The Lusi Eruption

---

The 29th of May 2006 several gas and mud eruption sites suddenly appeared along a fault in the NE of Java, Indonesia. Within weeks several villages were submerged by boiling mud. The most prominent eruption site was named Lusi. To date Lusi is still active. This disaster has forced 50.000 people to be evacuated and an area of more than 7 km² is covered by mud. The social impact of the eruption and its spectacular dimensions still attract the attention of international media reporting on the “largest mud eruption site on Earth”.

In 2006 I organized the first fieldwork to Indonesia to the Lusi eruption site. The Lusi eruption represents a unique opportunity to study the birth and evolution of a mud eruption, most of the studies on this type of structures are conducted during the dormant periods between eruptions. Our extensive fieldworks in 2006, 2007, 2008,  2009, 2010, 2011, 2012 aimed to define the origin of the erupted solids/fluids and the mechanisms that triggered this eruption.

Our investigations revealed that the Watukosek fault system reactivated after the 27-05-2006 Yogyakarta earthquake allowing the release of overpressured fluids along the fault planes. Sampling results indicate that the main source of clay and fluids was traced from the overpressured units located at ~1500 m depth. Further, analyses and modelling indicate that Lusi gas was generated at high temperatures (>220°C) with maturity and isotopic characteristics corresponding to the oil-prone Eocene, Ngimbang shales situated at 4,400 m. Hydrocarbon, CO2 and helium analyses are consistent with a scenario of deep sited (>4000 m) magmatic intrusions and hydrothermal fluids responsible for the enhanced heat that altered source rocks and/or gas reservoirs. The neighbouring magmatic Arjuno complex and its fluid-pressure system combined with high seismic activity could have played a key role in the Lusi genesis and evolution.

We collected sufficient data to initiate a synergetic study of the Lusi eruption combining field observations with analogue laboratory experiments complemented by analytical and numerical modelling. Our research gives totally new insights about the Lusi eruption and is applicable to numerous piercement structures occurring along strike-slip zones. Our data demonstrate that shearing is a very efficient mechanism for the release of overpressured fluids.