M.Sc. THESIS DEFENCE - Yuqiu Zhao
M.Sc. Thesis Defence
Yuqiu Zhao
Department of Earth and Environmental Sciences, Dalhousie University
Title: Unraveling the Seismic History of the Himalayas: Paleoseismological Insights from a Surface Rupture in Eastern Bhutan
Abstract: The Himalayan region is a significant seismic and orogenic zone, emerging from the collision and ongoing convergence between the Indian and Eurasian continental plates. The study of paleoseismology in this area encounters challenges due to the sparse historical records of earthquakes and the inadequacy of paleoseismic data. Notably, the literature suggests the existence of several potential seismic gaps within the Himalayas, such as in eastern Bhutan. Thus, investigating newly identified surface ruptures becomes essential for enhancing the earthquake catalogue of this region.
In eastern Bhutan, the seismic history was elucidated through the examination of a surface rupture at the Dungsam Chu site along the Main Frontal Thrust (MFT). This investigation has revealed evidence of at least three large paleo-earthquakes occurring along this fault over a span of more than 10,000 years. The initial two events (E2‒E3) are dated back to pre-medieval times, although their precise ages remain vague due to limited data. The most recent event (E1) is identified as the 1714 Bhutan earthquake, characterized by a surface rupture extending 175 to approximately 290 km, a maximum coseismic slip of around 10.5 m, and estimated magnitudes between 7.7 and 8.5. Furthermore, the recurrence interval of major Himalayan earthquakes was assessed using two approaches: (a) estimating the stress discharge of the 1714 M8.1 Bhutan earthquake and the time required to accumulate similar stress levels, based on current geodetic convergence rates; and (b) averaging the occurrence of events in Bhutan and Nepal.
Near our study area, paleoseismological findings suggest at least one significant medieval earthquake likely took place in the Bhutan Himalayas, such as the circa A.D. 1100 event with an Mw of over 8.7 or the A.D. 1255 earthquake exceeding Mw 8. Evidence indicates that the impact of these medieval earthquakes on the eastern Bhutan segment could have been masked by the 1714 M8.1 event or diminished by erosion and other surface processes. Alternatively, it is possible that these events did not reach the surface or extended further south into the foreland basin, as supported by Coulomb stress transfer calculations. The segmentation of the Main Himalayan Thrust (MHT), rather than an absence of evidence, suggests a lower probability of a singular, extensive rupture across the entire Himalayan front. Instead, it favors a tectonic scenario consisting of multiple significant events.
Examining Committee:
Dr. James Kirkpatrick, McGill University - External Examiner
Dr. Djordje Grujic, Dalhousie University - Supervisor
Dr. John Gosse, Dalhousie University - Reader
Dr Miao Zhang, Dalhousie University - Reader
Dr. Yana Fedortchouk, Dalhousie University - Departmental Representative
Time
Location
Milligan Room, 8007 LSC, Dalhousie University