Photograph by T. Leighley on 26 August 1981
Photograph by T. Leighley on 26 August 1981
Sketch by B. Myers
Scientists measure the distance between two benchmarks spanning the fault scarp of a thrust fault on the crater floor of Mount St. Helens. This scarp developed on the crater floor in 1981 as magma rose into the lava dome (backgound) before erupting onto its surface. The pressure exerted by the rising magma against rocks surounding the conduit caused the crater floor to fracture along a plane gently inclined toward the dome.
Material above the fault (person kneeling on upper surface) was pushed over material below the fault (person in lower right); in this view the fault scarp advances toward the lower right; arrow indicates direction of movement. Scientists measured an increasing rate of movement of this thrust fault before two eruptions in June and October 1981, which helped them to predict both eruptions accurately.
Photograph by P.W. LipmanScientist measures the vertical offset along a fault that moved as a consequence of a 7.2 magnitude earthquake on the south flank of Kilauea on November 29, 1975. The offset here is about 1.5 m; note fresh brown soil directly in front of the scientist. The total observed displacement along several similar faults of the Hilina fault system was at least 2.5 m (see image below for overview of the fault system). This earthquake resulted in dramatic ground movements along the south coast of Kilauea.
Photograph by D.A. Swanson in March 1971
Aerial view to the north of the Hilina fault system, which consists of a series of normal faults with scarps as high as 500 m on Kilauea's south flank. The large-scale offsets and associated seismicity in the south flank are thought to result from gravitational instability of this part of the volcano and from the repeated injection of magma into Kilauea's east rift zone. In contrast, the northern flank of Kilauea is relatively stable because it is buttressed by the mass of Mauna Loa Volcano.
Photograph by C.D. Miller in 1978
Aerial view of a glacial moraine along McGee Creek that has been offset about 15 m by part of the Hilton Creek fault system. Note cars in parking lot (lower left) for scale. The Hilton Creek fault is much larger than suggested by this relatively small offset--its scarp in places is about 1,100 m tall and forms part of the eastern front of the Sierra Nevada. The northern part of the fault terminates in Long Valley caldera, about 5 km NNE from this site.