Description
Foam rubber earthquake experiments provide a means to explore the sensitivity of near-fault ground motion to fault and rupture geometry, offering insights that would be difficult to achieve from the limited recordings available in the near-fault region of large, natural earthquakes. The foam rubber experiments simulate unilaterally propagating strike-slip earthquakes and the recorded waveforms are compared with those generated by dynamic three- dimensional simulations of the same experiments. Cross-validation of the foam rubber model with a numerical model helps reduce uncertainties in both modeling methodologies. Accelerometers are deployed on the free surface along lines parallel to strike, to characterize directivity-enhanced near-fault ground motion. The main features of the foam-model waveforms, such as the shape, duration, and absolute amplitude of the main acceleration pulses, are successfully reproduced by the numerical model. The main acceleration pulses in the foam and numerical models show similar decay with distance away from the fault. In addition, the fault- normal components in both models show similar strong directivity with increasing distance along fault strike. The directivity effect is evident in both peak accelerations and pseudo-spectral accelerations (response spectra) of both models. Furthermore, a comparison of the foam and numerical model response spectra with an empirical directivity model for earthquake strong motion spectra reveals good agreement at long periods. Both foam and numerical models over- predict short-period directivity effects, with the amount of over-prediction increasing systematically with diminishing period. It is speculated that this period-dependent difference is attributable to fault-zone heterogeneities in stress, frictional resistance, and elastic properties. These complexities, present in the earth but absent or minimal in the foam model (and in the numerical simulations of the foam model), can be expected to reduce rupture- and wave-front coherence, likely leading to reduced directivity.
