Interpretation of the piezometric fluctuations associated to the november 29, 2007 7.4 earthquake in Martinique

On November 29, 2007, at 3:00 pm local time (7:00 TU), a 7.4 earthquake occurred at about 30 km north from the island of Martinique, and was widely felt in all the Caribbean, from Jamaica to French Guyana. Piezometric levels changes (some abrupt rises or decreases), apparently correlated with the earthquake, were recorded by 12 of the 24 piezometers from the groundwater monitoring network of Martinique. Such reactions have already been observed notably in Japan, in the US or in Armenia. Variation of spring discharge or piezometric level changes induced by earthquake have been widely described, and synthesized. Most of them occur after the quake, nevertheless, some precursors have also been described. The present earthquake and its effects on piezometric levels are very interesting as the piezometric network is dense and the levels are not affected by external noise. The earthquake occurred after two dry weeks, and the local seismic activity was very low prior to the quake. Moreover, to our knowledge, it is the first time that hydrologic responses to an earthquake are reported and described in the Caribbean. The main objectives of this poster are to analyze the piezometric level changes observed in Martinique and attributed to this earthquake, to deeper characterize the earthquake mechanism and if possible to better understand the structure and functioning of the aquifers from Martinique. This poster will also spot piezometers that show probable precursors. These precursors consist in a decrease of the recession slope and/or of no more discernable barometric and/or tidal effects. These precursors begin approximately 30 to 75 h before the earthquake each at a different date. They all reveal a compression (a probable increase of the stresses) before the quake; the piezometers with such a precursor showing then systematically a co-seismic distension.