GPS-Based Monitoring of Surface Displacements in the Mud Volcano Area, Sidoarjo, East Java']Java

Since 29 May 2006, gas and hot mud has been gushing from the ground in Sidoarjo, East Java, Indonesia. As of late September 2006 scientists assume that the eruption may be a mud volcano forming, and may be impossible to stop. Surface displacements of the area, both in the vertical and horizontal directions, are expected due to this massive mud extrusion. GPS observations, both in campaign and continuous modes, were conducted to study this surface displacement phenomenon. GPS surveys were performed on up to about 20 stations using dual-frequency geodetic-type receivers with observation session lengths of about 5-7 h. Nine GPS campaigns have been conducted between June 2006 and June 2007. GPS continuous subsidence monitoring was conducted on five to ten stations, and started on 22 September 2006. A field survey to check the surface representation of the Subsidence phenomenon was also conducted. Based on the GPS-derived results it was determined that the area about 3-4 km around the extrusion centre experiences surface displacements. Rates of horizontal displacement are about 0.5-2cm/day, while vertical displacements are about 1-4 cm/day, with rate increasing towards the extrusion centre. These observed surface displacements might consist of ground relaxation due to mudflow, loading due to the weight of mud causing the area to compact, land settlement and/or reactivation of the geological structures.