Towards a better understanding of the impact of erosion on faultslip and seismicity
Résumé
Tectonics, climate and surface processes dictate the evolution of Earth’s surface topography.Topographic change in turn influences lithospheric deformation, but the temporal and spatialscales at which this feedback can be effective remains an open issue. Here, we make a synthesis ofrecent developments investigating how erosion impacts the stress-loading of faults and potentiallyinduces some earthquakes. We first show, using an elastic model for the lithosphere, that erosionrates of ca 0.1–20 mm.yr−1, as documented in active compressional orogens, can raise theCoulomb stress by ca 0.1–10 bar on the nearby thrust faults over an earthquake cycle, by changingboth the normal and tangential stress. This model also suggests that short-lived but intenseerosional events can represent a prominent mechanism for inter-seismic stress loading of faultsnear the surface. Indeed, we demonstrate that typhoon Morakot in 2009, which triggerednumerous landslides, was followed by a step increase in the shallow (< 15 km depth) earthquakefrequency and in the b-value, lasting at least 2.5 years. These observations suggest that theprogressive removal of landslide debris by rivers from southern Taiwan has increased the crustalstress rate and earthquake activity. Last, we use QDYN, a quasi‐dynamic numerical model ofearthquake cycles to investigate the effect of a large erosional event, such as typhoon Morakot, onseismicity. We show that erosional events with a duration shorter than the duration of anearthquake cycle can significantly increase the seismicity rate, even for small stress changes.Consistent with the increase in the b-value observed after typhoon Morakot, our results also showthat large erosional events with a period similar to the earthquake nucleation timescale canchange earthquake size distribution by triggering more small events. Overall, these modelling
results and observations highlight that short-lived but intense erosional events can lead toperceptible changes in shallow seismicity, affecting both earthquake frequency and size-distributions.