The Manila trench is the boundary between the Philippine Sea plate and the Sunda plate, along which the oceanic crust of the South China Sea is subducting eastward under the Philippines, the Luzon Strait, and southern Taiwan. Although no historical earthquake along the Philippine portion of the Manila trench has exceeded MS (magnitude) 7.6, we cannot rule out the possibility that M 9 ruptures may occur along the Manila trench, with recurrence intervals of 500 years or longer. Such an earthquake would significantly impact Luzon, one of the more densely populated islands in Southeast Asia, with 48 million inhabitants. Additionally, tsunami modeling has shown that such a rupture could lead to devastation of shorelines across much of the South China Sea. However, without knowledge of the rupture history or interseismic coupling over multiple seismic cycles, any conjecture about the seismogenic potential of the Manila trench is little more than speculation.
We are using coral microatolls in Luzon to reconstruct details of interseismic and coseismic deformation along the Manila trench. Microatolls form because prolonged subaerial exposure at times of extreme low water limits the highest level to which the coral colonies can grow. A microatoll’s growth history precisely records changes in relative sea level, and its annual banding permits precise resolution of the timing of those changes in relative sea level. From this, we can reconstruct the timing, extent, and magnitude of past deformation, both sudden (coseismic) and gradual (interseismic). This, in turn, allows us to infer a history of strain accumulation and release (earthquakes) along the subduction zone and upper-plate faults.
In related work, we are using remote sensing to help identify upper-plate structures that may be active over longer time scales.