A document by Rene Steinmann. Click on the document to view its contents.

We apply three different methods based on the analysis of the multi-component seismic data to detect seismovolcanic tremors and other seismovolcanic signals, to propose an approach to classify them and to locate their sources. We use continuous seismograms recorded during one year by 21 stations at the Piton de la Fournaise volcano (La Réunion, France). The first method allows to detect seismovolcanic signals based on stability in time of the inter-components cross-correlations function. Two other methods based on the simultaneous analysis of the whole network can be used to detect seismovolcanic signals and to locate their sources. In a second approach, the seismic wavefield is analyzed by calculating the width of the network covariance matrix eigenvalue distribution. The third method consists in performing the 3D back-projection of the inter-stations crosscorrelations in order to calculate the network response function. Simultaneous analysis of the parameters measured by the three different methods can be used to classify different types of seismovolcanic tremors. Our results demonstrate that all three methods efficiently detect seismovolcanic tremors accompanying the 2010 eruptions and the preceding pre-eruptive seismic swarms. Furthermore, methods 2 and 3 based on simultaneous analysis of the whole network detect a large number of volcanic earthquakes. Our location results show that each seismovolcanic tremor is located in a distinct region of the volcano, close to the eruptive site at a shallow depth and the preceding seismic crisis is located deeper at about the sea level under the summit crater.

Taupo Volcano, located in the central part of the TVZ (Taupo volcanic Zone), North Island of New Zealand, is one of the most productive Rhyolitic centres in the world. Although its last eruption occurred about 1800 years ago, 16 periods of unrest have been identified including surface deformation, hydrothermal eruptions, and seismic swarms since 1870. The town of Taupo lies on the north-eastern shore of the lake filling the caldera of the volcano and is located close to recent seismic swarms and local surface deformation episodes highlighted in this report. The aim of this work is to study the different periods of episodic deformation, contrasting with the long-term deformation of the Taupo region, in order to constrain the sources generating local deformation. For this, an analysis of GPS (continuous and campaign stations) and InSAR data (from two satellites, EnviSAT and ALOS) was conducted. After correcting the data for several external factors such as subsidence generated by water pumping in the Wairakei-Tauhara geothermal station and displacements associated with slow slip events along the Hikurangi subduction interface, periods of local deformation have been identified. We highlight two periods of uplift with rates of 10 mm/yr in 2004-2008 and in 2011-2013 accompanied by more or less rapid horizontal deformation punctuated by seismic swarms. The geodetic data were inverted to characterize the deformation sources using the GBIS software, allowing the use of different analytical models. In order to explain the different periods of deformation over time, at least three sources at different locations are needed, revealing the presence of different processes at depths ranging from ∼ 10 km to ∼ 0.5 km and whose causes can vary given the complexity of the tectonic context characterizing the region.