Farn Yuh Menq

Deep Shear Wave Velocity Profiles from Surface Wave Measurements in the Mississippi Embayment

Shear wave velocity ( VS ) profiles to depths of approximately 200 m were developed from active-source surface wave velocity measurements in the Mississippi Embayment region of the Central United States. Soil deposits in this region are hundreds of meters thick, but are poorly characterized at depths below 60 m. Measurements were performed at five locations in Arkansas and Tennessee with a maximum distance between sites of approximately 130 km. The median VS profile calculated from the five profiles is in good agreement with a generic reference VS profile for the Mississippi Embayment that has been used in recent site response studies. The near-surface VS profiles at the five sites were remarkably consistent with average shear wave velocities in the top 30 m (VS30), varying by less than 10%. Increasing variability between the VS profiles was observed at greater depths. The variability between VS profiles was shown to be correlated with changes in lithology at two of the sites where nearby lithologic information was available.

Inducing in situ, nonlinear soil response applying an active source

[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.

Shear-Wave Velocity Profiling of Strong Motion Sites That Recorded the 2001 Nisqually, Washington, Earthquake

The 2001 M 6.8 Nisqually, Washington, earthquake was recorded by more than 70 strong motion sites in and around the Puget Sound region. We have characterized the shear-wave velocity (VS) structure down to depths of 100 to 300 ft at the 32 permanent strong motion sites, which recorded the highest ground motions (peak horizontal ground accelerations [PGA] of 0.04 to 0.31 g), using the Spectral-Analysis-of-Surface-Waves (SASW) technique. Most of the surveyed sites are underlain by glacial till (Qvt) with the remaining sites on Holocene alluvium (Qal), glacial recessional (Qvr) and advance outwash deposits (Qva), or manmade fill/modified ground (m). VS30 values for Qvt and Qvr range from 1,266 to 1,769 ft/sec and 1,139 to 1,826 ft/sec, respectively, corresponding to NEHRP site class C. In general, a pattern of higher PGAs with lower VS30 was not observed suggesting that VS30 cannot account for all site effects on the 2001 Nisqually ground motions.

Induced Dynamic Nonlinear Ground Response at Garner Valley, California

We present results from a prototype experiment in which we actively induce, observe, and quantify in situ nonlinear sediment response in the near surface. This experiment was part of a suite of experiments conducted during August 2004 in Garner Valley, California, using a large mobile shaker truck from the Network for Earthquake Engineering Simulation (NEES) facility. We deployed a dense accel- erometer array within meters of the mobile shaker truck to replicate a controlled, laboratory-style soil dynamics experiment in order to observe wave-amplitude- dependent sediment properties. Ground motion exceeding 1g acceleration was pro- duced near the shaker truck. Thewave field was dominated by Rayleigh surface waves and ground motions were strong enough to produce observable nonlinear changes in wave velocity. We found that as the force load of the shaker increased, the Rayleigh- wave phase velocity decreased by as much as ∼30% at the highest frequencies used (up to 30 Hz). Phase velocity dispersion curves were inverted for S-wave velocity as a function of depth using a simple isotropic elastic model to estimate the depth depen- dence of changes to the velocity structure. The greatest change in velocity occurred nearest the surface, within the upper 4 m. These estimated S-wave velocity values were used with estimates of surface strain to compare with laboratory-based shear modulus reduction measurements from the same site. Our results suggest that it may be possible to characterize nonlinear soil properties in situ using a noninvasive field technique.