Deformation, lava dome evolution, and eruption cyclicity at Merapi volcano, Indonesia

Deformation monitoring results are reported here for the period 1988-1998 at Merapi volcano, one of the most active and dangerous volcanoes in Indonesia. Comprehensive databases of various geophysical parameters were concurrently studied and analyzed to 2000, and similar data were subsequently considered during periods of eruption crisis in 2001 and 2006. Of particular emphasis was the study of lava eruption rates based on dome volume estimates and seismic proxies for dome collapse volumes. The detailed study period of deformation includes a major resumption in lava effusion in January 1992 and major dome collapses in November 1994, January 1997, and July 1998. Monitoring techniques employed in the field are of two types. Translational movements were recorded via electronic distance measurements (EDM) on a summit trilateration network, slope distance changes measured to the upper flanks, and other data collected from 1988 to 1995. Tilt changes were detected by a summit and flank network of tilt stations that operated at various times from 1993 to 1998. A major consequence of the deformation results is the documentation of a significant 4-year period of deformation precursory to the 1992 eruption. Cross-crater strain rates accelerated from less than 3 x 10-6/day between 1988 and 1990 to more than 11 x 10-6/day just prior to the January 1992 activity, representing a general, asymmetric extension of the summit during highlevel 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. The Gendol breach, a pronounced depression formed by the juxtaposition of old lava coulees on the southeast flank, functioned as a major displacement discontinuity.; An elevated phase of magma production with respect to the long-term rate for the 20th Century characterized the activity at Merapi volcano, Central Java/Yogyakarta, Indonesia, for the period 1992-2006. Most large (0.2 - 3.4 x 106 m3) dome collapses or dome collapse episodes in the 1990s were initiated during elevated short-term extrusion rates, at Merapi typically 0.2 m3 s-1 or greater. Large collapses were often preceded by variable inflationary tilt of the crater rim, by increasing numbers of rockfalls and their associated seismicity, and by intensifying multiphase earthquake activity. Multiphase earthquakes, rockfall counts, and amplitude-duration data established from seismic records show varying positive correlations with extrusion rate. Pyroclastic flow and rockfall seismic amplitude-duration data were calibrated as proxies to enable estimates of collapsed lava volume. For the 20th Century as a whole, observed magma production rates suggest long-term cycles of ca. 30 years spacing that begin with heightened magma flux over 2 to 15 years with greater potential for explosive eruptive activity, followed by much-diminished mean magma flux in the concluding ∼15-25 years of the cycle. The data examined through 2006 suggest that if similar cycles are to continue well into the 21 st Century, activity at Merapi will now experience relatively lower magma production for the next ∼15 years, compared to the most recent eruptive phase during 1992-2006.