Split and cracked blocks and slabs - Landforms and Geology of Granite Terrains - page 276

Figure 11.21. A-tent with displaced slab beneath it, high on eastern slope of Wudinna Hill, northwestern

Eyre Peninsula, South Australia.

Figure 11.22. Suggested mechanism for translocation of boulder during the Kobe-Mt Rokka earthquake of

January, 1995.

At Mt Magnet, one A-tent is demonstrably associated with fault dislocation ( Fig. 11.12) and
one large, and many small, A-tents formed during an earth tremor (2.3 on the Richter Scale),
which affected part of Minnipa Hill, on northwestern Eyre Peninsula, on 19 January 1999.
Another line of evidence favouring compressive stress, and difficult to refute, concerns develop-
ments noted during the quarrying of Palaeozoic limestone in Ontario. Coates (1964) has reported
that the floor cracked and heaved, forming an A-tent the crest of which stood 2.4 m higher than it
did originally, and affecting rocks some 15 m to either side of the crestal fracture. Calculations
showed that pressure release could not quantitatively account for the upheaval, which was attrib-
uted to horizontal stress in the limestone. The forms described in this section are readily explained
as being associated with the release of compressive stress, particularly if any remnant stress due to
earth movements were ephemerally, but critically, increased during earth tremors. The horizontal
components of earthquake motion play an important part in the initiation of landslides and, in gen-
eral terms, the sudden release of strain energy in the rock resulting from a high horizontal stress
field could induce both failure of the surface rock and sufficient shaking to produce instability.