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Fault Branching

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Fault BranchingReferences – Fault BranchingFault Branching ModelConfiguration of a preexisting branched fault system and prestress state Fault-normal precompression is dominant, Ψ > 45o, allowing rupture to continue along bend paths primarily to the extensional side Fault-parallel precompression is dominant, Ψ < 45o, allowing rupture to continue along bend paths primarily to the compressional side Rupture ModesCase Histories of Earthquakes on Branched FaultsSlide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13DenaliLandersSlide Number 16Kobe1979 Imperial ValleySlide Number 19KokoxiliSlide Number 21LuzonLuzonConclusionsFault BranchingPaul SomervilleURS PasadenaReferences – Fault Branching• Kame, N. and T. Yamashita (1999). Simulation of spontaneous growth of dynamic crack without constraints on the crack tip path. Geophys. J. Int. 139, 349-358.• Kame, N., J.R. Rice and R. Dmowska (2003). Effects of prestress state and rupture velocity on dynamic fault branching. J. Geophys. Res. 108, ESE 13-1 – 13-21.Fault Branching Model• Dynamic rupture modeling of strike-slip faulting shows that the rupture prefers to bend than to run straight (Kame and Yamashita, 1999)• On pre-existing strike-slip faults with branches (Kame et al., 2003), the rupture mode depends on • ! = angle between the direction of maximum compressive stress (Smax) and the fault strike• " = angle between the main fault and the branch fault• vr = rupture velocity (expressed as a fraction of the shear wave velocity cs)Configuration of a preexisting branched fault system and prestress stateFault-normal precompression is dominant, ! > 45o, allowing rupture to continue along bend paths primarily to the extensional sideFault-parallel precompression is dominant, ! < 45o, allowing rupture to continue along bend paths primarily to the compressional sideRupture ModesMode Description Examples 1 Rupture on both main fault and branch fault 1979 Imperial Valley 1995 Kobe 1992 Landers 2 Rupture only on branch fault 1990 Luzon 2002 Denali 3 Rupture only on main fault 2001 Kokoxili (Kunlunshan)Case Histories of Earthquakes on Branched FaultsEVENT MAIN FAULT BRANCH FAULT REFERENCE 1979 Imperial Valley Imperial Brawley Kame et al., 20031990 Luzon Philippine Digdig Rantucci, 1994 1995 Kobe Suwayama (Gosukebashi)Okamoto Sekiguchi et al., 2000 1992 Landers Johnson Kickapoo Kame et al., 20032001 Kokoxili (Kunlunshan) Kunlun Kitadan Bhat et al., 2007 2002 Denali Denali Totschunda Bhat et al., 20042002 Denali1992 Landers1995 Kobe1979 Imperial Valley2001 Kokoxili?1990 Luzon?2002 Denali1992 LandersDenaliLanders1995 Kobe1979 Imperial ValleyKobe1979 Imperial Valley2001 Kokoxili?Kokoxili1990 Luzon?LuzonLuzonConclusions• We have analyzed six events involving rupture on branched faults which represent three possible modes of fault branching behavior. • In three cases, including the 1979 Imperial Valley, 1995 Kobe, and 1992 Landers earthquakes, such rupture occurred on both the main fault and the branch fault, all consistent with Kame et al. (2003).• In the 1990 Luzon and 2002 Denali earthquakes, rupture proceeded onto the branch fault but stopped on the main fault at the branch point. – The Denali earthquake observations are consistent with Kame et al. – The Luzon earthquake observations are potentially consistent, but the uncertainty in the stress field orientation renders this inconclusive with current data.• In the 2001 Kokoxili (Kunlunshan) earthquake, the main fault continued to rupture without branching onto the Kitadan fault. The Kokoxili earthquake observations are potentially consistent with Kame et al., but the uncertainty in the stress field orientation renders this inconclusive with current


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