DETERMINATION OF CONSTITUTIVE RELATIONS OF FAULT SLIP BASED ON SEISMIC-WAVE ANALYSIS

Constitutive laws define the boundary conditions on fault plane and go vern many aspects of earthquake failure. Although several constitutive laws have been formulated based on laboratory rock experiments and ap plied to theoretical studies in various fields, no actual relation dur ing a natural earthquake has been determined. The 1995 Kobe earthquake is suitable for detailed kinematic analysis, and this enables the fir st evaluation of constitutive relations for a natural earthquake. In t his study, we determine spatiotemporal slip distribution on an assumed fault plane of the 1995 Kobe earthquake by waveform inversion and the n solve elastodynamic equations using a finite difference method to de termine the stress distribution and constitutive relations on the faul t plane. An inversion method based on Bayes theorem is employed to obt ain a spatiotemporal slip distribution, and enables us to ensure the o bjective uniqueness of the solution with numerous parameters and smoot hing constraints. This slip distribution is then used as part of the b oundary condition in the finite difference calculation The time histor ies of slip and shear stress obtained then provide a constitutive rela tion at each point on the fault plane. They show slip weakening relati ons almost everywhere on the fault plane, while slip rate dependency i s not clear. The slip weakening behavior has a clear depth dependency indicating that the slip weakening rate (d tau/du) is smaller in the s hallow crust than that in the deep crust. This may be associated with the paucity of shallow seismicity observed in the source region of thi s earthquake as reported for many mature fault systems.