The 1965–1970 seismic episode in the Hainaut coal basin (Belgium): a key period to analyse the triggered nature of a century-long seismic activity

Damaging earthquakes happened in the Hainaut coal basin (Belgium) in the 19th and 20th centuries but stopped after coal mining. We relocated 54 Hainaut earthquakes of the period 1965 and 1985 by using regional and local seismic phase measurements from recordings in and around Belgium. These new results finalise the Hainaut earthquake catalogue, derived from macroseismic data for the 1887–1965 period and instrumental records after 1965. This updated database allows us to discuss the origin of this intriguing seismicity. Computed focal depths show that the strongest earthquakes before 1985 occurred inside the Upper Palaeozoic part of the Brabant Parautochton, probably below the deepest parts of the coal mines. The shallowness of this seismicity, its spatial link with the coal basin, the compressive to strike-slip focal mechanisms of the stronger earthquakes between 1965 and 1985 and the large quantity of coal mass removal all match with worldwide observations of mining-triggered earthquakes. The earthquake mechanisms agree with the local, compressive to transpressive stress regime derived from differences in upper crustal density and topography that make the Hainaut area more sensitive to fault reactivation at shallow depths, as observed in similar tectonic settings elsewhere in the world.