Mechanism of the ML4.0 25 April 2016 earthquake in southwest of France in the vicinity of the Lacq gas field

Abstract

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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Acknowledgments

We thank two anonymous reviewers for their comments, which improved our manuscript.

Funding

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).

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Correspondence to Hideo Aochi.

Appendix

Appendix

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.

Fig. 18
figure18

Waveform (ground velocity in cm/s) fitting between the observation and synthetics at three stations for the 2 September 2013 earthquake with focal depth of z = 4 km. Synthetic (red curves) represent the final solution after the 21st generation, and the thin gray lines show the intermediate solutions. The seismograms in velocity are filtered between 16 and 32 s. The time 0 is taken at 12:36:35. The obtained source parameter is summarized in Table 1(a)

Fig. 19
figure19

PGV (peak ground velocity) of each component of the ground motions simulated for the focal mechanism obtained at 4 km depth. The PGV is calculated after a filter up to 1 Hz. The relative position of station URDF with respect to the supposed source position is shown. Small triangles show the synthetic receiver positions

Fig. 20
figure20

a Misfit between the observation and synthetics for the 10 s. The area is limited by PGV(NS)/PGV(EW) > 5 and 0.016 cm/s < PGV(NS) < 0.143 cm/s. Initially, supposed URDF station position is shown by green. Two local minimums are shown in red and green. b Comparison of the waveforms filtered between 0.02 and 1 Hz. c Particle motion of the first 5 s

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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

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Keywords

  • 25 April 2016 earthquake
  • Near-field ground motion
  • Peak ground velocity
  • Lacq gas field