Epicenter distribution and magnitude of earthquakes in fold-thrust belts: insights from sandbox models

Scaled analogue models are used to illustrate the effect of basal friction and erosion on fault activity and hence on epicentre distribution and magnitude of earthquakes in the sedimentary cover of active fold-thrust belts. Model results suggest that in fold-thrust belts shortened above low-friction ductile decollements (rock salt or over-pressured mudstone), low- to moderate-magnitude earthquakes (M-w = 5.3-5.6), distributed over a wide area, occur along several long-lived thrust faults. Conversely, in areas shortened above high-friction decollements large-magnitude earthquakes (M-w = 6.6-6.8), distributed over a narrow zone are likely to occur along few short-lived thrust ramps. Calculated magnitude of earthquakes from models and their distribution are in agreement with recorded earthquake pattern from the Zagros mountain belt, which is partially shortened above a ductile decollement of Hormuz salt formation. Model results also showed that erosion reactivates older inactive thrusts and promotes formation of out-of-sequence thrusts.