Microcracks promote earthquakes
New research by British scientists has now provided an insight into the role of the water in the rock in causing earthquakes. As they report in the journal “Nature”, microcracks in the rock divert tensions and thus prevent the pressure from being relieved, for example through larger fractures or water leaks. As a result, the earthquake probability increases.
Scientists led by Dan Faulkner from the University of Liverpool examined an extensive fault in the Atacama Desert in northern Chile. Here they measured the density of so-called “micro-cracks” in the rock. The movements of the subsoil along the plate boundary create tensions in the rock and these create fine cracks. Usually water collects in these cracks and between the different rock areas of the fault. As the tension in the rock increases, so does the pressure of this fluid. This, in turn, can increase the risk of an earthquake. Most of the time, however, the rock breaks up before this happens, the water escapes and the pressure decreases again as a result.
Loma Prieta: Strong tremor despite relatively low tension
“The problem with predicting earthquakes is that we know so little about how faults work,” explains Faulkner. “Over the years, we have found that even low stresses acting on the earth's crustal plates can trigger large tremors. One example is the Loma Prieta earthquake in 1989. It caused massive devastation, even though there was previously very little tension on the plate boundary. ”The British researchers now investigated more closely why this relief mechanism does not always work and wanted to find out which factors decide when the water can and when not. They mainly focused on the microcracks in the rock. Under laboratory conditions, they exposed these alternating pressures to observe how the cracks changed the reaction of the material.
Micro cracks prevent relief fractures
It was found that the microcracks significantly changed the elasticity of the rock. Lines of stress that would normally appear at right angles to the fault were deflected by the cracks and instead ran at a 45 degree angle. When this happens, however, the likelihood of the rock giving way and breaking is reduced and the pressurized water cannot escape. The result: Relief does not take place, so the earthquake probability remains high. ”In theory, very strong tensions are required to move the rock along a fault, but if pressurized water or gas comes in between, it works like this a lubricant and facilitates movement between the plates - and thus an earthquake, ”explains Faulkner. “So far, the problem with this theory has been that the rock should actually break under such high water pressures and the lubricant effect is canceled out again. However, our current study shows that many small cracks around the fault can change the stresses in the rock in such a way that larger fractures, which could provide a way out for the water, are less likely. ”Where the pore water and the gases in such faults come from, however, needs to be researched in further studies. Faulkner and American colleagues are currently drilling at sample points along the San Andreas Fault in California, which should help to better understand the mechanics of the fault zones and how earthquakes occur.
(Source: University of Liverpool, December 14, 2006, Image: USGS / MMCD)