HR: 08:30h
AN: OS21E-03 INVITED     [Abstracts]
TI: Landslide Basal Friction as Measured by Seismic Waves
AU: * Brodsky, E E
EM: brodsky@ess.ucla.edu
AF: Dept of Earth and Space Sci. UCLA, 595 Charles Young Dr., Los Angeles, CA 90095 United States
AU: Gordeev, E
EM: gord@emsd.iks.ru
AF: Geophys. Survey of Russian Acad. of Sci., av. Piip, 9, Petropavlovsk-Kamcha, 683006 Russian Federation
AB: Dynamical predictions of landslide runout require measurements of the basal friction. We present the first seismically determined bounds on the frictional coefficients for three large volcanic landslides. 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. We studied three landslides (Bezymianny, Russia 1956, Sheveluch, Russia 1964 and Mount St. Helens, USA 1980) that were all followed immediately by eruptions. The landslide masses vary by a factor of 5. We test the data against the hypothesis of a constant value of apparent friction. The apparent friction $\mu_{app}$ is defined as the ratio of the amplitude of the horizontal single force to the weight of the landslide. Since the Mount St. Helens seismic source is very well-constrained, we use the amplitude of this landslide force drop as a starting point. We calculated the value of $\mu_{app}$ for Mount St. Helens using previous seismic results and the geological data. We then test whether or not the other two landslides are consistent with the same value of $\mu_{app}$. We use teleseismic and regional seismic data to show that all three landslides are consistent with an apparent coefficient of friction of 0.2 which corresponds to an actual areally-averaged frictional coefficient of 0.2--0.6. We find that the apparent friction is independent of the quantity of hot gas subsequently released, i.e., all the landslides are consistent with $\mu_{app}$=0.2 even though some had directed blasts and others did not. In addition, the data rule out viscous flow as the constitutive model for basal shear. The seismic data are consistent with a model where the amplitude of the shear force scales with landslide mass, but they are inconsistent with a model where the force scales with landslide area.
DE: 8419 Eruption monitoring (7280)
DE: 7209 Earthquake dynamics and mechanics
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting