by . In February 2005, the Royal Navy vessel surveyed the seabed around the earthquake zone, which varies in depth between . The survey, conducted using a high-resolution, multi-beam sonar system, revealed that the earthquake had made a considerable impact on the topography of the seabed. thrust ridges created by previous geologic activity along the fault had collapsed, generating landslides several kilometres wide. One such landslide consisted of a single block of rock some high and long. The momentum of the water displaced by tectonic uplift had also dragged massive slabs of rock, each weighing millions of tonnes, as far as across the seabed. An oceanic trench several kilometres wide was exposed in the earthquake zone. The TOPEX/Poseidon and Jason-1 satellites happened to pass over the tsunami as it was crossing the ocean. These satellites carry radars that measure precisely the height of the water surface; anomalies in the order of were measured. Measurements from these satellites may prove invaluable for the understanding of the earthquake and tsunami. Unlike data from tide gauges installed on shores, measurements obtained in the middle of the ocean can be used for computing the parameters of the source earthquake without having to compensate for the complex ways in which proximity to the coast changes the size and shape of a wave. Tsunami The sudden vertical rise of the seabed by several metres during the earthquake displaced massive volumes of water, resulting in a tsunami that struck the coasts of the Indian Ocean. A tsunami that causes damage far away from its source is sometimes called a teletsunami and is much more likely to be produced by the vertical motion of the seabed than by horizontal motion. The tsunami, like all others, behaved differently in deep water than in shallow water. In deep ocean water, tsunami waves form only a low, broad hump, barely noticeable and harmless, which generally travels at high speed of ; in shallow water near coastlines, a tsunami slows down to only tens of kilometres per hour but, in doing so, forms large destructive waves. Scientists investigating the damage in Aceh found evidence that the wave reached a height of when coming ashore along large stretches of the coastline, rising to in some areas when travelling inland. Radar satellites recorded the heights of tsunami waves in deep water: maximum height was at two hours after the earthquake, the first such observations