Abstract
Land subsidence is a global threat to coastal areas worldwide, including the North Java coastal area. Of many areas experiencing land subsidence in North Java, the rate of land subsidence in Pekalongan has matched the high subsidence rates usually found in big cities. The rate of land subsidence in Pekalongan far exceeds the sea-level rise, resulting in a looming threat of land loss. The devastating impacts of land subsidence are the manifestation of its subsurface movement. Therefore, it is essential to understand the subsurface to elucidate the mechanism of land subsidence. Previous studies on land subsidence in Pekalongan are mainly related to subsidence rate monitoring and have not elaborated on the subsurface condition. This paper reviews the Pekalongan subsurface geology based on available literature to provide insight into the land subsidence problem. The results revealed that the land subsidence occurs in the recent alluvial plain of Pekalongan, consisting of a 30-70 m thick compressible deposit. Possible mechanisms of land subsidence arise from natural compaction, over-exploitation of confined groundwater, and increased built areas. As the seismicity of the study area is low, tectonic influence on land subsidence is considered negligible. It is expected that the offshore, nearshore, and swamp deposits are still naturally compacting. As the surface water supply is minimal, over-exploitation of groundwater resources from the deltaic and Damar Formation aquifers occurs. In the end, future research direction is proposed to reduce the impacts of the subsidence hazard.
Highlights
The deltaic region is highly susceptible to risks exposed from local human activities, sea-level rise, land subsidence, storms and flooding, and an increase in groundwater salinity
In Pekalongan, the active fault extends for 16 km with a deformation rate of 4.5 mm/year (National Center for Earthquake Studies, 2017).A probabilistic earthquake hazard study by Ashadi et al (2015) shows that the peak ground acceleration (PGA) in Pekalongan bedrock due to subduction and crustal fault-related earthquake is 0.14-0.15g with a 10% probability of exceedance in 50 years corresponding to the return period of 475 years
The influence of tectonic to subsidence is temporarily omitted until the existence of an active fault is proven.The high subsidence rate is related to the larger thickness of the compressible deposit
Summary
The deltaic region is highly susceptible to risks exposed from local human activities, sea-level rise, land subsidence, storms and flooding, and an increase in groundwater salinity. Global sea-level rise poses a threat to the coastal lowland, a more critical hazard is forthcoming: the subsidence of flat deltaic plains, in Southeast Asia (Schmidtz, 2015). In Southeast Asia and globally, Indonesia counts as one of the countries with a highly exposed population to land subsidence and sea-level rise hazards, with nearly 60% of its population living near the coast (Cao et al, 2021; Herrera-García et al, 2021). Land subsidence in North Java affects almost the entire north coast, the coastal cities, such as Jakarta, Bekasi, Pekalongan, Semarang, Surabaya (Andreas et al, 2019a; Sarah & Soebowo, 2018).
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