Morphological impacts of low magnitude seismic events on granite: Viewing certain landforms with new eyes

Several landforms developed in massive brittle rocks located in cratonic settings, and formerly thought to be caused by external agencies, are now attributed to low magnitude seismic events or earth tremors. Field observations and particularly monitoring of specific sites have shown that the frequent joggling of shield areas has given rise to miniature scarps that define dislodged segments. Furthermore, sheeting fractures, previously well known as offloading joints, are planes of dislocation and are initiated during low energy earthquakes. A-tents or pop-ups were in past times widely attributed to insolation and less commonly to offloading, but they too, and like sheet structures and rock wedges, are formed by compressive stress applied either instantly during earth tremors or gradually, over decades. Many of these forms develop after exposure but some A-tents are almost certainly initiated at the weathering front. Wedges also develop in the subsurface. Rock bursts and slabs and blocks are displaced or split during earth tremors. It is not suggested that lithostatic pressure can be overlooked but applied compression as well as reduced lithostatic loading are necessary for the development of sheet fractures and the other forms cited.