Gas release from the LUSI eruption site: large scale estimates.

Abstract:

The spectacular Indonesian Lusi mud eruption started in May 2006 following to a 6.3 M earthquake striking the island of Java. Previous studies investigated the mechanisms of reactivation of the Watukosek fault system that crosses Lusi locality and continues to the NE of Java. Results show that the quake triggered lateral movement of this strike-slip system resulting in several aligned eruptions sites including Lusi. Geochemical studies of the erupted fluids reveal a mantle signature and point to a connection with the neighboring Arjuno-Welirang volcanic complex indicating that Lusi is a sedimentary hosted geothermal system.

In order to better understand 1) the geometry of the Lusi subsurface plumbing system, 2) to estimate the type and the amount of gas released, and 3) how tectonic structures may control this activity, we conducted a comprehensive survey around the Lusi crater. We sampled more than 60 seepage sites to analyze the composition of the gas released and conducted a flux measurements survey of over 350 stations (CO₂ and CH₄). In addition we completed three CO₂, CH₄, radon profiles (120 points) perpendicular to the NE-SW oriented Watukosek strike-slip fault system and complemented that with geoelectric surveys.

Results show that the whole area is characterized by diffused gas release through seeps, fractures, microfractures and soil degassing. Overall the highest gas flux were recorded at stations crossing the fractured zones that coincide with the position of the Watukosek fault system. The fractures release mainly CO₂ (with peaks up to 400 g/m²day) and display higher temperatures (up to 41°C). This main shear zone is populated by numerous seeps that expel mostly CH4. Flux measurements in the seeping pools reveal that φCO₂ is an order of magnitude higher than that measured in the fractures, and two orders of magnitude for φCH₄. Radon measurements vary from 30 and 90 Bq/m³ on the edges of the study area to 6000 Bq/m³in the proximity of the faulted zone suggesting the presence of preferential leakage pathways towards the surface. The higher gas measurements in the faulted areas coincide with the anomalies recorded by the geoelectric survey.

Statistical analyses over the 7 km² area allowed estimating the full amount of gas release. Estimates from the crater zone suggest an order of magnitude higher than the surroundings.