Eastern Australia’s submarine landslides: implications for tsunami hazard between Jervis Bay and Fraser Island

Abstract

A hazard assessment of submarine landslide-generated tsunami for the east Australian continental slope between Jervis Bay and Fraser Island is presented. Submarine landslides are present in water depths of approximately 400 to 3500 m along the entire length of continental margin, but are increasingly prevalent northward of Coffs Harbour without clustering at any particular water depth. Two hundred sixty individual submarine landslide scars that are greater than 1 km in width have been identified. Of these, 36 have been calculated to produce a tsunami flow depth equal to or greater than 5 m at the coastline for an assumed landslide downslope velocity of 20 ms⁻¹. Landslides that are both thick (> 100 m) and wide (> 5 km) have the greatest potential to generate the largest coastal flow depths (> 10 m). The water depth of a landslide’s centre of mass strongly influences the onshore height of the tsunami’s surge with the larger events generated in shallower water depths (~ 500–1500 m). The maximum flow depth at the coastline is larger for thicker (50–250+ m) canyon landslides which occur on steeper slopes (> 4°), compared to thinner (< 50 m) plateau landslides which generally produce smaller tsunami. Maximum inundation distances and run-up heights of 1.6 km and 22 m respectively have been calculated for landslide velocities of 20 ms⁻¹ and these values vary significantly depending on local coastal topography. There is no evidence for a submarine landslide large enough and young enough to have generated a Holocene age megatsunami for the east coast of Australia.

Appendices

Appendix 1

Table 4 Sensitivity analysis results of Eq. (1) summarizing parameter combinations required to produce ~ 5 m flow depth at coastline in water depths between 500 and 2500 m for landslide blocks with dimensions similar to those outlined in Appendix Table 5. A conservative landslide velocity (v_s) of 20 ms⁻¹ was used and distance to the coastline was averaged to ~ 50 km

Table 5 Catalogue of the submarine landslides investigated within the study area including morphometric parameters for each submarine landslide listed (see Figs. 3, 4, 5, and 6 for landslide locations). Each slide is given a unique ID number. Measured values include slide thickness (t), slide length (L), slide width (W), water depth (h₀) and distance to the coastline (r). Slide thickness is the maximum thickness within the slide scar taken by replacing the material apparently missing (i.e. by maintaining the continuity and shape of the adjacent slope and projecting it above the slide scar). Slide length is the distance from the head of the slide to the toe of the slide. Slide width is the average of measurements taken every 500 m down the slide scar, perpendicular to the slide axis. Water depth is taken from the centre of mass for each slide. Distance to the coastline is measured from the head of the slide to the directly adjacent coastline

Table 6 Maximum expected flow depth at the coastline (F_d(0)) for all submarine landslides investigated within the study area over a range of maximum slide velocities (v_s): 10 ms⁻¹; 20 ms⁻¹; 30 ms⁻¹; 40 ms⁻¹

Table 7 Inundation distance (X_max) and run-up (R(X_max)) values for each identified submarine landslide over a range of slide velocities (v_s): 10 ms⁻¹; 20 ms⁻¹; 30 ms⁻¹; 40 ms⁻¹. Manning’s coefficient is 0.03 (urbanized/built land)

Appendix 2

Representative examples of coastal topography profiles for each region (green). The maximum run-up (R(X_max)) and inundation distance (X_max) for each slide is shown on the coastal topographic profile for slide velocities 20 ms⁻¹ (dark blue) and 40 ms⁻¹ (light blue). For profile locations, see Figs. 9, 10 and 11

Fig. 9

Region 1—profiles adjacent to slides a R1–2, b R1–3 and c R1–4

Fig. 10

Region 2—profiles adjacent to slides a R2- and b R2–3

Fig. 11

Region 3—profiles adjacent to slides a R3–2, b R3–3, c R3–4 and d R3–6

Fig. 12

Region 4—profiles adjacent to slides a R4–4, b R4–6 and c R4–7

Fig. 13

Region 4—profiles adjacent to slides a R4–1, b R4–11 and c R4–16; the profiles are representative of topography that include Morten Island or Fraser Island (see Fig. 6)

Fig. 14

All regions—representative examples of coastal topography profiles adjacent to slides where maximum inundation distance (X_max) values exceed 1.5 km: a R2–2, b R3–13, c R4–13. Only 10 slides produce X_max values > 1.5 km and only when the slide velocity is ≥ 40 ms⁻¹