HR: 1340h
AN: S13B-1301    [Abstracts]
TI: 2007 Mw=6.6 Niigata Chuetsu-Oki earthquake ruptured on a fault strongly unclamped by the 2004 Mw=6.6 Niigata Chuetsu shock
AU: * Toda, S
EM: s-toda@aist.go.jp
AF: Active Fault Research Center, AIST, Site 7, Higashi, 1-1, Tsukuba, 305-8567, Japan
AB: The highly damaging 2004 and 2007 earthquakes struck 40 km apart in an active fold and thrust belt along the Japan Sea coast of Honshu. Both earthquakes slipped immature reverse faults that are probably reactivated normal faults. The 2004 event ruptured several antithetical faults, and produced an unusually vigorous and widespread aftershock sequence, which including four M≥6 events within the first week. We argue that the high rate of aftershocks results from slip on blind high-angle reverse faults. Unlike reverse faults that cuts the earth's surface, or low-angle thrust faults, slip on steeply-dipping blind reverse faults impart high stresses to the surrounding crust. This hypothesis raises the question as to whether the 2007 earthquake is a delayed aftershock or in some sense an event promoted by the 2004 earthquake. If the fault friction is high (coefficient ~0.8), we find that the 2007 fault was subjected to a Coulomb stress increase of up to 0.3 bars by the 2004 earthquake. However, the Coulomb stress increase is found only if the 2007 rupture occurred on the NW-dipping nodal plane (strike/dip/rake of 228°/42°/95°), and only if the hypocenter lies at depth greater than about 15 km. These conditions are reasonable, but not certain. Regardless of the depth and nodal plane, the 2007 fault was subjected to an unclamping stress, and this could be as large as 0.9 bars. Unclamping may be very important for faults with little cumulative slip, because they presumably have rough surfaces. It is also possible that crustal fluids were driven into the unclamped regions following the 2004 earthquake, slowly lubricating the 2007 and surrounding faults. Although much closer together, the 1983 Mw=6.7 Coalinga, California thrust earthquake strongly unclamped the nearby Nunez reverse fault, which ruptured in a Mw=6.0 earthquake 2 months later. Thus the strongest link between the two Niigata earthquakes is coseismic unclamping.
DE: 7200 SEISMOLOGY
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8100 TECTONOPHYSICS
DE: 8164 Stresses: crust and lithosphere
SC: Seismology [S]
MN: 2007 Fall Meeting