Coastal subsidence increases vulnerability to sea level rise over twenty first century in Cartagena, Caribbean Colombia

Cartagena is subsiding at a higher rate compared to that of global climate-driven sea level rise. We investigate the relative sea level rise (RSLR) and the influence of vertical land movements in Cartagena through the integration of different datasets, including tide gauge records, GPS geodetic subsidence data, and Interferometric Synthetic Aperture Radar (InSAR) observations of vertical motions. Results reveal a long-term rate (> 60 years) of RSLR of 5.98 ± 0.01 mm/yr. The last two decades exhibited an even greater rate of RSLR of 7.02 ± 0.06 mm/yr. GPS subsidence rates range between − 5.71 ± 2.18 and − 2.85 ± 0.84 mm/yr. InSAR data for the 2014–2020 period show cumulative subsidence rates of up to 72.3 mm. We find that geologically induced vertical motions represent 41% of the observed changes in RSLR and that subsidence poses a major threat to Cartagena’s preservation. The geodetic subsidence rates found would imply a further additional RSLR of 83 mm by 2050 and 225 mm by 2100. The Colombian government should plan for the future and serve as an example to similar cities across the Caribbean.


Mud diapirism and tectonics in the Cartagena region. Mud intrusion (diapirism) and volcanism are characterized
by fluid-rich and fine-grained sediments that ascend within a lithological succession due to their buoyancy. These geologic features and processes have long been associated with the occurrence of petroleum, gas, regional volcanic and earthquake activity, and collisional tectonic settings. Most importantly, mud volcanoes occur along convergent plate margins or collisional settings where fluid-rich sediment is accumulated in deep-sea trenches at high rates. Such deposits then enter the subduction factory, where liquids and volatiles are released due to compactional stress and temperature. After these fluids are released, sediment compaction and further subsidence occur 1 .
In the Cartagena region, early descriptions of these mud volcanoes date back from the 1960s 2,3 . Later, other geologic assessments classified the coral reef islands around Cartagena, including Tierra Bomba island ( Supplementary Fig. S1b), as diapiric domes due to upheaval mechanisms during the late Holocene 4-8 . Based on recent seismic profiles 9 (Supplementary Fig.  S1c) and studies 10-13 , many diapiric formations and domes have been mapped in Cartagena Bay 14 ( Supplementary Fig. S1b), showing that mud diapirism-induced subsidence in Cartagena is much greater than previously thought. Approximately more than 45 mud domes, hereafter pop marks, have been identified in the bottom of the bay (Supplementary Fig. S1b).
Further evidence of recent tectonism in the Cartagena region includes the upheaval of coastal terraces at Tierra Bomba at a rate of 3.11 mm/yr, the upheaval of a diapiric dome with a Pleistocene reef, locally known as La Popa Hill 8 , and different local tectonic faults like the Dique fault ( Supplementary Fig. S1b), an important component of the neotectonic activity in the Cartagena region 4 . Also, many mud-volcanoes have had eruptions during recent times in Cartagena. For example, a major event in the El Rodeo Volcano occurred in May 2014. This event produced major structural damages in buildings as well as land subsidence in the area ( Supplementary Fig. S2). There is no doubt that the complex geology of Cartagena, which is characterized by compressional and transpressional tectonics, active diapirism and mud volcanism, is a key factor affecting the vertical land motions observed in the coastal bay and city.

Satellite altimetry data of absolute sea level (ASL) across the Caribbean offshore area of Colombia.
Significant trends and values similar to the absolute global mean sea level rate (2.9 ± 0.1 mm/yr) 14 were found over the region covering the northern and southern parts of the Caribbean offshore area. Overall, the whole Caribbean marine area of Colombia has witnessed an increasing mean ASL trend of 2.96 ± 0.12 mm/yr during the 1993-2015 period. Trends of ASL rise range from 2.83 ± 0.18 mm/yr in the northern region of the Guajira Peninsula, to 3.22 ± 0.22 mm/yr in the offshore area of Cartagena ( Supplementary Fig. S3).
Relative sea level observations from pressure sensors. To compare and contrast the relative sea level trends between the tide gauge and the southern part of Cartagena Bay, we made water level observations at four stations (Fig. 1c,  Supplementary Fig. S4). Time series measurements of water level elevation were obtained using several near-bottom pressure sensors. The instruments were deployed at the bottom of each station, programmed to calculate a 2-min average every 10 min, and remained moored from April to October 2013.
Hourly relative sea level (RSL) data from the tide gauge at CIOH (Fig. 1c) during the same period of April-October 2013 were obtained from the Hydrological and Environmental Institute of Colombia (IDEAM), which was previously the institution in charge of the tide gauge. These RSL tide gauge data were related to water level observations from the pressure sensor stations through linear fit-transfer equations. To obtain the long-term RSL trend for 1952-2000 at the location of each pressure sensor ( Supplementary Fig S4a), each transfer equation was applied to the RSL series for the 1952-2000 period. Overall, the long-term RSLR trends for 1952-2000 in the southern part of the bay varied between 3.55 and 5.32 mm/yr ( Supplementary Fig. S4), values that are very similar to previous rates of RSLR 15