Plain Language Summary Beneath the Central Andes, small earthquakes occur along a band 30 km within the subducting Nazca tectonic plate that slowly sinks below the South American plate. Such earthquake bands occur worldwide, and it has been proposed that they happen, because there is water bound in the solid rocks there, which would be released when the subducting plate heats up while it sinks into the Earth. We test this hypothesis by determining a rock property that can indicate whether liquid water is present from the arrival times of earthquake waves. We then theoretically calculate how this property would be for different kinds of rock that can be present at this specific location within the Earth. It results that the rock property that we derived in the earthquake band can best be explained by the presence of liquid water that is stored in thin interconnected veins there. Other researchers saw in laboratory experiments, in computer simulations, and in the field that rocks that release water look exactly like this, so that their and our pieces of the puzzle fit together well. With this knowledge, we can understand better how water sinks into the Earth and how it finds its way back to the surface.