Hussein, Alwii

The Lusi mud eruption started in May 2006 following to a 6.3 M earthquake striking the Java Island. In theframework of the Lusi Lab project (ERC grant n◦308126) we carried out geochemical surveys in the Sidoarjodistrict (Eastern Java Island, Indonesia) to investigate the gas bearing properties of the Watukosek fault systemthat crosses the Lusi mud eruption area. Soil gas (222Rn, CO2, CH4) concentration and flux measurements wereperformed 1) along two detailed profiles (∼1km long), trending almost W-E direction, and 2) inside the Lusiembankment (about 7 km2) built to contain the erupted mud.Higher gas concentrations and fluxes were detected at the intersection with the Watukosek fault and the antitheticfault system.These zones characterized by the association of higher soil gas values constitute preferential migration pathwaysfor fluids towards surface. The fractures release mainly CO2(with peaks up to 400 g/m2day) and display highertemperatures (up to 41◦C). The main shear zones are populated by numerous seeps that expel mostly CH4. Fluxmeasurements in the seeping pools reveal thatφCO2is an order of magnitude higher than that measured in thefractures, and two orders of magnitude higher forφCH4.An additional geochemical profile was completed perpendicularly to the Watukosek fault escarpement (W-Edirection) at the foots of the Penanngungang volcano. Results reveal CO2and CH4 flux values significantlylower than those measured in the embankment, however an increase of radon and flux measurements is observedapproaching the foots of the escarpment.These measurements are complemented with a database of∼350 CH4 and CO2flux measurements and somesoil gas concentrations (He, H2, CO2, CH4 and C2H6) and their isotopic analyses (δ13C–CH4,δD–CH4 andδ13C–CO2). Results show that the whole area is characterized by diffused gas release through seeps, fractures,microfractures and soil degassing. The collected results shed light on the origin of the seeping gases.Statistical analyses over the 7 km2 area allowed us to estimate the full amount of gas currently released.Flux estimates from the crater zone suggest an order of magnitude higher than those measured from the surround-ing region

The spectacular Indonesian Lusi mud eruption started in May 2006 following to a 6.3 M earthquake striking the island of Java (Mazzini et al., 2007). Previous studies investigated the mechanisms of reactivation of the Watukosek fault system that crosses Lusi locality (Mazzini et al., 2009) 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 (Mazzini et al., 2012) indicating that Lusi is a sedimentary hosted geothermal system. In order to estimate the amount of gas that is being released around the Lusi crater ( 7 km2), we recently conducted a survey of over 300 stations (CO2 and CH4 flux measurements) using a closed-chamber flux-meter system and collected gas samples to analyze the composition of the seeps. In addition 20 soil gas concentrations were collected using a steel probe driven into the ground to a depth of 0.7-0.8 m to avoid the major influence of meteorological variables. Results show that the highest CO2 flux is present along the NE-SW oriented Watukosek fault (with peaks up to 400 g/m2day) and along the E-W oriented Siring antithetic fault (with peaks up to 110 g/m2day). The pools have overall a CH4-dominated composition, while the dry fault-related fractures are CO2-dominated which is in agreement with higher recorded temperatures at these sites. Flux measurements in the seeping pools reveal that CO2 flux is an order of magnitude higher than that measured in the fault zones, and two order of magnitude for CH4 flux. C02 and CH4 microseepage is occurring in significant amount throughput the mud-covered area with average values of 297 and 95 g/m2day, respectively. CH4 flux shows the highest values in the W and NW sector of the Lusi area, while CO2 flux highlights the presence of three areas characterized by high values, in the northern, southern and western sectors. Methane concentrations show high values (maximum value 1490 ppm) in the eastern part of Lusi crater, while CO2 show low values (maximum value 11580 ppm) along the west Lusi profile.