The other day, I heard a very interesting talk by Prof. Fineberg (from my physics department) about dry friction, and in particular, how the contact surface between two objects begins to separate as objects begin to move relative to each other.
In the talk, he described how three types of separation waves (i.e., cracks) can form and propagate through the contact surface. Two of the waves are known from crack theory: One is a wave traveling at a somewhat subsonic speed, the other is an ultra-sonic wave, eh, crack. Both describe how one object starts to move with respect to the other. Their experiments, however, demonstrated the existence of a third type of separation wave, unknown in standard crack theory. Unlike the other two, it has a very low propagation speed.
Now why is this slow mode so interesting? Fault lines are essentially contact surfaces between two objects which can move relative to each other. If the onset of motion in the lab can take place through the slow mode, why can't it be also in fault lines?
The surprising answer is that it appears that it can. Just a few years ago, seismologists began noticing that large movements along fault lines could take place over night. Motion over these "large" few centimeter distances should have been felt as a very large earthquakes. However, instead of headline news, even sensitive seismometers record nothing. The only movements (of millimeters an hour) are detected through GPS measurements. It appears that these silent earthquakes are the large scale manifestations of the slow modes seen by Fineberg and his students in the lab.
These results clearly raise a few particularly interesting questions:
- Are the silent earthquakes indeed the manifestation of the slow modes?
- What is the physical mechanism giving rise to these slow modes? They don't appear in normal cracks, only in the contact surface between separate objects.
- If we did understand the physical origin, could we
- Why are they called silent earthquakes? Shouldn't they be christened a more appropriate name, such as earthcreeps?
Incidentally, if you're interested you can take the little poll for the appropriate name that these "silent earthquakes" should have.
A few relevant links: