We infer temporal changes in the elastic properties of the edifice of Merapi volcano (Java, Indonesia) before its eruption in 1998 by analyzing multiply scattered elastic waves excited by a repeatable controlled seismic source. A pre-eruptive increase of shear wave velocity, which correlates well with pre-eruptive seismicity and dome-growth is revealed. The method can be used as a "pressure-gauge" for pressure changes inside of volcanoes, because increasing pressures in rocks are known to cause proportionally increasing elastic wave velocities.