UP Geologists Report Subsidence Rates in Select PH Metropolitan Cities

By: Eunice Jean C. Patron

Land subsidence, or the gradual sinking of the ground, threatens not only Metro Manila but also other cities.

In Photo: (left) Extreme damage running along the length of a road in Marilao, Bulacan is visible. (right) A reconnaissance fieldwork near the trace of the West Valley Fault showing cracks on the road with vertical displacements of 5-7 cm and an aperture of 12 cm (Photo credit: Sulapas et. al., 2024)

Excessive groundwater extraction, rapid urbanization, tectonic motion, and the natural compaction of sediments contribute to land subsidence worldwide. This issue is 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.

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 in major Philippine cities from 2014 to 2020.

Their land subsidence analysis showed that Bulacan, which is located in Greater Manila, has the highest sinking rate at 109 millimeters (mm) per year. This was 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,” Sulapas said. “Cities are also more vulnerable to land subsidence because the subsurface strata, or the rocks beneath the cities, are relatively younger.”

These sinking areas are mostly found in industrial and commercial zones, where large and expansive man-made structures intensify subsidence. Monitoring these areas is crucial, as subsidence increases flood risks, damages buildings and infrastructure, and causes economic losses. This hazard also increases the vulnerability of residents in coastal communities to high tides and storm surges aggravated by climate change.

“It seems like industrial complexes require a lot of water to keep their operations running. The same can be said for residential areas,” Sulapas noted.

The study recommends sustainable groundwater use and improved water management practices. Sulapas mentioned that previous studies on land subsidence were recognized by the National Water Resources Board (NWRB). “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,” Sulapas added. “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?”

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.

The research, titled “Ground subsidence in major Philippine metropolitan cities from 2014 to 2020,” was published in the International Journal of Applied Earth Observation and Geoinformation, which publishes original papers that apply earth observation data to the inventory and management of natural resources and the environment. (PR)

References:

Sulapas, J. J., Ybañez, A. A., Marasigan, K. M., Grageda, J. M., & Lagmay, A. M. (2024). Ground subsidence in major Philippine metropolitan cities from 2014 to 2020. International Journal of Applied Earth Observation and Geoinformation. https://doi.org/10.2139/ssrn.4779261