HR: 11:35h
AN: S11H-06    [PDF]
TI: Slip Transfer from the Denali to Totschunda Faults During the 3 November 2002 $M_w$ 7.9 Denali, Alaska, Earthquake
AU: * Bhat, H S
EM: bhat@esag.harvard.edu
AF: Div. of Eng. and Appl. Sci., Harvard Univ., 29 Oxford Street, Cambridge, MA 02138 United States
AU: Dmowska, R
EM: dmowska@esag.harvard.edu
AF: Div. of Eng. and Appl. Sci., Harvard Univ., 29 Oxford Street, Cambridge, MA 02138 United States
AU: Dmowska, R
EM: dmowska@esag.harvard.edu
AF: Dept. of Earth and Planet. Sci., Harvard Univ., 20 Oxford Street, Cambridge, MA 02138 United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Div. of Eng. and Appl. Sci., Harvard Univ., 29 Oxford Street, Cambridge, MA 02138 United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Dept. of Earth and Planet. Sci., Harvard Univ., 20 Oxford Street, Cambridge, MA 02138 United States
AU: Kame, N
EM: kame@esag.harvard.edu
AF: Dept. of Earth and Planet. Sci., Kyushu Univ., Hakozaki 6-10-1, Higashi-ku, Fukuoka, 812-8581 Japan
AB: We analyze dynamic slip transfer from the Denali to the Totschunda faults. This adopts methodology from earlier studies (Poliakov et al., 2002; Kame et al., 2003) in which it was shown that the propensity of the rupture path to follow a fault branch is determined by the preexisting stress state, branch angle and incoming rupture velocity at the branch location. Even though the studies rely on 2D numerical simulations, we think that they describe correctly the first order of phenomena associated with dynamic branching along geological fault systems. Here we check that theory on the Denali-Totschunda rupture process. This is a more complete analysis than Bhat et al. (2002). The $M_w$ 7.9 Denali earthquake, which was mainly a right lateral strike-slip event, occurred on 3 November 2002 and ruptured for about 340 km, with the last 76 km being on the Totschunda Fault which branches off from the Denali Fault at an angle of about $15\deg$ to the extensional side. The rupture path chose Totschunda, exclusively, beyond the Denali-Totschunda branching point. We have no evidence on prestress directions very near the branch, but Ratchkovski and Hansen (2002, 2003) have recently evaluated stress directions for interior Alaska including near the Denali fault. The principal stress closest to the branch is almost fault-normal with the local direction of the Denali fault (Ratchkovski, 2003). Earlier works on the stress field in central Alaska suggest that the prestress inclination with Denali was around $70\deg$ in the area of branching. Thus we use the values of $70\deg$ and $80\deg$ in our numerical simulations. The average rupture velocity seems to be about 0.8 $c_s$ (Kikuchi and Yamanaka, 2002), although the velocity as the branch was approached is not yet constrained. As it is not yet clear what was the rupture velocity at the branching point, other than that it was rather high (Eberhart-Phillips et. al., 2003), we use 0.6 $c_s$, 0.8 $c_s$, 0.9 $c_s$ and even 1.4 $c_s$ as parameters in our simulations. We numerically simulated slip transfer from Denali to Totschunda by the methodology of Kame et al. (2003) which uses a 2D elastodynamic boundary integral equation model of mode II rupture with self-chosen path along a branched fault system. The strength of the faults is assumed to follow a Coulomb law with friction coefficient which slip-weakens from its static to dynamic value. All our simulations for sub-Rayleigh rupture velocities, except one, predict that the rupture path branches off along Totschunda without continuation along Denali. For the case when the prestress inclination is $70\deg$ and incoming rupture speed at branching is 0.9 $c_s$, there is also a continuation of rupture along Denali beyond the branching point, at a speed slower than that along Totschunda. However when the prestress inclination is steeper, at $80\deg$, the rupture chooses Totschunda exclusively when its velocity near the branching point is around 0.9 $c_s$. We also see exclusive continuation of rupture on Totschunda when the rupture is super-shear, 1.4 $c_s$.
DE: 7209 Earthquake dynamics and mechanics
DE: 8020 Mechanics
DE: 8123 Dynamics, seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting