Adi Saric

1^st Class Honours

The 2,500 km long Himalayan-Tibetan orogen is characterized by areas of
low seismicity known as seismic gaps. One potential seismic gap is that of the
Bhutan Himalaya, where seismicity is more than four times less than the mean
seismicity of the HSB. Different hypotheses have been proposed as to the cause of
the putative seismic gap, but almost all are directly related to the Shillong Plateau of
eastern India. This uplifted pop-up structure of the eastern Himalaya is bounded by
two reverse faults, the Oldham and Dauki. The 1897 Mw~8.1 Great Assam
earthquake occurred along the Oldham fault and is thought to have altered the
stress environment, potentially resulting in the formation of a stress shadow
contributing to the seismic gap of the Bhutan Himalaya to the north.

This study attempts to model the 1897 Great Assam earthquake using
Coulomb 3.3, a graphic-rich deformation and stress-change software. The goal is to
model the potential Coulomb stress change that resulted from the earthquake using
known geologic and seismic parameters. Determining the Coulomb stress change
imparted to nearby receiver faults as a result of slip along the Oldham fault is the
most important aspect of this research study, as nearby faults in the region could
pose a future threat for seismic hazards.

This topic of research is of the utmost importance as stress taken up by
nearby receiver faults could be released in the near future through seismic events.
Seismic hazards pose a serious threat to the region, as population and population
density is very large in the Brahmaputra valley to the north of the Shillong Plateau,
and especially that of Bangladesh to the south. Furthermore, infrastructure is not
well equipped to handle such seismic hazards. Therefore, it is crucial to investigate
the Coulomb stress change that occurred as a result of the earthquake in order to
determine the future location of possible seismic events.

Results suggest that Coulomb stress change, as a result of the 1897 Great
Assam earthquake, did contribute to the formation of a stress shadow that coincides
with part of the putative seismic gap along the Bhutan Himalaya. The resulting
stress shadow coincides with the frontal ramp of the MFT south of the Bhutan
Himalaya. The drop in Coulomb stress could have had a potentially significant effect
on seismicity along the seismogenic detachment in the area. The lack of seismicity
due to a drop in Coulomb stress would result in less stress transfer to the basal
detachment segment of the MFT leading to prolonged stress and strain
accumulation, in turn contributing to lower seismicity levels. Furthermore,
Coulomb stress change was imparted to nearby strike-slip faults in the region with
the majority incurring a small decrease in Coulomb stress. Large increases in
Coulomb stress were imparted to the Dauki fault making it the highest risk for a
future seismic event. Future work and research should be aimed at determining a
time frame for the recurrence of seismic events, especially along the Dauki fault.

Keywords:

1) Coulomb envelope: The linear failure envelope predicted by the Coulomb
fracture criterion
2) Shear fracture: Fracture with detectable wall-parallel displacement.
Different from faults in that it only consists of a single fracture, while faults
are composed of a number of linked fractures
3) Extension fracture: Fracture formed by extension perpendicular to the
fracture walls
4) Shear stress: Stress acting parallel to a plane of reference
5) Normal stress: Stress or stress component acting perpendicular to the
surface of reference
6) Brittle Deformation: Deformation by fracturing (discontinuous
deformation)
7) Seismogenic Zone: The zone of frequent earthquakes in the crust, which is
the middle and lower part of the brittle crust, where the crust is the strongest
8) Mohr Circle: Circle in the Mohr diagram that describes the normal and
shear stress acting on planes of all possible orientations through a point in
the rock

Pages: 126
Supervisor: Djordje Grujic