Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> On 2 October 2020, the Maritime Alps in southern France were struck by the devastating Storm Alex, which caused locally more than 600 mm of rain in less than 24 h. The extreme rainfall and flooding destroyed regional rain and stream gauges. That hinders our understanding of the spatial and temporal dynamics of rainfall–runoff processes during the storm. Here, we show that seismological observations from permanent seismic stations constrain these processes at a catchment scale. The analysis of seismic power, peak frequency, and the back azimuth provides us with the timing and velocity of the propagation of flash-flood waves associated with bedload-dominated phases of the flood on the Vésubie River. Moreover, the combined short-term average to long-term average ratio and template-matching earthquake detection reveal that 114 local earthquakes between local magnitude <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>M</mi><mi mathvariant="normal">L</mi></msub><mo>=</mo><mo>-</mo><mn mathvariant="normal">0.5</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="bcb46319d0a86e32cfd5c86d7c5ae0d9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="nhess-22-1541-2022-ie00001.svg" width="52pt" height="12pt" src="nhess-22-1541-2022-ie00001.png"/></svg:svg></span></span> and <span class="inline-formula"><i>M</i><sub>L</sub>=2</span> were triggered by the hydrological loading and/or the resulting in situ underground pore pressure increase. This study shows the impact of Storm Alex on the Earth's surface and deep-layer processes and paves the way for future works that can reveal further details of these processes.

Highlights

  • Extreme weather events might trigger an extreme response of the Earth’s surface and subsurface processes, e.g., in the form of rapid and disastrous flash-floods (e.g., Khajehei et al, 2020), mass movements (Stoffel and Huggel, 2012), and/or seismogenic 15 underground stress changes (e.g., Rigo et al, 2008)

  • We focus our flood-dynamics analysis on the Vésubie river because (1) the Vésubie catchment has been one of the most strongly affected in the region (Figure 1, B1, B2) and (2) the seismic station coverage is adequate in this catchment with three seismic stations being located in the proximity: SPIF (3-component velocimeter), BELV (3-component accelerome70 ter) and TURF (3-component velocimeter) at respectively about 1,570, 630 and 5,970 m from the Vésubie river

  • All three seismic stations (SPIF, BELV, and TURF) show elevated noise levels during the 24 h period starting at 07:00 UTC October 2, 2020, that overlap with the duration of storm Alex (Carrega and Michelot, 2021) (Figure 1D)

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Summary

Introduction

Extreme weather events might trigger an extreme response of the Earth’s surface and subsurface processes, e.g., in the form of rapid and disastrous flash-floods (e.g., Khajehei et al, 2020), mass movements (Stoffel and Huggel, 2012), and/or seismogenic 15 underground stress changes (e.g., Rigo et al, 2008). These processes contribute to societal and environmental risks and are an important agent in landscape evolution. Seismic methods have the potential to monitor surface and subsurface processes associated with extreme weather events. Other observations display a punctual increase of seismic activity following an exceptional rainfall episode, for example in the Swiss Alps (Roth et al, 1992), German Alps (Kraft et al, 2006), and southern France (Rigo et al, 2008)

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