1. Earthquakes, elevations and continental plateaux
An investigation into the absence of large thrust earthquakes at high elevations within fold-
and-thrust belts.
C.R.M Goddard1, M.B Allen2, N De-Paola2, Dr S.Nielsen2 and C. Saville2
1
Department of Earth Sciences, Oxford University, S Parks Rd, Oxford OX1 3AN
2
Department of Earth Sciences, Science Labs, Durham University, Elvet Hill, Durham DH1 3LE
Large thrust earthquakes (M ≥ 5) are rare at high elevations in continental fold-and-thrust belts.
Data from the Zagros, Himalayas, Qilian Shan and Longmen Shan are analysed to explore three
hypotheses for this phenomenon:
1. Fold-and-thrust belts deform via the critical Coulomb wedge model, which favours thrusting
at lower elevations.
2. Fold-and-thrust belts are controlled by minimum work processes, which suppress thrusts at
high elevations.
3. A thickened ductile zone within the sedimentary layer of fold-and-thrust belts inhibits
earthquake propagation.
Elevation vs magnitude analysis revealed abrupt cut-off elevations for M ≥ 5 earthquakes. Cut-
offs from different localities were then compared to the elevations of plateau edges in order to explore
Hypothesis 1. The cut-offs correlate with a change in slope at an elevation below the plateau edge.
Higher elevation regions of the fold-and-thrust belts continue to converge aseismically until reaching
a limiting elevation. The mechanism by which the fold and-thrust belts continue to deform
aseismically above the seismicity cut-off is unclear: basal decollement aseismic creep of the wedge is
one possible explanation. This work supports a modified brittle-ductile critical Coulomb wedge having
a controlling influence on seismicity. In order to explore the relevance of Hypothesis 3, dual lithology
triaxial loading experiments were conducted as an analogue to study the control of brittle-ductile
behaviour on fault propagation. Brittle faults were found to propagate aseismically into a ductile
medium offering an alternative explanation for the high elevation regions of aseismic convergence.
The results of this study suggest that the individual hypotheses noted above cannot solely be used
to explain the empirical relationship between large thrust earthquakes and elevation. Instead, a
hybrid of Hypotheses 1 and 2 is proposed in which ductile deformation in the critical Coulomb wedge
limits M ≥ 5 earthquakes but convergence continues aseismically until the formation of a plateau with
its height determined by minimum work processes.