Mechanisms that control seismic activity in low strain rate areas such as western Europe remain poorly understood. For example, in spite of low shortening rates of <0.5 mm/yr, the Western Alps and the Pyrenees are underlain by moderate but frequent seismicity detectable by instruments. Beneath the elevated part of these mountain ranges, analysis of earthquake focal mechanisms indicates extension, which is commonly interpreted as the result of gravitational collapse. Here we show that erosional processes are the predominant control on present-day deformation and seismicity. We demonstrate, using finite element modeling, that erosion induces extension and rock uplift of the elevated region of mountain ranges accommodating relatively low overall convergence. Our results suggest that an erosion rate of ∼1 mm/yr can lead to extension in mountain ranges accommodating significant shortening of <3 mm/yr. Based on this study, the seismotectonic framework and seismic hazard assessment for low strain rate areas need to be revisited, because erosion-related earthquakes could increase seismic hazard.

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