Investigating Amplitude Dependent Sediment Properties using a Vibroseis Truck and a Micro-array of Accelerometers

Amplitude dependent (i.e., nonlinear) properties of sediment play an important role in our understanding of sediment behavior during earthquake strong ground motions. I present my ongoing research investigating amplitude dependent sediment properties using a large Vibroseis truck with a nearby micro-array of accelerometers in Garner Valley, California and Austin, Texas. Previous work on these datasets have shown a shift in the resonance frequency of the upper soil layer to lower frequencies, interpreted to represent amplitude dependent modulus reduction within the sediment (Pearce et al., 2004). That study used the Vibroseis truck to produce a harmonic source that contained a range of frequencies to ensure the resonant frequency of the near-surface soil layer was excited. The process and analysis was similar to laboratory studies on material nonlinearity using resonant bar experiments (Ostrovsky and Johnson, 2001). I use the datasets to study basic wave propagation away from the Vibroseis truck focusing on using a harmonic source that is held at a fixed frequency and analyzing basic observables of traveling wave propagation away from the Vibroseis truck as a function of distance and driving amplitude. Initial results show wave propagation to be quite complex; for instance, the presence of higher-order harmonics, the wavefield exhibiting beating in space, and a surprisingly high degree of spatial variability of waveforms within a small array area. Wavefield complexity may be due to complexities in the Vibroseis source radiation, inconsistent instrument response due to coupling issues during large ground motions, and nonlinear wave propagation. It has proven challenging confidently distinguishing between the possible sources influencing wave propagation. Nonetheless, horizontal phase velocity, ground motion attenuation, and frequency spectrum appear to exhibit some degree of amplitude dependence.