Southern California Earthquake Center Fault System Behavior Southern California Earthquake Center Annual Meeting September 18, 2000

Themes Earthquakes and criticality Stress triggering Rupture complexity Complexity of earthquake sequences Fault zone fine structure (damage, segmentation)

Accelerating moment release preceded M>6.5 on S.A. system Clustering of intermediate events Effect is clearest for largest events Region size scales with event size Bowman & Sammis

Landers Earthquake Rupture Dynamic Model (Peyrat et al.) Kinematic Model (Wald et al.) Can fit strong motion waveforms with dynamic (spontaneous rupture) model, slip-weakening friction Rupture highly sensitive to prestress “Criticality” condition for rupture growth Connectivity of high prestress regions is key to event size

Landers Strong Motion Waveforms Dynamic Model vs. Recorded (Peyrat et al.)

Accelerating seismicity, complex dynamics of individual earthquakes, both suggest long range correlations in stress field prior to largest events Transition in ground motion modeling: from kinematic to dynamic modeling New questions raised. Require advances in observation, and more advanced numerical models. Examples Nature of fault friction How much does geometrical complexity contribute to rupture complexity and strong motion characteristics? Parametrization of dynamic model-based ground motion simulations?

Fault Zone Trapped Waves Landers (Li et al., 1994) Damage zone dimensions ~100m Wavespeed degradation ~30-50% Fault trace discontinuities associated with waveguide discontinuity at depth

Fault Zone Healing (Li et al., 1998)

Important time-dependent effects recognized Experiments + numerical simulations advanced our understanding of fault zone fine-structure Level and extent of fault-zone damage Persistence of fault discontinuities Important time-dependent effects recognized Poroelasticity? Viscoelasticity? Frictional dynamics?

But wait, . . .there’s more. . . Long-term clustering of large events (E. Mojave) Continuum complexity (frictional dynamics vs. geometrical disorder) Thrust fault dynamics (theory, observation, implications for strong motion) Lab experiments to validate numerical models of rupture