Ph.D. Cambridge University, U.K., 1970
Professor, Geological Sciences
Bilham’s research includes: Application of space geodesy and strain and tilt instrumentation to monitor deformation of the Earth at plate boundaries, mostly in southern Asia and western North America; Design and operation of new geophysical instruments to monitor strain and tilt in the Earth; Archival research into historical earthquakes; Statistics of urban earthquakes and global seismic hazards.
Current Research: Great earthquakes at the ends of the Himalayan Arc
The characterization of future damaging earthquakes in the Himalaya depends on quantifying tectonic convergence rates, investigating the history of previous earthquakes, and identifying segments most likely to rupture. The two most recent catastrophic earthquakes in the Himalaya occurred near the ends of the arc—in Assam in 1950 and in Kashmir in 2005—regions now believed to be relatively safe from an immediate recurrence of catastrophic shaking. These earthquakes stressed, however, adjoining regions. Global Positioning System (GPS) geodesy in Nepal reveals that India converges with southern Tibet at approximately 18 millimeters per year, with stress concentrated near a nucleation zone 18 kilometers beneath the high mountains. Earthquakes occur when this nucleation zone is stressed beyond the ability of Himalayan faults to resist southward slip. We are focusing our measurement efforts at the ends of the arc where no historical earthquake has occurred recently.
GPS results from Bhutan in 2012 reveal that a continental block the size of Colorado has broken from the Indian Plate and is slowly rotating clockwise, carrying with it the Brahmaputra River Valley. From a consideration of the resulting convergence
between southern Tibet and this block, we deduce that the seismic behavior of the eastern Himalaya resembles that of the central
Himalaya. We recognize three contiguous segments near Bhutan where earthquakes greater than magnitude 8 could occur
separately or in combination. Ominously, in at least one of these segments, the last earthquake, circa 1,000 years ago, may have
slipped 18 meters; hence, a great earthquake here in the next century would not be unexpected.
At the other end of the Himalayan Arc, our GPS measurements record a very different style of convergence. In Kashmir, we
find no well-defined nucleation zone, slower convergence rates, and a doubling in the width of the Himalaya. The slow rates suggest that long intervals occur between great earthquakes, but the greater width permits the earthquakes’ maximum magnitude
(Mmax) to approach 8.9 Mmax. Recent earthquakes here, however, suggest that convergence may be absorbed by smaller
faults, hosting approximately 7.5 momentmagnitude earthquakes that cut through to the surface of the Kashmir Valley instead of
shifting the entire Himalaya southward.
Professor Bilham is a professor at the University of Colorado Boulder.