Land subsidence in Jakarta and Semarang Bay - The relationship between physical processes, risk perception, and household adaptation

Sea level rise (SLR) is among the most pressing challenges for urban coastal areas. While geocentric (eustatic) SLR receives widespread attention in politics and media, relative SLR at the coast, mainly caused by land subsidence, is still comparatively under-researched despite much higher rates. This paper introduces a combined natural and social science study to bring subsidence more to the forefront of coastal hazard research. We use data from radar altimetry, GNSS controlled tide gauge stations, and InSAR mapping to characterize regional and relative SLR at Jakarta and Semarang Bay, and focus-group discussions and a standardized household survey to analyze risk perceptions and adaptation. Our analysis of InSAR, radar altimetry, and corrected tide gauges clearly identifies subsidence as the major coastal threat in our study areas. The InSAR analysis for Semarang shows stable trends of subsidence up to -100 mm/a. For Jakarta, our analysis reveals more complex spatial and temporal patterns with rates around 60 mm/a; revealing significant changes to previous studies. Our analysis of radar altimetry data since 1993 shows a moderate regional SLR of 2.1 mm/a off Semarang and 3.2 mm/a off Jakarta. The InSAR data are integrated into our statistical analysis of household responses towards subsidence. We found, that in contrast to fast-onset events, constantly proceeding subsidence becomes normalized in peoples' perceptions and responses are integrated into day-to-day habits. Thus, risk perception is a far lesser determinant of responses towards subsidence than it is for fast-onset events. Hence, our results relativize former assumptions that risk perception and not actual exposure lead to action. Moreover, we found that local people are not willing to vacate highly exposed areas. Their views need to be included in municipal disaster risk reduction, the urgency of which clearly lies on mitigating subsidence effects rather than on building protection against regionally rising sea levels.