Inducing in situ, nonlinear soil response applying an active source

Abstract

[1] It is well known that soil sites have a profound effect on ground motion during large earthquakes. The complex structure of soil deposits and the highly nonlinear constitutive behavior of soils largely control nonlinear site response at soil sites. Measurements of nonlinear soil response under natural conditions are critical to advancing our understanding of soil behavior during earthquakes. Many factors limit the use of earthquake observations to estimate nonlinear site response such that quantitative characterization of nonlinear behavior relies almost exclusively on laboratory experiments and modeling of wave propagation. Here we introduce a new method for in situ characterization of the nonlinear behavior of a natural soil formation using measurements obtained immediately adjacent to a large vibrator source. To our knowledge, we are the first group to propose and test such an approach. Employing a large, surface vibrator as a source, we measure the nonlinear behavior of the soil by incrementally increasing the source amplitude over a range of frequencies and monitoring changes in the output spectra. We apply a homodyne algorithm for measuring spectral amplitudes, which provides robust signal-to-noise ratios at the frequencies of interest. Spectral ratios are computed between the receivers and the source as well as receiver pairs located in an array adjacent to the source, providing the means to separate source and near-source nonlinearity from pervasive nonlinearity in the soil column. We find clear evidence of nonlinearity in significant decreases in the frequency of peak spectral ratios, corresponding to material softening with amplitude, observed across the array as the source amplitude is increased. The observed peak shifts are consistent with laboratory measurements of soil nonlinearity. Our results provide constraints for future numerical modeling studies of strong ground motion during earthquakes.

Refined thresholds for non-linear ground motion and temporal changes of site response associated with medium-size earthquakes

“…Generally, non‐linear response has a strong correlation with the level of ground motion (e.g. Hartzell 1998; Su et al 1998; Johnson et al 2009) along with rock type and other characteristics of the sites (e.g. Beresnev & Wen 1996; Hartzell 1998; Trifunac et al 1999; Tsuda et al 2006).…”

Characterizing the nonlinear interaction of S- and P-waves in a rock sample

“…The propagating pump wave is common to all in-situ methods [6][7][8][9] and has also been tested for a DAE method in Ref. 27.…”

“…5 Actively induced nonlinear responses have been observed in-situ at the scale of a few meters. [6][7][8][9] Laboratory measurements are also helpful in understanding the nonlinear elastic response. Of particular importance is the role of additional compliance due to micro-cracks including anisotropy and fluid saturation effects.…”

Field Observations

“…Experiments described in reference [4] employing a large, active shaker source show strong elastic nonlinear response in near-source sediments. The site of the experiment, located near Austin, Texas, involved 11 m of young, unconsolidated point bar sediments above bedrock (point bars are deposits formed along the inside of a river bend).…”