02 Feb 2023
 | 02 Feb 2023
Status: a revised version of this preprint was accepted for the journal NHESS.

Boulder transport and wave height of a seventeenth century South China Sea tsunami on Penghu Islands, Taiwan

Neng-Ti Yu, Cheng-Hao Lu, I-Chin Yen, Jia-Hong Chen, Jiun-Yee Yen, and Shyh-Jeng Chyi

Abstract. 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.

Neng-Ti Yu et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on nhess-2022-293', Anonymous Referee #1, 16 Feb 2023
    • AC1: 'Reply on RC1', Neng-Ti Yu, 22 Feb 2023
      • RC2: 'Reply on AC1', Anonymous Referee #1, 23 Feb 2023
        • AC2: 'Reply on RC2', Neng-Ti Yu, 24 Feb 2023
      • RC3: 'Reply on AC1', Anonymous Referee #1, 23 Feb 2023
        • AC3: 'Reply on RC3', Neng-Ti Yu, 24 Feb 2023
          • RC4: 'Reply on AC3', Anonymous Referee #1, 24 Feb 2023
            • AC5: 'Reply on RC4', Neng-Ti Yu, 19 Mar 2023
  • RC5: 'Comment on nhess-2022-293', Anonymous Referee #2, 10 Mar 2023
    • AC4: 'Reply on RC5', Neng-Ti Yu, 13 Mar 2023

Neng-Ti Yu et al.

Neng-Ti Yu et al.


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Short summary
Rock debris moved by tsunami waves that probably followed the 1661 earthquake in SW Taiwan are found. The long-awaited question of whether large tsunamis could impact on the South China Sea coasts is answered. The 3 m tall waves are powerful enough to carry big rocks, up to 2 t in weight and 1.7 m in length, landward for 30 m in distance and onto a 2.5 m high cliff. The earthquake and tsunami must be huge compared to the modern record set by the 1994 M6.4 earthquake and 0.4 m high tsunami.