Lava discharge rate and effusive pattern at Piton de la Fournaise from MODIS data

We analysed the effusive activity of 15 eruptions of Piton de la Fournaise by using the data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) during 2003-2007 period. Thermal anomalies were detected by adjusting the values of the Normalised Thermal Index (NTI) threshold [Wright, R.L, Flynn, H., Garbeil, A.J.L, Harris Pilger. E., 2002. Automated volcanic eruption detection using MODIS. Remote Sens. Environ. 82: 135-155.] in accord to the seasonal and topographic variations of the background temperature at Piton de la Fournaise. Each satellite overpass, has been classified according to the presence of clouds and the satellite viewing geometry. The analyzed data indicate that at Piton de la Fournaise only 10% of MODIS overpasses occurred in optimal conditions (clear sky and satellite zenith <40 degrees). However this classification allowed us to use the 4 pm radiance to estimate time average of lava discharge rates (TADR) for most of the analysed eruptions. These data has been used to compare the inferred effusive pattern with the real time seismic amplitude data (RSAM) recorded during the same events. We found that summit eruptions, characterised by a lower discharge rate (<2 m(3)s-(1)), induce a separation of the gas phase which translate into a decoupling between TADR and RSAM. This regime coupled with the low magma ascent rate and the typical steady effusion of these eruptions imply a very low pressurization of the shallower reservoir which is consistent with a direct overflow of the magma supplied from depth. Conversely, the distal eruptions show a TADR linearly correlated with the RSAM thus indicating a more homogeneous degassing regime and the absence of gas accumulation process within the shallow reservoir. The high initial TADR (>20 m(3)s(-1)) and the following vanishing phase indicate that distal eruptions are triggered when a critical overpressure is reached (>5 MPa). This last seems to be increased during the whole analysed period up to the onset of the April 2007 which led to the collaps of the Dolomieu crater. Finally we propose that the recent activity of PdF may result from a persistent loading of the shallow reservoir with a magma supply rate which increased from 1135 kg s(-1) to 4310 kg s(-1) during the period between 2003 and 2007. (C) 2008 Elsevier B.V. All rights reserved.