A study conducted by geologists from the University of the Philippines-Diliman (UPD) warned that land subsidence caused by factors such as climate change, excessive groundwater extraction, rapid urbanization, tectonic motion, and the natural compaction of sediments threatens to erase from the map not just cities in Metro Manila but also nearby metropolitan cities.
Jolly Joyce Sulapas, Audrei Anne Ybañez, and Dr. Alfredo Mahar Francisco Lagmay of the University of the Philippines – Diliman College of Science (UPD-CS), along with Kayla Milcah Marasigan and Julian Marie Bernice Grageda of the UP Resilience Institute Nationwide Operational Assessment of Hazards (UPRI-NOAH), analyzed land subsidence or the gradual sinking of the ground in major Philippine cities from 2014 to 2020.
Their study, titled "Ground subsidence in major Philippine metropolitan cities from 2014 to 2020" that was published in the International Journal of Applied Earth Observation and Geoinformation, revealed that Bulacan has the highest sinking rate at 109 millimeters (mm) per year, followed by Cavite and Laguna.
It is also the first study to examine land subsidence in other cities, with rates of 11 mm per year in Metro Cebu, 38 mm per year in Metro Davao, 9 mm per year in Metro Iloilo, and 29 mm per year in Legazpi City.
“The high population density of cities makes the information more relevant to the public, as its impact on people is greater. Cities are also more vulnerable to land subsidence because the subsurface strata, or the rocks beneath the cities, are relatively younger," Sulapas said.
Land subsidence is also worsened by rising sea levels due to climate change, leading to frequent and severe flooding especially during high tides or typhoons, as well as local contexts of hazard management regulations.
Sulapas explained that areas that have recorded the highest sinking rates are mostly found in industrial and commercial zones where large and expansive man-made structures exacerbate subsidence.
What's next?
Aside from determining the areas that are most at risk of sinking, the geologists have also analyzed the most effective ways of preventing or mitigating land subsidence in these communities which include improved and sustainable groundwater use and water management practices.
Sulapas noted that the National Water Resources Board (NWRB) has recognized previous studies regarding land subsidence and has stepped up its efforts in controlling them.
“The Board then identified zones where groundwater usage would be restricted,” she said.
“My co-authors were part of a previous study on subsidence in Metro Manila and surrounding provinces. This was during the year 2010. So more than a decade later, what we wanted to do is to update the deformation data. In the case of the Greater Manila area, what happened after more than a decade?" she added.
"Moving forward, the team of researchers plans to incorporate additional data, such as groundwater extraction in the major Philippine metropolitan cities covered in their study, and correlate it with existing information. They also intend to continuously update their data to include the years 2021 to the present, collaborating with other researchers interested in contributing to the study over the next decade and beyond." UPD said.