Subsidence and uplift
2.7 Subsidence and uplift
Five years after the mud eruption, the area near LUSI has subsided at a considerable rate. Buildings and houses near the eruption site have completely disappeared under layers of mud. However, in the east and northeast uplift is occurring. To measure both the subsidence and uplift, four survey campaigns were conducted (Table 2):
25 GPS Dec. 2007
July 2006
March 2007
30 Total Station Dec. 2008
April 2009
15 GPS Dec. 2008
Table 2. Four survey methods to measure elevation near LUSI MV Data from these four surveys was used to show the changes in elevation, subsidence and
uplift, as well as horizontal movement over time. Subsidence contour maps were created using GIS software by interpolating the measurement data. The results showed an almost concentric pattern shown in Figure 17. The subsidence started as a crack in the ground that continued to grow and decrease its elevation. The existence of subsidence was evidenced by, among other things, the pattern of ground cracks, tilting of houses, cracking of flyover and bridges, as well as collapsing of buildings. The direction of the cracks varies depending on its location. In the Renokenongo area, southeast of LUSI, the cracks direction is NE- SW, whereas in West Siring area, west of LUSI, the cracks are North-South. Subsidence and horizontal movements indicate the dynamic geological changes in the area. These movements have caused reactivation of pre-existing faults or newly formed faults. The continued movements along faults would likely result in the emergence of more fractures and gas bubbles (see figures 17 and 18). Subsidence continues as the mud eruptions progress. The subsidence might result from any combination of ground relaxation due to mudflows, loading due to the weight of mud causing the area to compact, land settlement, geological structural transformation and tectonic activity (Abidin et al., 2007). Based of field measurements, areas up to 3 km from the main eruption vent are experiencing subsidence to some degree. Presently however, due to much reduced volumes of mud eruption, the measured rate of subsidence on the West side of main eruption vent indicate a decrease from the original 25 cm/month when LUSI was very active in the first year, to less than 5 cm/month. If the decreasing trend continues, the affected subsidence area will likely decrease from earlier prediction of more than 3-4 km.
Mud Volcano and Its Evolution
Fig. 17. LUSI post eruption map. The subsidence contour is status as of January 2010, constructed by interpolating the measurement data, and was created using GIS software. The contour showed an almost concentric pattern. The area West of the main vent was subsiding faster than other areas. The map also shows fractures distribution around LUSI. East of the main vent, fractures trend NE - SW, whereas West of the main vent the fracture trend is North-South. The Gas bubble distribution around LUSI status in May 2011 where more than 220 gas bubble locations have been recorded since the start of LUSI eruption in May 2006. Presently only a few are still active.
Earth Sciences
Fig. 18. Photo showing subsidence and collapse of the retaining mud dyke northeast of the LUSI main vent that occurred on 21 May 2008. In some parts, where slumping and subsidence occurred, local small scale faulting at the edge of subsiding wall occured. The continued subsidence proves very difficult to maintain the dyke.