Search

The widespread tsunami risks in the South China Sea have diverse origins from trench megathrust to intraplate earthquake or landslide and remain poorly understood due to the scarce historical and geological records. The cliff-top paleotsunami gravels and basalt boulders on Penghu Islands in the Taiwan Strait present facies constraints on sediment transport, wave estimates from incipient motion formulas, and stratigraphic links to the probable sources. The boulders are supported by a pumice-bearing mud matrix that reflects a suspension-rich turbulent flow process and the typical rolling–saltation transport that results from bore-like waves. Calibrating for ancient sea level height and 100 year surge indicates that the storm waves that are likely to form in the shallow interisland bathymetry only enable boulder sliding–rolling and are incapable of the 2.5 m high cliff-top deposition. The estimated minimum height of tsunami waves is also insufficient and needs to add to 3.0 m high for a minimum cliff-top overflow of 0.5 m depth for terminal rolling before deposition. Coeval gravels in two other outcrops also record the time and extent of tsunami deposition, and are characterized by beach-derived bioclasts and stranded pumices, sharp base, matrix support, poor sorting, and elevation reaching above the 100 year surge. The gravels mark the local minimum wave run-ups and reach 2.4–4.0 m above sea level. The 1575–1706 radiocarbon age of the studied boulder suggests a probable tie to the disastrous 1661 earthquake in the SW Taiwan Orogen and the megathrust source in the northern Manila Trench.

The coasts around the South China Sea are highly populated and threatened by the ubiquitous tsunamigenic structures of the Manila trench-arc system. However, owing to limited historical records and rare geological findings of tsunamis, our comprehension of and preparation for regional tsunami risks are far from sufficient. Located in the NE South China Sea, the Penghu Islands in Taiwan’s western offshore area are selected for a geological tsunami survey because of their advantages of lengthy coastlines and abundant upper Holocene deposits. Three gravel layers of marine incursion events were identified and correlated between three far-separated coastal sections based on the constraints of radiocarbon dating. Intercalated with talus, soil, and dune sand, the gravel layers are poorly sorted, matrix-supported, up to boulder in size, and dominated by marine bioclastics and rounded basalts, which are identical to modern beach gravels. The deposits are located 3–4 m above present sea level, which is significantly higher than the local high tide levels and maximal typhoon surges of 2–3 m high. Furthermore, the deposits are distributed extensively on south-facing shores, indicating relatively large and abnormal inundation events from the South China Sea. Based on the radiocarbon dating results of the event deposits, three tsunamis likely occurred in the periods from the 4th to 6th, 11th to 12th, and 15th to 16th centuries. A recurrence interval of 400–500 years is accordingly associated with these event deposits. These findings demonstrate great tsunami risks exist in the South China Sea basin and call for more attestations and future studies. The major contribution of this study is also supported by the AD 1076 Guangzhou tsunami, which is comparable in age to the middle event deposit.

We have collected two shallow gas hydrate samples at two sites having different geological settings off southwest Taiwan during the cruise MD214 in 2018. The first core site, MD18-3542, is on the South Yuan-An East Ridge at ~ 1200 m water deep, where a structural unconformity covered by fine-silt sediments appears at ~ 5.5 m below the seafloor. The second core site, MD18-3543, is close to the Good-Weather Ridge at ~ 1100 m water deep, where a gas-related pockmark structure and authigenic carbonates are present at shallow strata with fine-silt sediments near the seafloor. Sediment properties of core MD18-3542 are distinctively different above and below the layer corresponding to the unconformity. Both cores show obvious gaps or voids in the lower core halves. The core features could be linked to the dissociated methane upward migrating from deep strata. Core site settings with upwelling methane would favor the formation of shallow gas hydrates. At site MD18-3542, the shallow hydrate could be formed due to high concentration methane kept beneath the unconformity covered by fine-silt sediments. At site MD18-3543, the shallow hydrate could be formed due to an extremely high flux of upwelling methane trapped either beneath the authigenic carbonates or fine-silt sediments.

Located in the western circum-Pacific seismic zone, Taiwan is an ideal area for recording tsunamis that originate in the tectonically and volcanically active systems of the Ryukyu Arc, Okinawa Trough, and Luzon Arc. However, because of Taiwan's short historical record, there is only credible evidence for one disastrous tsunami, which occurred in northern Taiwan in AD 1867. To better understand the western Pacific tsunamis, an Upper Holocene deposit of a coastal plain inundated by the AD 1867 tsunami was investigated to search for geological evidence of tsunamis. Based on facies characteristics and sediment composition, we identified two sharp-based, decimetre-thick marine quartzose sandy layers that fine upward and landward, intercalated with floodplain marsh sand and mud. The fluvium are rusty red, rich in volcanic clasts and Fe, and comparable to modern riverine sediments of local andesitic provenance. Extending 0.8 km inland, the marine quartzose sandy layers are light-coloured and rich in quartz and Si, have high Ca/Ti values, and are comparable to the modern beach–dune sediments of regional siliciclastic provenance. Radiocarbon dating indicates that the marine sandy layers are correlated with the AD 1867 tsunami and an older event at the boundary between the 17th and 18th centuries. Both events were probably associated with reported earthquakes involving normal faulting. In addition, the distribution patterns of the two marine sandy layers are similar, suggesting that the inundation distances, run-ups, and flow heights of the two events are comparable. This indicates that the disastrous tsunamis were frequent in the area between northern Taiwan and the Okinawa Trough of the western circum-Pacific seismic zone.

In order to scrutinize the development, process and control of an early post-rift siliciclastic fill-up wedge, sequence stratigraphy was applied to the Oligo-Miocene coastal-shelf strata of a mid-Tertiary rift basin in Taiwan. Four sequences at the million-year/3rd-order scale, constrained by microfossil biohorizons and radioactive dates, were identified and correlated regionally. The lower sequence belongs to the fault-bounded Oligocene syn-rift wedge. The middle two comprise the lower Miocene early post-rift wedge, onlapping onto the Mesozoic basement highs. The upper sequence is the lower Miocene late post-rift drape on the continental margin. The middle two sequences record a process of active topographic transformation through successive coastal-shelf progradations. At first, the remnant rift topographic low received drastic progradations from both the inner and outer highs. During the later progradations principally from the inner high, the topographic low was filled up for the successive drape. This wedge formation was characteristically short-lived and high in sedimentation rate, when accommodation space and sediment supply both increased. The initial accommodation space of the remnant rift deep shelf was augmented by rapid thermal subsidence and long-term eustatic sea level rise. The sediment supply, in spite of a decrease in provenance exposure due to onlapping, was promoted by periodic exposures due to 3rd-order eustatic sea level falls, and additionally by a warm and humid climate. These falls are age concordant with the sequence boundaries and thus evidently important for the sequence boundary formations and sediment dispersal.