Ground deformation at Merapi Volcano, Java, Indonesia: distance changes, June 1988-October 1995

Edifice deformations are reported here for the period 1988-1995 at Merapi v olcano, one of the most active and dangerous volcanoes in Indonesia. The st udy period includes a major resumption in lava effusion in January 1992 and a major dome collapse in November 1994. The data comprise electronic dista nce measurements (EDM) on a summit trilateration network, slope distance ch anges measured to the upper flanks, and other data collected from 1988 to 1 995. A major consequence of this study is the documentation of a significan t 4-year period of deformation precursory to the 1992 eruption. Cross-crate r strain rates accelerated from less than 3 x 10(-6)/day between 1988 and 1 990 to more than 11 x 10(-6)/day just prior to the January 1992 activity, r epresenting a general, asymmetric extension of the summit during high-level conduit pressurization. After the vent opened and effusion of lava resumed , strain occurred at a much-reduced rate of less than 2 x 10(-6)/day. EDM m easurements between lower flank benchmarks and the upper edifice indicate d isplacements as great as 1 tn per year over the four years before the 1992 eruption. The Gendol breach, a pronounced depression formed by the juxtapos ition of old lava coulees on the southeast Rank, functioned as a major disp lacement discontinuity. Since 1993, movements have generally not exceeded t he 95% confidence limits of the summit network. Exceptions to this include 12 cm outward movement for the northwest crater rim in 1992-1993, probably from loading by newly erupted dome lava, and movements as much as 7 cm on t he south flank between November 1994 and September 1995. No short-term prec ursors were noted before the November 1994 lava dome collapse, but long-ter m adjustments of crater geometry accompanied lava dome growth in 1994. Shor t-term 2-cm deflation of the edifice occurred following the November 1994 d ome collapse. (C) 2000 Elsevier Science B.V. All rights reserved.