Résumé
Extreme rainfall events can trigger numerous landslides in mountainous areas and a prolonged increase of riversediment load. The resulting mass transfer at the Earth surface in turn induces stress changes at depth, whichcould be sufficient to trigger shallow earthquakes. The 2009 Morakot typhoon represents a good case study as itdelivered 3 m of precipitation in 3 days and caused some of the most intense erosion ever recorded. Analysis ofseismicity time-series before and after the Morakot typhoon reveals a systematic increase of shallow (i.e. 0-15 kmof depth) earthquake frequency in the vicinity of the areas displaying a high spatial density of landslides. Thisstep-like increase in frequency lasts for at least 2-3 years and does not follow an Omori-type aftershock sequence.Rather, it is associated to a step change of the Gutenberg-Richter b-value of the earthquake catalog. Both changesoccurred in mountainous areas of southwest Taiwan, where typhoon Morakot caused extensive landsliding. Thesespatial and temporal correlations strongly suggest a causal relationship between the Morakot-triggered landslidesand the increase of earthquake frequency and their associated b-value. We propose that the progressive removalof landslide materials from the steep mountain landscape by river sediment transport acts as an approximatelyconstant increase of the stress rate with respect to pre-typhoon conditions, and that this in turn causes a step-wiseincrease in earthquake frequency. To test this hypothesis, we investigate the response of a rate-and-state fault tostress changes using a 2-D continuum elasto-dynamic model. Consistent with the results of Ader et al. (2013), ourpreliminary results show a step-like increase of earthquake frequency in response to a step-like decrease of thefault normal stress. We also investigate the sensitivity of the amplitude and time-scale of the earthquake frequencyincrease to the amplitude of the normal stress change and to rheological parameters. Our study offers new insightson the potential influence of extreme erosional events on the short-time scale dynamics of faults and earthquakes