontal Thrust (MFT), the Main Boundary Thrust (MBT), and the Main Central Thrust (MCT) span the entire length of the Orogen and merge at depth into the Main Himalayan Thrust (MHT) – the basal detachment of the Himalaya. The majority of seismicity is concentrated along a belt located approximately 100 km north of the mountain front. In a section of the eastern Himalaya (the Bhutan Himalayan region) the seismic belt is interrupted and there have been no major seismic events in this area in written record. Since the geodetic convergence rates in the eastern Himalaya are higher than in the west, and the lithology does not change significantly, the lack of seismicity in this area is puzzling. This study uses records of crustal seismicity to determine and quantify changes in seismic strain along strike of the Orogen.
The Himalaya was separated into five geographic regions and fault-slip inversion was performed on the corresponding seismic data. Three crustal fault regimes were identified: thrust, strike slip, and normal. Events belonging to the normal fault regime are located along the southern margin of the Tibetan Plateau. From west to east, the normal faults indicate predominantly E-W extension which is interpreted to be the result of faulting along the South Tibetan Grabens. Other results show a prominent thrust fault regime in the seismic belt of the western to central Himalaya. Most of these events yield solutions compatible with thrusting along a ramp of the MHT. In contrast, a strike-slip faulting regime is dominant both in the eastern Himalaya (east of 87°E) and to the south of the Himalaya in the Shillong Plateau. The latter is the only elevated area outboard the Himalaya and is one of the most seismically active areas covered in this study. There appears to be a direct link between the seismic strain conditions that contribute to the thrust and strike slip faulting regimes; however, results show that thrust faults dominate the western regions of the Himalayas, while strike slip faults dominate the eastern regions. Calculations were performed on the seismic data to quantify the total amount of energy released on these respective fault types, as well as the total amount of slip that has occurred. The conclusion was that the increased convergence rates in the eastern Himalaya may be accounted for by the increase in strike slip faulting activity in this region of the Orogen. This result may suggest that the MHT in the eastern Himalaya is not accumulating as much seismic strain from the higher convergence rate as previously theorized; rather, the plate convergence is being released as slip along transverse faults in the Indian crust.
Keywords: Himalaya, crustal seismicity, fault slip inversion, seismic energy, slip rate, seismic gap
Pages: 85
Supervisor: Djordje Grujic