Earthquakes (e.g., Seed, 1968; Biscontin et al., 2004) or storms (e.g., DeGraff, 1994) can trigger the gravity sliding of soft sediments. Attempts to describe the mechanical links between earthquakes or storms and gravity sliding have tended to emphasize the role of fluid pressure. For example, Biscontin et al. (2004) and Hutton et al. (2004) suggest that landslides can be triggered by excess pore pressure generated by earthquake loading. Similarly, Kokusho (1999), Kokusho and Kojima (2002), and KoKusho and Fujita (2002) suggest mat gravity sliding of sediments can occur above layers of water created by liquefaction during earthquakes.

This paper demonstrates a simple, reusable, and inexpensive experiment for the triggering of gravity-driven sliding of soft sediments, which illustrates the importance of liquefaction and overpressuring. It is a development of the method presented by Peacock (2003) to illustrate the development of overpressuring in soft sediments, and uses the same materials, simply modifying the set-up of the experiment. The experiment is therefore an ideal teaching tool for illustrating a range of geological processes.

SIMPLE ANALOGUE EXPERIMENT

The set-up of the experiment is shown in Figure 1(A). Sand, mud and water are mixed together in a transparent, sealed 2-liter plastic bottle ( 121 mm wide, 82 mm thick and 200 mm high). Various types of sand have been used with success, but the mud should be course enough to settle within the space of a few hours. Lucustrine silt has been found to work better than esturine muds, while soil commonly contains too much organic matter. A few grammes of salt are added to speed up the settling of the mud, and a few milliliters of bleach are added to stop organisms from growing in the bottle. The relative quantities of the sediments do not appear to affect the results, but there should be enough sand for liquefaction to be significant, and the mud should not be 10 mm thick. The water above the settled sediments should be tens of millimeters deep. The following procedure is followed:

1. The bottle containing the sediments and water is shaken vigorously until all of the sediments are liquefied.

2. The bottle is held still for a few seconds so the sand can start to settle.

3. It is tilted so the top surface of the sand has a dip of a few degrees.

4. The bottle is left on a stable surface for several hours, until the mud has settled and the water is clear (Figure 1A). The mud must be allowed to settle undisturbed because even slight disturbances during or after settling cause premature liquefaction.

5. The sand compacts and liquefies when the bottle is gently squeezed or knocked.

A typical sequence of events illustrated in Figure 1. Water is expelled from the sand and trapped under the mud, developing a layer of water (Figure 1B). Extension fractures rapidly develop upslope, with buckles developing down slope and increasing in amplitude as sliding of the mud continues (Figure 1C). Thrusts commonly develop in the buckles to accommodate shortening, with strike-slip faults accommodating variations in movement within the mud. The water layer beneath the mud starts to escape into the overlying water, forming muddy plumes (Figure 1D). Such a sequence typically takes several seconds.

The experiment presented in Figure 1 has the following characteristics: (1) it is a closed system, in a sealed bottle; (2) liquefaction occurs as the sand is disturbed, rather than as water is pumped through the sand (e.g., Nichols et al., 1994); (3) it is simple and inexpensive; and (4) it is reusable. It is therefore ideal for demonstrating gravity sliding, especially for teaching purposes.

GRAVITY SLIDING AND THE DEVELOPMENT OF OVERPRESSURED FLUIDS

Overpressured fluids (e.g., Thomeer and Bottema, 1961; Osborne and Swarbrick, 1997) operate in various geological processes. For example, the crucial role of fluid pressure in controlling thrust faulting was recognized by Hubbert and Rubey (1959), who showed that large thrust sheets can move on gently-dipping fault surfaces only if the fluid pressure is close to, or exceeds, that of the overburden. The overpressurized fluid effectively supports the mass of the overlying thrust sheet. Evidence for overpressured fluids in thrust sheets includes sub-horizontal veins in the walls of thrusts, these having initiated as cracks caused by overpressured fluid (e.g., Sibson, 1989; Teixell et al., 2000).