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Following the eruption of January 1992, episodes of lava dome growth accompanied by generation of dome-collapse nuees ardentes occurred in 1994–1998. In addition, nuees ardentes were generated by fountain-collapse in January 1997, and the 1998 events also suggest an explosive component. Significant tilt and seismic precursors on varying time scales preceded these events. Deformation about the summit has been detected by electronic tiltmeters since November 1992, with inflation corresponding generally to lava dome growth, and deflation (or decreased inflation) corresponding to loss of dome mass. Strong short-term (days to weeks) accelerations in tilt rate and seismicity occurred prior to the major nuees ardentes episodes, apart from those of 22 November 1994 which were preceded by steadily increasing tilt for over 200 days but lacked short-term precursors. Because of the combination of populated hazardous areas and the lack of an issued warning, about 100 casualties occurred in 1994. In contrast, the strong precursors in 1997 and 1998 provided advance warning to observatory scientists, enabled the stepped raising of alert levels, and aided hazard management. As a result of these factors, but also the fortunate fact that the large nuees ardentes did not quite descend into populated areas, no casualties occurred. The nuee ardente episode of 1994 is interpreted as purely due to gravitational collapse, whereas those of 1997 and 1998 were influenced by gas-pressurization of the lava dome.

Edifice deformations are reported here for the period 1988–1995 at Merapi volcano, one of the most active and dangerous volcanoes in Indonesia. The study period includes a major resumption in lava effusion in January 1992 and a major dome collapse in November 1994. The data comprise 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. A major consequence of this study is the documentation of a significant 4-year period of deformation precursory to the 1992 eruption. Cross-crater strain rates accelerated from less than 3×10 −6 /day between 1988 and 1990 to more than 11×10 −6 /day just prior to the January 1992 activity, representing 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 2×10 −6 /day. EDM measurements between lower flank benchmarks and the upper edifice indicate displacements as great as 1 m per year over the four years before the 1992 eruption. The Gendol breach, a pronounced depression formed by the juxtaposition of old lava coulees on the southeast flank, functioned as a major displacement discontinuity. Since 1993, movements have generally not exceeded the 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 the south flank between November 1994 and September 1995. No short-term precursors were noted before the November 1994 lava dome collapse, but long-term adjustments of crater geometry accompanied lava dome growth in 1994. Short-term 2-cm deflation of the edifice occurred following the November 1994 dome collapse.

Significant fracture zone deformation has been imparted to Tertiary lavas on the peninsula (Flateyjarskagi) located between Eyjafjordur and Skjalfandi in north central Iceland. The region lies immediately south of the Flatey fault, one of a series of WNW trending, left-stepping faults comprising the right-lateral, oblique slip Husavik-Flatey fault system (HFFS). The HFFS defines the present southern margin of the 70 to 80-km-wide Tjornes Fracture Zone (TFZ). Lava bedding, dikes, and faults in the western half of Flateyjarskagi display a progressive 0°–110° clockwise change of trend and steepening of lava dip angles to 15°–45° over an 11-km-wide zone nearest the Flatey fault. Accompanying this structural curvature is an increase in fault and dike frequency and amygdule/vein mineralization. Northeast striking faults develop predominant downthrows to the east approaching the TFZ, accommodating the steep lava dips via repetition of stratigraphic section and normal fault-bounded block rotations. Although the possibility cannot yet be conclusively ruled out that structural trends formed in curved orientations, the simplest interpretation suggests that the lava pile was tectonically rotated by heterogeneous simple shear. Maximum on-land shear strain has been crudely estimated at γ = 3.5; estimates of shear displacement suggest that the TFZ was active for at least 2 m.y. sometime prior to 6–7 Ma, at a strain rate of approximately 5.5×10−14 s−1. Antithetic faulting and associated flexural bending may have been the means by which a great part of the proposed simple shear rotations were accomplished. WNW extension produced by rotational normal faulting is interpreted to have accommodated systematic variations in shear strain parallel to the direction of shear. Paleomagnetic and structural evidence suggest that rocks of Tertiary age on Tjornes peninsula immediately to the east also have been tectonically rotated, implying an original shear zone width of at least 25 km.