LUSI MITIGATION IMPLICATIONS OF BPLS AND OTHER SUBSIDENCE MEASUREMENTS

Abstract

Since the beginning of LUSI eruption, extrusion of mud has been accompanied by deformation of the land surface extending out as much as three km from the main vent. Deformation generally consists of subsidence, however also includes uplift in certain areas and minor surface faulting and cracking as surface blocks of sediment move. BPLS monthly leveling since 12/2007 by laser surveying gives the most reliable measure of vertical movement around the perimeter of the mud ponds. This has been augmented by differential GPS measurements that extends farther out. Subsidence beneath the mud within the levees is difficult to be measured accurately. Rapid subsidence, rising mud, and continuous addition of dirt to maintain the internal dikes make it impossible to maintain a system of benchmarks in the inner area. Shape of the surface beneath the mud can be estimated by indicators such as early GPS measurements, tilting of partially buried buildings, and the rate at which tiers of sandbags have disappeared beneath the mud. We have synthesized all available data to produce a contour map of estimated total deformation for the first 35 months of eruption despite limitations of the data such as variable quality, contradictions, poor point distribution, and observations over variable time intervals.

Pre-eruption land surface near LUSI was a nearly level plain about 4 m above MSL sloping about 0.45 m/km NE toward the sea and away from the artificially high channel and levees of the Porong River. Land around the vent has subsided in the form of an asymmetrical shallow funnel where the gentle tilt of about 10 m/km around the margins increases dramatically near the vent. Early ITB continuous differential GPS measurements of horizontal movement indicated a point of maximum subsidence about 250 m northwest of the main vent. Our contours indicate 65 million m³ of mud is presently stored. About 60% occupies the subsidence depression, the rest is confined by the dikes. About 200 m south of the vent is a flexure zone trending generally east-west that separates fast subsidence from slower subsidence to the south. This flexure has caused difficulty in mitigation by blocking flow of mud southward towards disposal points along the Porong River.

On the west side of the subsidence cone, flexure toward the vent area has stretched the ground surface, breaking it with steep north-northeast trending fractures that localize methane gas venting. These resulting blocks settle unevenly, creating horsts and grabens with offset less than 15 cm. This fracturing probably extends east beneath the levees although it is not clearly expressed there. Transverse to this pattern near the former toll road bridge is a 100 m-wide zone characterized by the highest subsidence rates outside the levees. This subsidence of about 1.5 m in three years is much less than an estimated 40 m subsidence beneath the mud near the vent, but lies across natural drainage and causes local flooding. At present rates of subsidence, this area will be below sea level in four years and become a pond unless constantly pumped. Until the road realignment is completed, most of the commercial traffic of East Java must travel through this area, and the proposed realignment still lies perilously close to northwesterly expanding subsidence. Increased methane venting and ground cracking has made West Siring Village too hazardous for continued habitation.

Keywords: Subsidence, mitigation, LUSI, mud, eruption