Airport plan raises earthquake risks

The relocation of Taipei International Airport (Songshan Airport), which Taipei mayor Chiang Wan-an (蔣萬安) proposed on Thursday, has been debated for years. Mayoral candidates have previously advocated preserving the site as a park and green space, which I would have no objection to. However, plans for the site to undergo intensive development would risk overloading the land’s carrying capacity and seriously exacerbate the severity of potential earthquakes.

While a major offshore megathrust earthquake would severely affect Taipei and New Taipei City, movement along the Shanchiao Fault (山腳斷層) could mean an earthquake or subsidence event taking place right underneath the city, pushing the capital toward the point of destruction beyond repair.

As extensive land consolidation in low-lying areas contributes to fault risks, one cannot assume that the Shanchiao Fault would remain inactive. Consider the current state of the Taipei Basin: It has been 50 years since land consolidation began in the low-lying areas of New Taipei City’s Sinjhuang (新莊) and Wugu (五股) districts; plans have been made to raise the ground level across Shezidao (社子島); extensive filling is under way for the Wenzizun (塭仔圳) development project scheduled for completion next year; and large-scale land reclamation also lies ahead for Guandu Plain’s agricultural area. If a major development project follows the relocation of Songshan Airport, it would inevitably mean further land consolidation projects.

What is particularly concerning is that all these development projects are concentrated in areas with elevations close to sea level, which is precisely where movement along the Shanchiao Fault could cause the basin to subside. Here, extensive landfilling reaching the equivalent of two stories in height increases stress on the fault’s seismogenic zone by approximately 1.2kg per square centimeter.

For reference, the “stress drop” associated with the change is shear stress on a fault after each rupture usually only amounts to several to several dozen kilograms per square centimeter. The weight of vast quantities of fill and subsequent development could therefore be enough to destabilize the Shanchiao Fault, triggering a major earthquake directly beneath the city. At the very least, it could substantially shorten the fault’s recurrence interval, bringing the next earthquake forward.

The last earthquake along the Shanchiao Fault occurred in 1694. In its aftermath, the basin subsided and its northwestern half was inundated, forming a 5m-deep lake. This became known as the Kangxi Taipei Lake. If this were to happen again, today’s Taipei Basin could suffer severe soil liquefaction and ground deformation, damaging drainage systems, rupturing utility pipelines and causing buildings to collapse. Even underground MRT and high-speed rail tunnels could sustain serious damage, making post-disaster reconstruction exceptionally difficult.

Given these circumstances, the central government should act quickly to regulate or even ban large-scale land consolidation and development in the low-lying areas of the Taipei Basin. Particular care should be taken in deciding how the land would be used following the relocation of Songshan airport. Reckless urban expansion should not become a force that raises the risk of a devastating earthquake directly beneath the city.

Lee Chyi-ti is a professor at National Central University’s Graduate Institute of Applied Geology.