Secrets of Earthquakes Unearthed
Researchers from the Racah Institute of Physics created mini-tremors in the lab, and discovered the process which triggers earthquakes. It all happens when fracture meets friction
A team of researchers from the Racah Institute of Physics challenged the basic laws of physics, figured out a new way to look at friction and fracture processes - and made a number of scientific achievements that could one day help us save both lives and resources.
Their first achievement was discovering how materials fracture and ultimately fail. They noticed that fractures always start with an innocent looking microscopic crack. But under certain conditions it begins to rapidly increase in size, and as it grows, it spawns a cloud of microscopic cracks around it. Those additional cracks propagate as well, and ultimately cause the material to break down.
This intricate process has been very hard to track in the past, since it not only occurs at a microscopic level, but also at nearly the speed of sound. But the team, led by Prof. Jay Fineberg, managed to slow down the speed of sound by a factor of 1000 or so in the lab, and – by using high-speed cameras – obtained the first real look of how it takes place.
Their innovative method allowed the researchers to better understand why, when and how cracks reach a certain critical behaviour, and why the motion of cracks becomes so complex. It also enabled them to shed light on the mechanisms that determine strengths and weaknesses of materials, and may possibly help engineers design stronger and more cost-effective structures in the future.
How do the ideas of fracture help them understand both friction and earthquakes?
For centuries, scientists believed that the amount of force needed to start an object sliding across a surface is equal to the frictional force that keeps them stuck together. It was also thought that the friction didn’t depend on the contact areas between the two bodies, but by the ratio between the forces pushing sideways and pushing down.
Those laws, which were first described by Leonardo da Vinci in the 15th century, fell apart when Prof. Fineberg’s team put them to the test. By using miniature sensors, lasers and high-speed cameras, the team was able to measure when and where two blocks really touch each other. Surprisingly, they found that the relatively slow motion that characterizes friction is actually mediated by cracks that rapidly propagate along the contact surface and break the contacts between the blocks. They were thus able to prove that friction caused by the sliding of two contacting blocks (or tectonic plates, in nature) can only take place when the connections between the surfaces are first fractured or broken. Or in the case of earthquakes, they are triggered only after rapid cracks propagate along the contacting surfaces of the tectonic plates.
In addition to that discovery, by watching the process as it unfolds at the onset of friction, the Fineberg team has also managed to identify different types of earthquakes – ranging from slowly moving ones to supersonic varieties – which have parallels in nature. Though it’s still impossible to predict tremors, this new paradigm may enable researchers to detect the types of earthquakes to which an area is prone – and in the future to design, accordingly, safer buildings.
Prof. Fineberg's project on Fracture and Friction: Rapid Dynamics of Material Failure, received a 2,265,400 Euro grant under the 7th Framework programme.
Sequence of photographs of the tip of a crack moving at approximately 50% of the material’s sound speed, as the crack undergoes a microscopic branching instability. The eventual length of the microscopic branch formed is a few 10’s of microns, yet, as can be seen in this sequence, its formation has a enormous effect on the crack’s dynamics.