Effects of off-fault inelasticity on near-fault directivity pulses

Yongfei Wang, & Steven M. Day

Published August 12, 2019, SCEC Contribution #9429, 2019 SCEC Annual Meeting Poster #288

Rupture directivity strongly affects spatial variations in ground-motion amplitude and duration around faults and leads to dominant pulse-like fault-normal horizontal ground motions (for prevailing subshear-rupture events) causing a large amount of damage to structures. It is important to understand how directivity-enhanced velocity pulses behave very close to the rupture surface, where plastic yielding is likely to affect their amplitudes and waveforms. This paper assesses the extent to which plastic yielding, which is absent in standard kinematic models, may systematically affect the amplitude, frequency content, and distance scaling of directivity pulse. We use some simple 2D modeling experiments to gain a general understanding of how rupture kinematics are reflected in near-fault pulses, and then simulate strike-slip ruptures in 3D with and without plastic yielding. As an initial step toward assessing the sensitivity of the results to rupture complexity and degree of coherence, we repeat the 3D simulations on a fault with geometric roughness. We find that each of the four 3D models (flat and rough faults, with and without off-fault yielding), scaled to approximately magnitude 7, predicts a fault-normal pulse with characteristic behavior of observed pulses (periods in the range 2-5 second, amplitudes increasing with distance in the forward-directivity direction but approaching a limiting amplitude). Plastic yielding systematically reduces pulse amplitude and increases its dominant period, relative to models that neglect off-fault yielding. Yielding saturates near-fault peak ground velocity (PGV) as for a larger stress drop and epicentral distance, alternatively interpreting observed magnitude saturation of PGV near a magnitude of 7, and show period-dependent distance taper and along-strike saturation of directivity-induced amplification, undermining the commonly used wedge-shaped directivity amplification. In addition, off-fault plasticity strongly suppresses the otherwise very strong high-frequency acceleration pulses that otherwise appear in the fault-parallel acceleration when local supershear rupture transients occur, suggesting a mechanism of the apparent absence of the expected Mach wave signature from supershear ruptures. The incorporation of nonlinear material models by dynamic earthquake rupture simulations into the analysis of seismic pulse dynamics can better understand and more accurately predict near-fault ground motions.

Wang, Y., & Day, S. M. (2019, 08). Effects of off-fault inelasticity on near-fault directivity pulses. Poster Presentation at 2019 SCEC Annual Meeting.

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