Da Vinci was wrong about friction: Earthquake study

In a breakthrough, scientists have discovered that fracture and friction are closely interrelated, a finding that may help describe the mechanics that drive earthquakes.
The study overturns a 500-year-old theory given by Leonardo da Vinci. He described how rough blocks slide over one another, providing the basis for our understanding of friction to this day.
The phenomenon of fracture was always considered to be something totally different.
But new research by Professor Jay Fineberg and graduate student Ilya Svetlizky, at the Hebrew University's Racah Institute of Physics, has demonstrated that these two seemingly disparate processes of fracture and friction are actually intimately intertwined.

The findings create a new paradigm that's very different from the da Vinci version, and, according to the researchers, give us a new understanding of how earthquakes actually occur.
Fineberg and Svetlizky produced "laboratory earthquakes" showing that the friction caused by the sliding of two contacting blocks can only occur when the connections between the surfaces are first ruptured (that is, fractured or broken) in an orderly, "organised" process that takes place at nearly the speed of sound.

Before any motion can occur, the blocks are connected by interlocking rough contacts that define their interface.
In order for motion to occur, these connections have to be broken. This physical process of breaking is called a fracture process.

This process is described by the theory of crack propagation, say the researchers, meaning that the stresses (or forces) that exist at the front edge of a crack become highly magnified, even if the overall forces being applied are initially quite small.
"The insights gained from our study provide a new paradigm for understanding friction and give us a new, fundamental description of the mechanics and behaviour that drive earthquakes, the sliding of two tectonic blocks within natural faults," said Fineberg.
"In this way, we can now understand important processes that are generally hidden kilometres beneath the Earth's surface," Fineberg said.

The research was published in the journal Nature.