Stress and mass changes during the 2011-2012 unrest at Kawah Ijen volcano, East Java, Indonesia

Kawah Ijen volcano (East Java, Indonesia) has been equipped since June 2010 with 3 broadband seismometers, temporary and permanent short-period seismometers. While the volcano did not experience any magmatic eruption for more than a century, several types of unrests occurred during the last years. Apart from the seismometers, temperature and leveling divers have been immerged in the extremely acidic volcanic lake (pH ~ 0). While finding instruments capable of resisting in such extreme conditions has been particularly challenging, the coupling of lake monitoring techniques with seismic data improves the understanding and monitoring of the volcanic-hydrothermal system. To detect small velocity changes, the approach developed by Brenguier et al. (2008) and Clarke et al. (2011) has been implemented to monitor Ijen volcano. First, the influence of several parameters detrimental to the recovering of the cross correlation function will be discussed (i.e.: different types of seismometers and their azimuthal distribution, presence of volcanic tremor in different frequency bands). At Kawah Ijen, the frequency band that is less affected by the volcanic tremor and the seasonal fluctuations at the source ranges between 0.5-1.0 Hz. Moreover, a stack of 5 days for the current CCF gives reliable results with low errors and allows to detect fluctuations which are missed using a 10-day stack. We will then present the results of this technique compared to other seismic parameters (e.g.: seismo volcanic events spectral analysis) and temporal changes in lake temperature, color or lake levels that occurred during 2011-2012 crises that were the strongest ever recorded by the seismic monitoring network. An unrest commenced in October 2011 with heightened VT (Volcano Tectonic) earthquakes and low frequency events activity, which culminated mid-December 2011. This unrest was correlated with an enhanced heat and hydrothermal fluids discharge to the crater and significant variations of the relative velocities (-1%). This suggests an important build-up of stress into the system. VT earthquakes opened pathways for the fluids to ascend, by increasing the permeability of the system, which latter allowed the initiation of monochromatic tremor (MT) when the steam/gases interacted with the shallow portions of the aquifer. Our calculations evidence a higher contribution of steam in March 2012 that might explain the increase of the MT frequency when bubbles were observed at the lake surface. This period was also characterized by short-lived but strong velocity variations, related to water level rises containing important amount of bubbles, and important heat and mass discharges into the lake (2100 kJ/kg and 255 kg/s, respectively).