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We provide structural and paleomagnetic evidence for major distributed rotational shear adjacent to and including the active Husavik-Flatey transform fault in the southern part of the Tjornes Fracture Zone of north Iceland. In the Flateyjarskagi peninsula immediately south of the fault, dike and bedding trends gradually change over about 11 km in approaching the transform fault main trace. In map view this pattern is seen as a progressive clockwise curvature of structural elements that equals or in some cases exceeds 90°. We present paleomagnetic analyses that verify large clockwise rotations of the same order as the curvature of structural elements observed. We compare our field and paleomagnetic observations with results from careful analog modeling that demonstrate how regional curvature of structures results from distributed strain that dominates the early stages of transform shear deformation, prior to the appearance of a transform fault system comprised of discrete shears. Our paleomagnetic results were obtained from one relatively undisturbed reference site, and three sites of deformed rocks in Flateyjarskagi and two in Tjornes peninsula. These results indicate large vertical-axis rotations, and the progressive deflection of dike trends throughout northern Flateyjarskagi also serves as a separate quantitative estimate of vertical-axis rotation. The clockwise vertical-axis rotation estimates of 71°–97° from dikes are less than those deduced from paleomagnetism, but the order of magnitude is correct and we attribute the differences as mainly reflecting imperfect bed attitude adjustments used in analyses. The problem is typical for deformed rocks from high-latitude sites with steeply inclined paleomagnetic vectors. We consider our field measurements of dikes as more accurate indicators of true tectonic rotation. This regional deformation of Miocene rocks occurred early in the evolution of the TFZ transform, probably beginning ca. 7 Ma, when an older rift zone present in western North Iceland jumped to its present NVZ location in northeast Iceland. The early deformations involved shear strain distributed over a region about 20-km broad and produced the curved structures. This phase was followed by the sequential development of more narrowly distributed discrete shears, generating the initial Husavik-Flatey transform fault.

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.

More than 50 volcanic debris flows or lahars were generated around Mt Merapi during the first rainy season following the nuees ardentes of 22 November 1994. The rainfalls that triggered the lahars were analyzed, using such instruments as weather radar and telemetered rain gauges. Lahar dynamics were also monitored, using new non-contact detection instrumentation installed on the slopes of the volcano. These devices include real-time seismic amplitude measurement (RSAM), seismic spectral amplitude measurement (SSAM) and acoustic flow monitoring (AFM) systems. Calibration of the various systems was accomplished by field measurements of flow velocities and discharge, contemporaneously with instrumental monitoring. The 1994–1995 lahars were relatively short events, their duration in the Boyong river commonly ranging between 30 min and 1 h 30 min. The great majority (90%) of the lahars was recognized at Kaliurang village between 13:00 and 17:30 h, due to the predominance of afternoon rainfalls. The observed mean velocity of lahar fronts ranged between 1.1 and 3.4 m/s, whereas the peak velocity of the flows varied from 11 to 15 m/s, under the Gardu Pandang viewpoint location at Kaliurang, to 8–10 m/s at a section 500 m downstream from this site. River slopes vary from 28 to 22 m/km at the two sites. Peak discharges recorded in various events ranged from 33 to 360 m3/s, with the maximum value of peak discharge 360 m3/s, on 20 May 1995. To improve the lahar warning system along Boyong river, some instrumental thresholds were proposed: large and potentially hazardous lahars may be detected by RSAM units exceeding 400, SSAM units exceeding 80 on the highest frequency band, or AFM values greater than 1500 mV on the low-gain, broad-band setting.

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.