Studied zone with the three-fault system and the surface quarry near Le Rhône river (with texture map and exaggerated topography). Credit: André Burnol, Author provided
On 11 November 2019, a
surface-rupturing earthquake occurred at a very shallow depth (about 1 km) near
Montélimar city, along the Rhône Valley in southeast France. The earthquake
caused significant damage in the villages of Le Teil and Saint-Thomé, with the
partial collapse of hundreds of old houses built with ancient stones and damage
to sites of historical and religious heritage.
The question was raised shortly
after about the potential influence of a nearby surface quarry. In contrast to
a purely mechanical view, we investigated in this study the hypothesis of
"hydroseismicity" developed by John K. Costain (1930–2015), assuming
that this earthquake was triggered by the impact of meteoric water
transfer from the ground surface. We simulated the hydraulic recharge linked to
the infiltration of water in the fault zones in the decade preceding the earthquake.
We verified via a
double-permeability model that the intersections between two or multiple faults
were the most probable location zones for the hypocenter of an earthquake
triggered by a hydraulic recharge. We finally compared this estimated hydraulic
overpressure to the impact of stress transfer from the ground
surface due to
the extraction of rocks from the surface quarry over about two centuries.
Two separate numerical modelings (mechanical and hydraulic) of the same
structural geology
The scientists, working on a
research program in partnership with the quarry owner, conducted pure
mechanical modeling to take into account not only the evolution of the surface
topography, but also the history of excavated volumes of rocks since 1850.
The study was carried out in two
stages. From January to December 2020, the first program involved constructing
the first 3D geological model of the area using two new seismic lines and field
observations (including a new interpretation of InSAR data acquired via
Sentinel-1 satellite). This work combining geology and geophysics was completed
by the first two-dimensional hydraulic simulations (2D vertical).
Surface soil moisture data acquired
by the European SMOS satellite and modeling of unsaturated zone height variations
using the ComPASS code showed that the water infiltration had an
impact on piezometric levels, with maximum hydraulic overpressures at the fault
intersections.
The second phase of the study, from
February to May 2021, confirmed and refined the results of the initial stage, particularly
with regard to the location of the zone of maximum hydraulic overpressure (at
the intersection of the three-fault system). The best estimation of the
overpressure at depth was calculated by using in situ soil moisture (at a 30 cm
depth) as surface conditions during the period 2015–2019 before the seismic
event.
Evolution of the pressure at the intersection of
the three-fault system before the earthquake of 11 November 2019 at Le Teil
(France). Credit: André Burnol, author provided
Comparison of two potential triggering factors
The two separate numerical
approaches that were carried out by the researchers show that the hydraulic
effect is about 2.5 times larger than the cumulative effect of mechanical
stress release due to the mass removal from the surface quarry. The researchers
show that, while the tectonic origin for the Le Teil earthquake is not in
doubt, it may have been triggered by intense rainfalls in the month preceding
the seismic event. This study suggests a rapid hydraulic triggering mechanism
on an intersection of faults at a shallow depth.
This highly original,
transdisciplinary project, combining geology, geophysics, geomechanics,
seismology and hydrogeology, opens up a new field of research into
hydroseismicity in the context of a possible evolution of the dynamics of the
hydrologic cycle due to climate change.
Such hypotheses on hydroseismicity
in the intraplate domain had already been advanced in the U.S. and China over
the past 20 years; it was here confirmed in mainland France.
More broadly, this earthquake not
only revealed the need to reassess the hazard in this area in the vicinity of
industrial facilities in the Rhône Valley, but also launched a French debate on
the risk of surface-rupturing shallow earthquakes in sensitive areas
(including nuclear power plants).
This story is part of Science X Dialog, where researchers can report findings from their
published research articles. Visit this page for information about ScienceX Dialog and how to
participate.
by André Burnol
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