Occurrence of 1 ka-old corals on an uplifted reef terrace in west Luzon, Philippines: Implications for a prehistoric extreme wave event in the South China Sea region

Abstract

Recent 230Th dating of fossil corals in west Luzon has provided new insights on the emergence of late Quaternary marine terraces that fringe west Luzon Island facing the Manila Trench. Apart from regional sea level changes, accumulated uplift from aseismic and seismic processes may have influenced the emergence of sea level indicators such as coral terraces and notches. Varied elevations of middle-to-late Holocene coral terraces along the west Luzon coasts reveal the differential uplift that is probably associated with the movement of local onland faults or upper-plate structures across the Manila Trench forearc basin. In Badoc Island, offshore west of Luzon mainland, we found notably young fossil corals, dated at 945.1 ± 4.6 years BP and 903.1 ± 3.9 years BP, on top of a ~5-m-high reef platform. To constrain the mechanism of emergence or emplacement of these fossil corals, we use field geomorphic data and wave inundation models to constrain an extreme wave event that affected west Luzon about 1000 years ago. Our preliminary tectonic and tsunami models show that a megathrust rupture will likely lead to subsidence of a large part of the west Luzon coast, while permanent coastal uplift is attributed to an offshore upper-plate rupture in the northern Manila Trench forearc region. The modeled source fault ruptures and tsunami lead to a maximum wave height of more than 3 m and inundation distance as far as 2 km along the coasts of western and northern Luzon. While emplacement of coral boulders by an unusually strong typhoon is also likely, modeled storm surge heights along west Luzon do not exceed 2 m even with Typhoon Haiyan characteristics. Whether tsunami or unusually strong typhoon, the occurrence of a prehistoric extreme wave event in west Luzon remains an important issue in future studies of coastal hazards in the South China Sea region.

Late Quaternary uplift across northwest Luzon Island, Philippines constrained from emergent coral reef terraces

“…In a study by Ramos et al . () on the occurrence of coral boulders on the reef terraces in Badoc Island, an in situ fossil coral which was collected on a 4.9‐m‐high terrace surface yielded an age of 6.87 ± 0.02 kyr BP. No dateable corals were collected on TII and TIV.…”

“…(), and Ramos et al . () are also shown. (B) Mid‐Holocene corals of northwest Luzon as highlighted from Figure (A).…”

“…() for Currimao, and Ramos et al . () for Badoc Island. New elevation and age data of fossil coral reef terraces in Currimao support the previous observation of Shen et al .…”

“…Agricultural activities, as well as climate, could have also modified the terrace surfaces in which fossil corals were destroyed or removed. While younger than mid‐Holocene corals were not found, this does not preclude the hypothesis that episodes of terrace emergence in the past few millennia have possibly occurred (Ramos et al, ). Sources of tectonic uplift in northwest Luzon will be discussed further in the following sections.…”

Introduction to thematic collection “Historical and geological studies of earthquakes”

“…Such sudden coastal uplifts are preserved as marine terraces. Ramos et al (2017) studied marine terraces on the west coast of Luzon Island, Philippines. While this coast faces the Manila Trench, where occurrence of giant interplate earthquakes and associated tsunami have been speculated, the fault modeling suggests the coast will subside during such a giant earthquake.…”