The EDM measurement at the summit area of Merapi volcano had been done periodically in 1988, 1990, 1991, and 1992. By using the data of trilateration, horizontal strain components were calculated to evaluate strain pattern changes with time. During the period of 1988 - 1990, seismic and eruptive activities were rather low. In the period of 1990 - 1991, HFA and HFB type earthquakes increased and…