The source mechanism of the ML 4.0 25 April 2016 Lacq earthquake (Aquitaine Basin, South-West France) is analyzed from the available public data and discussed with respect to the geometry of the nearby Lacq gas field. It is one of the biggest earthquakes in the area in the past few decades of gas extraction and the biggest after the end of gas exploitation in 2013. The routinely obtained location shows its hypocenter position inside the gas reservoir. We first analyze its focal mechanism through regional broad-band seismograms recorded in a radius of about 50 km epicentral distances and obtain EW running normal faulting above the reservoir. While the solution is stable using regional data only, we observe a large discrepancy between the recorded data on nearby station URDF and the forward modeling up to 1 Hz. We then look for the best epicenter position through performing wave propagation simulations and constraining the potential source area by the peak ground velocity (PGV). The resulting epicentral position is a few to several km away to the north or south direction with respect to station URDF such that the simulated particle motions are consistent with the observation. The initial motion of the seismograms shows that the epicenter position in the north from URDF is preferable, indicating the north-east of the Lacq reservoir. This study is an application of full waveform simulations and characterization of near-field ground motion in terms of an engineering factor such as PGV. The finally obtained solution gives a moment magnitude of Mw 3.9 and the best focal depth of 4 km, which corresponds to the crust above the reservoir rather than its interior. This position is consistent with the tendency of Coulomb stress change due to a compaction at 5 km depth in the crust. Therefore, this earthquake can be interpreted as a relaxation of the shallow crust due to a deeper gas reservoir compaction so that the occurrence of similar events cannot be excluded in the near future. It would be necessary to continue monitoring such local induced seismicity in order to better understand the reservoir/overburden behavior and better assess the local seismic hazard even after the end of gas exploitation.
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We thank two anonymous reviewers for their comments, which improved our manuscript.
This study has been funded by internal Research & Development fund from BRGM. The calculation of Green’s function was carried out at French National Supercomputing Center (GENCI/CINES) under the grants c2016-046700 (2016), A0010406700 (2017), and A0030406700 (2017-18).
This appendix shows the detailed analysis for the 2 September 2013 ML 4.0 earthquake mentioned in the discussion section. We apply the same procedure as in the text for the 25 April 2016 ML 4.0 earthquake. Figure 18 shows the waveform fit for the focal mechanism inversion, supposing 4.0 km focal depth. There are only two stations ATE and PYLO available at epicentral distance of about 50 km. The station URDF (a few kilometers from the supposed epicenter) is not used in the inversion. Figure 19 shows the PGV map from the nearfield ground motion using the obtained focal mechanism. From the observation, the PGV in EW, NS, and UD is 0.008, 0.048, and 0.019 cm/s, respectively. Figure 20 represents the grid search of better positions of source-receiver. The NS movement is dominant and this information limits the probable area. The PGV is much smaller than the 25 April 2016 earthquake and briefly consistent with our simulation of Mw 3.25 source. The position #00142, namely the epicenter located in the south-west by a few kilometers, represents the minimum misfit and consistent with the polarity of the particle motions.
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Aochi, H., Burnol, A. Mechanism of the ML4.0 25 April 2016 earthquake in southwest of France in the vicinity of the Lacq gas field. J Seismol 22, 1139–1155 (2018). https://doi.org/10.1007/s10950-018-9758-5
- 25 April 2016 earthquake
- Near-field ground motion
- Peak ground velocity
- Lacq gas field