HR: 0830h
AN: S41C-0101    [PDF]
TI: Fault Jumping and Bilateral Propagation as a General Mechanism of Backward Branching: Case Study of Transition from the Kickapoo to the Homestead Valley Fault in the 1992 Landers Earthquake
AU: * Fliss, S
EM: sonia.fliss@polytechnique.org
AF: Division of Engineering and Applied Science, Harvard University, Cambridge, MA 02138
AU: * Fliss, S
EM: sonia.fliss@polytechnique.org
AF: Option de Mecanique, Ecole Polytechnique, & Corps des Telecoms, route de Saclay, Palaiseau, 91128 France
AU: Bhat, H S
EM: bhat@esag.harvard.edu
AF: Division of Engineering and Applied Science, Harvard University, Cambridge, MA 02138
AU: Dmowska, R
EM: dmowska@seismology.harvard.edu
AF: Division of Engineering and Applied Science, Harvard University, Cambridge, MA 02138
AU: Dmowska, R
EM: dmowska@seismology.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Division of Engineering and Applied Science, Harvard University, Cambridge, MA 02138
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138
AB: To unravel the directivity of past large earthquakes along complex fault systems, with offsets and branches, we address the mechanics of "backward" branching ({\it Dmowska et al., EOS}, 2002). In the 1992 Landers earthquake, such a 4-km long backward branch formed along the southeastern end of the Homestead Valley Fault (HVF), after transfer of slip to it from the Johnson Valley Fault (JVF) via the Kickapoo Fault (KF) (Sowers et al., BSSA, 1994); the main Landers rupture continued to the north along the HVF. Direct bending of a right-lateral rupture path through a strongly obtuse angle, to form a backward branch, is disallowed by the stress field at a dynamic rupture front ({\it Poliakov et al., JGR}, 2002). An alternative mechanism must be identified. We propose that rupture arrests at the termination of one fault segment, allowing a {\it Harris-Day} ({\it JGR}, 1993) jump to an adjoining segment which then ruptures bilaterally; the end propagating oppositely to the arrested rupture forms the backward branch. Using a 2D elastodynamic BIE formulation incorporating slip-weakening rupture ({\it Kame et al., JGR}, 2003), we numerically reproduced backward branching for the JVF-KF to HVF geometry. When the rupture stopped at the northern termination of the KF, it jumped to the HVF and then evolved bilaterally, most extensively to the north in continuation of the main rupture, but also along the southeastern HVF, which curves away from the KF to form the backward branch. A 2D rupture model such as we use may often be justified when length scales of phenomena modeled are small compared to the thickness of the seismogenic zone, as in this case here. We showed that stresses radiated to the curved HVF, while the rupture tip was still propagating along the JVF-KF system, would be unlikely to nucleate rupture on the HVF. Rather, the jump was made possible by the much higher stresses radiated when the rupture stopped at the northern termination of the KF. Those stresses succeeded in nucleating on the HVF because the two fault traces are close to parallel there; the less parallel orientation of the curved HVF further to the southeast would not have allowed jumping. Evidence for the termination of the KF and for the lack of its direct connection with the HVF, hence for the necessity of a jump to transfer rupture, is provided by the following: Detailed surface slip mapping ({\it Sowers et al., BSSA}, 1994) which showed a gap, relative relocations of aftershocks which projected fault planes to seismogenic depths ({\it Felzer and Beroza, GRL}, 1999), and fault zone trapped wave studies ({\it Li et al., JGR}, 1994) which showed non-communication between the JVF-KF and HVF. Such a jump followed by bilateral propagation provides a general mechanism of forming rupture segments running backwards by comparison with the general direction of propagation. During the Landers 1992 event two other such cases followed the JVF-KF to HVF transition just discussed, namely one from the northern HVF to the southeastern Emerson Fault, and then another from the northwestern Emerson to southeastern Camp Rock Fault.
DE: 7209 Earthquake dynamics and mechanics
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8020 Mechanics
DE: 8123 Dynamics, seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting