Friction
Since friction is the major resisting force during an earthquake, it is
the key to understanding how earthquakes grow, how large waves they
generate and how big they are going to be. We are currently measuring
fault roughness in the field and also doing laboratory experiments on
rocks to try and establish what dynamical regime best describes the
friction of rocks during earthquakes. Mechanisms like hydrodynamic
lubrication (Brodsky and Kanamori, 2001) may help to explain low
friction on faults during rupture.
Seismic records of landslides also provide insight into natural
friction processes. A landslide generates seismic waves by both
shearing and loading the surface as the mass moves
from a steep to a shallow slope. The effective force system is a
horizontal single force. The amplitude of the seismic waves is
proportional to the force drop during the landslide, just as during an
earthquake the seismic wave amplitude is proportional to the seismic
moment, i.e., the force drop multiplied by the source dimension. For
landslides we know an additional variable that is unknown for the
earthquake case. We know the gravitational driving force of the
landslide while the magnitude of the tectonic forces that drive
earthquakes are generally unknown. Therefore, we can find the absolute
value of the frictional force for landslides whereas we are unable to
perform this calculation for earthquakes. In Brodsky et al. 2003, we found that three large
volcanic landslides were all consistent with an apparent coefficient of
friction of 0.2. |
