# The M6 1799 Vendée earthquake (France) : a multidisciplinary investigation of the active fault

Caroline Kaub, Laurent Bollinger, Laurent Geoffroy, Julie Perrot, Pascal P. Le Roy, Christine Authemayou

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Caroline Kaub, Laurent Bollinger, Laurent Geoffroy, Julie Perrot, Pascal P. Le Roy, et al.. The M6 1799 Vendée earthquake (France) : a multidisciplinary investigation of the active fault. Rencontres scientifiques et techniques RESIF 2019, Nov 2019, Biarritz, France. hal-02446624

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Submitted on 21 Jan 2020

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Distributed under a Creative Commons Attribution 4.0 International License
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UNIVERSITE
BRETAGNE
LOIRE

## 1. Context of the M6 1799 Vendée intraplate earthquake

The coastal Vendée (France) is located to the south of a main structure of the Armorican Massif, the 600-km-long South Armorican crustal Shear Zone (SASZ) (1) [fig. 1]. This “stable continental region” (2) is affected by a system of dominantly NW-SE trending shear zones and faults, inherited from a long and polyphased tectonic history since Variscan times.

<figure><img src="image_6.png" /><figcaption>Fig. 1: Macroseismic intensity of the M6 1799 earthquake.</figcaption></figure>

This area currently presents a moderate background seismic activity, but was affected by a significant historical earthquake (magnitude M ~ 6) on the 25th of January 1799. The mesoseismic area probably influenced by strong site effects in the nearby Neogene basin (Marais Breton) is located along an onshore/offshore discontinuity bounding northward the basin, the Machecoul fault (MF) [fig. 2].

The perception area stretched from Paris to Bordeaux and made this event the largest documented in the western part of France [fig. 1]. Earthquake parameters have been determined from macroseismic data (3) and present a wide variability of magnitude and depth couples : from Mw 5.8 to 6.6 and depth around 20 km (4 and 5).

Can we connect this main earthquake to a specific fault ?

<figure><img src="image_7.png" /><figcaption>Fig. 2: Main faults of the Marais Breton and Baie de Bourgneuf and epicentral location (3), modified from (6).</figcaption></figure>

## 3. Machecoul Fault trace and morphology

<figure><img src="image_8.png" /><figcaption>Fig. 4: Onshore-offshore extension of the Machecoul fault signature.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 5: Incision and captures of the footwall and hanging wall.</figcaption></figure>

Complex fault system geometry [fig. 4 & 6] :

* Onshore morphological trace very tenuous, disappearing gradually eastward

* Slope break highly anthropized

* Relay structures

* Offshore signature expressed up to the seafloor, with two parallel faults and upward flexured HW [fig. 6]

* **Segmentation** : several N110 trending segments ranging from 2.5 to 12.5 km length onshore, and a maximum 7.5 km length offshore

* **SW dipping normal faults system** : no accurate estimation of onshore dip, around 60° dip for the main segment offshore [fig. 6]

Mophometric analysis [fig. 5] :

* Low incision and residual relief of the FW

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preserved Pliocene paleosurface ?

* Epigenic river (Le Tenu) : FW uplift ?

* 4 minor captures in the FW [fig. 5]

* 1 main capture in the HW [fig. 5]

* 2 km wide offshore sub-basin

This pattern of active fault and basin orientation is observed all along the north French Atlantic coast and could be connected to tectonic activity since early Cenozoic (9).

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 6: HR bathymetry and interpreted seismic lines. See bathymetry location on [fig. 3].</figcaption></figure>

**References :**
(1) Bitri, A. et al. (2003). C. R. Geosciences. (2) Johnston, A. C. et al. (1994). The earthquakes of stable continental regions. (3) BRGM-EDF-IRSN http://www.sisfrance.net/ (4) Stucchi et al. (2013) Jour. Seism., 17(2):523-544 (5) Manchuel, K. et al. (2017). Bull. Earthqu. Eng. (6) Limasset et al. (1992) Ann. Bret. Pays Ouest, vol. 99, 97-116 (7) BRGM http://infoterre.brgm.fr/page/banque-sol-bss (8) BGI http://bgiobs-mip.fr/fr (9) Delanoë Y., 1988. Géol. de la France. (10) Baize, S. et al. (2013). Bull. Soc. Géol. France, 184(3):225-259.

**Acknowledgements** : Baltzer A. for bathymetry data exchange of the Loire estuary & Tiberi C. and Plasman M. for their expertise with gravity processing and interpretation.

## 2. An onshore/offshore multidisciplinary approach

Investigation of the epicentral area of the 1799 earthquake to identify active faults potentially responsible for this event and quantify their recent activity combining outcomes from [fig. 3] :

* **40 km²** of new HR bathymetry (Geoswath interferometric sonar - RETZ2) and topography (RGEALTI, IGN) data

* Machecoul fault morphology

* **700 km** of new seismic profiles (Sparker source single channel streamer, CHIRP echo sounder – RETZ1 & 2)

* Existing onshore drill database (BSS, (7)) to stress Neogene depot centers in the Marais Breton.

* Available onshore gravity data (BGI, (8)) to calculate Bouguer anomaly and 2D modeling near the MF

* basin geometry & sediment infilling.

* **10 stations** temporary seismological experiment (MACHE) to complement 4 years of regional catalog

* current seismicity

<figure><img src="image_13.png" /><figcaption>Fig. 3: Location map of all the techniques.</figcaption></figure>

## 4. Basin infilling

<figure><img src="image_14.png" /></figure>

* 5 mGal negative Bouguer anomaly in the Marais Breton

<figure><img src="image_15.png" /></figure>

* **Shallow sedimentary basin**

Plio-quaternary depocenter repartition study :

* Poorly represented onshore, widely offshore [fig. 7]

* Thickness gradient both toward the MF and westward along the MF

* No syntectonic deformation figures observed on seismic reflexion profiles [fig. 6]

<figure><img src="image_16.png" /></figure>

* Pre-existing topography passively filled during the Neogene or tectonic control of the sedimentation by the MF ?

<caption>Fig. 7: Neogene to Quaternary sedimentary thickness (m).</caption>

## 5. Seismicity

### CHALLANS EARTHQUAKE SEQUENCE (2012)

* 27 April 2012 and located in the Marais Breton [fig. 8]

* Approximately 30 events in 17 days, with $M_c$ ranging from 0.5 to 2.2, and a range depth of 20-25 km

* Largest event occurred on 28 April 2012

* No focal mechanisms

### MACHE NETWORK (2016 – 2017)

* 58 events located in 22 months with a $M_L$ ranging from 0.3 to 3 [fig. 8]

* Diffuse microseismicity

* Majority of the activity in the HW

* Depth of earthquakes between 5 and 22 km, at least (both intermediate and deep seismicity)

* 8 normal and strike-slip focal mechanisms [fig. 8]

* Stress inversion : $NW-SE \sigma_{Hmax} = \sigma_1$ or $\sigma_2$ and well constrained $\sigma_3$

<figure><img src="image_17.png" /><figcaption>Fig. 8: Seismicity characterized by 3 different network of 10 stations (2011-2017).</figcaption></figure>

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## 6. Conclusions

* Complex fault geometry : segmentation, relay structures onshore and offshore.

* Apparent tectonic control of Plio-Quaternary depocenters : recent reactivation of an old structure.

* The Machecoul fault is probably an active fault : non-negligible microseismic activity in Marais Breton.

* Possible implication of the Machecoul fault in the 1799 event :

* Noirmoutier fault dips to the SW and is therefore an unlikely source for the 1799 earthquake (6), requiring a macroseismic epicenter several kilometers further South West

* Foreshock felt only in Machecoul city could indicate a rupture on a fault nearby

* Machecoul faultogenic segment length compatible with a M6 event

* Thick seismogenic depth compatible with macroseismic hypocentral estimations (3, 4, 5)

* MACHE focal solutions compatible with quaternary deformation (10)