The NASA-ISRO Synthetic Aperture Radar (NISAR) mission, launched in July 2025, has detected rapid land subsidence in Mexico City, with specific areas sinking over two centimeters per month between October 2025 and January 2026. This joint satellite mission utilizes advanced radar imaging to map surface deformation with high precision. Mexico City, constructed on an ancient lakebed composed of soft sediments, experiences this severe ground sinking primarily due to excessive groundwater extraction from its underlying aquifer system. The latest radar data serves as a critical tool for urban planning and disaster risk reduction in the region.
Understanding Land Subsidence
Land subsidence refers to the gradual settling or sudden sinking of the Earth’s surface due to the removal of subsurface materials.
Mechanisms and Causes
- Groundwater Depletion: Excessive pumping of water from underground aquifers reduces pore water pressure. This causes the surrounding soil and sediment layers to compact permanently.
- Soil Composition: Alluvial, lacustrine (lake-derived), and organic soils are highly compressible. When water is drained, these soft sediments collapse under the weight of urban infrastructure.
- Anthropogenic Load: The heavy structural weight of dense urban buildings accelerates the compaction of weak subsurface strata.
The NISAR Mission and Technical Capabilities
The NISAR satellite is a pioneering Earth-observation project developed jointly by the National Aeronautics and Space Administration (NASA) and the Indian Space Research Organisation (ISRO).
Sensor Configurations
- Dual-Frequency Radar: The satellite carries both an L-band Synthetic Aperture Radar (developed by NASA) and an S-band Synthetic Aperture Radar (developed by ISRO).
- L-Band Capability: The L-band radar features a long wavelength of approximately 24 centimeters. This allows the signal to penetrate dense vegetation canopies, clouds, and darkness to map the ground surface.
- S-Band Capability: The short-wavelength S-band radar specializes in heavy weather penetration and provides high-resolution data on surface roughness and near-surface changes.
Monitoring Surface Deformation
- Interferometry (InSAR): NISAR employs Interferometric Synthetic Aperture Radar techniques. By comparing radar images taken from the exact same orbital position at different times, scientists measure millimeter-scale changes in land elevation.
- Observation Frequency: The satellite tracks the entire globe every 12 days, providing regular, repeated data points to map ongoing hazards like sinkholes, landslides, and subsidence.
Impacts on Mexico City
The geographic and historical context of Mexico City aggravates the structural risks associated with ground sinking.
Historical Background and Geological Vulnerability
- Lake Texcoco Bed: The Aztec city of Tenochtitlan, which later became Mexico City, was built on an island within Lake Texcoco. The Spanish empire drained the lake system to expand the settlement, leaving behind a deep foundation of soft, water-saturated clay.
- The Angel of Independence: Built in 1910, this iconic monument serves as a visible gauge of subsidence. The base of the monument was originally flush with the street. Because the surrounding ground has sunk over time, city authorities have built more than twenty concrete steps around its base to maintain access.
Infrastructure and Environmental Risks
- Structural Damage: Differential subsidence, where different parts of a building sink at different rates, fractures building foundations, ruptures sewage lines, and breaks drinking water pipes.
- Increased Flood Risk: Sinking topography alters natural drainage gradients, making urban areas highly vulnerable to severe seasonal flooding.
Comparative Overview of Globally Sinking Cities
| City | Primary Cause of Subsidence | Observed Sinking Rate | Key Mitigation Strategies |
| Mexico City, Mexico | Aquifer depletion on a compressible clay lakebed | Up to 20–50 cm per year in specific zones | Groundwater regulation, rainwater harvesting |
| Jakarta, Indonesia | Excessive groundwater extraction, heavy building load | Up to 15–25 cm per year in northern coastal areas | Construction of sea walls, moving the capital to Nusantara |
| Venice, Italy | Natural tectonic settling, historical groundwater pumping | 1–2 mm per year historically | MOSE storm surge barrier system, ban on artesian wells |
| Joshi Math, India | Aquifer discharge, toe erosion, heavy construction on moraine soil | Variable seasonal acceleration | Ban on major construction, drainage management |
IASPOINT Booster Facts for UPSC
- Orbit Type: NISAR operates in a sun-synchronous, low-Earth orbit (LEO) at an altitude of approximately 747 kilometers.
- SweptSAR Technique: The mission utilizes a specialized imaging technique called SweptSAR to map a wide swath of land (over 240 kilometers) at high resolution.
- Geological Formations: Aquifers in lacustrine environments consist of fine-grained aquitards (clay and silt layers) interspersed with coarser aquifers. The compaction happens mostly within the aquitards when water pressure drops.
- Nusantara Shift: Jakarta’s severe subsidence crisis prompted the Indonesian government to officially pass a law to relocate its capital city to Nusantara on East Kalimantan island.
- Indian Context: India monitors its own land subsidence zones, such as Joshimath in Uttarakhand and specific patches in Delhi-NCR, using ISRO’s Cartosat satellites and European Space Agency’s Sentinel-1 radar data.