*3.2. Propagation Speed of ISWs*

Two MODIS images were available on 12, 14 and 19 April (Figure 1), and the 175-min interval of imaging time makes it possible to accurately calculate the propagation speed of ISWs (Figure 3c).

In the LS, a clear ISW packet (S3) was observed from the MODIS images on 14 April (Figure 1c,d). Within 175 min, the leading part of the wave center propagated northward for ~27 km, and the average speed was calculated to be 2.54 m/s accordingly. This value is very close to the ISW speed of 2.5 m/s obtained by Lindsey et al. [30] with a shorter 10-min time steps. Moreover, based on the assumption that the time interval of ISW generation in the LS is consistent with the tidal cycle, Susanto et al. [17] estimated the speeds of northward-propagating ISWs from the Lombok Strait as 1.97 and 1.96 m/s using ERS-1/2 SAR images of 23 and 24 April 1996 and Karang et al. [29] obtained a speed of 2.05 m/s using Landsat 8 image of 17 May 2015. It can be seen that the speed of ISW in the LS is not the same at different periods, and here we show a variation range of about 0.5 m/s.

**Figure 3.** (**a**) The colored curves are crests of all leading ISWs in Figures 1 and 2. Gray dotted line is the 850 m isobath. The propagation path of ISWs and their generation site are plotted by thin gray line and a red dot, respectively. The pink ellipse marks the approximate area of the submarine wreck inferred from the reported information. (**b**) Bathymetry along the ISWs propagation path. (**c**) The average propagation speed between two points. Different colors correspond to ISWs in (**a**).

In the BS basin, there are three observed ISWs (S1, 4 and 5) whose speeds decreased from south to north along the propagation path. The images on 19 April (Figure 1e,f) showed that the wave crest center of S5 moved at a mean speed of 2.69 m/s between 7.80◦ S and 7.57◦ S where the average water depth was 1274 m, and further to the northwest of the basin, the images on 12 April (Figure 1a,b) suggested that the wave crest center of S1 propagated at a mean speed of 2.24 m/s between 7.59◦ S and 7.40◦ S. Over the shallow terrain west of the Kangean Islands, the propagation speed of S6 severely slowed down to a mean value of 0.71 m/s in the area with an average water depth of 155 m. The above calculations show a clear decreasing trend of propagation speed as ISWs shoaled from the deep basin of the BS onto the continental shelf. Through numerical simulation, Ningsih et al. [22] showed an ISW propagation speed range in the BS of 0.71–2.67 m/s, especially consistent with our estimated result. Using ALOS PALSAR images, Matthews et al. [21] defined ISW mean speeds between two wave packets about 1.6 to 2.3 m/s by measuring the distances between the leading signals in adjacent wave packets generated 12.4 h apart.

Figure 2a shows that S3 reached the continental shelf to the west of the Kangean Islands at 12:42 on April 15. Compared with the images taken on 14 April (Figure 1c,d), it took more than a day for the wave to travel from the LS to the continental shelf. During the over 27-h process from formation to shoaling in shallow water, the ISW traveled nearly 200 km and the average propagation speed was about 2 m/s.
