Geochemical surveys in the Lusi mud eruption

Abstract

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