Earthquake in a lab helps scientists decipher knowledge of physics of friction

Scientists at an “earthquake laboratory at CalTech increased the knowledge of the physics of friction that drive thrust-fault earthquakes, often the world’s largest quakes.

By using photography and placing two blocks of a material that has similar frictional properties as rock, they can study thrust-fault earthquakes in a way not possible otherwise because most of the action in the quake happens too deep inside the earth, CalTech reported on its website.

"Simulating earthquakes in a lab lets us observe how these brief and violent events grow and evolve by ‘slowing down' their motion through high-speed photography and optics," Ares Rosakis, the Theodore von Karman Professor of Aeronautics and Mechanical Engineering, who runs the facility, told CalTech.

Rosakis introduced the concept of laboratory earthquakes with former Caltech Seismology Laboratory director Hiroo Kanamori, John E. and Hazel S. Smits Professor of Geophysics, Emeritus.

With of digital image correlation, the researchers could measure shifts in the location of individual points during the simulated thrust-fault earthquake. They learned how strain and stress dynamically evolve throughout the material. That enabled them to map a rupture moving up a fault, how it interacts with the ground surface and even how propagated waves affected itself, CalTech reported.

Fault normal stress, the compressive force keeping the fault locked tight, changed in strength with rapid cycling, which changed the fault’s resistance to slipping, making it more likely to cause a quake. With what they learned they challenged the assumption that friction locking plates together along a fault is proportional to the fault-normal stress. Instead, when the rupture interacts with the earth’s surface a significant lag happens between changes in fault-normal stress and the sheer resistance. The two are not proportional, the researchers told CalTech.

"This implies the presence of a complex history-dependent mechanism governing friction in the presence of rapid fault normal stress, which are characteristic of thrust-fault configurations," Rosakis told CalTech.