Daniel Lavallée

Hysteretic and Dilatant Behavior of Cohesionless Soils and Their Effects on Nonlinear Site Response: Field Data Observations and Modeling

In this study we present evidence that nonlinearity can be directly observed in acceleration time histories such as those recorded at the Wildlife Refuge and Kushiro Port downhole arrays from the 1987 Superstition Hills, California, and the 1993 Kushiro-Oki, Japan, earthquakes, respectively. These accelerograms and others compiled in this study present a characteristic waveform composed of intermittent high-frequency peaks riding on a low-frequency carrier. In addition, soil amplification of the surface records is strongly observed compared to their downhole counterpart; this is contrary to the expected amplification reduction produced by the nonlinear soil behavior. Laboratory studies show that the physical mechanism that produces such phenomena is the dilatant nature of cohesionless soils, which introduces the partial recovery of the shear strength under cyclic loads. This recovery translates into the ability to produce large deformations followed by large and spiky shear stresses. The spikes observed in the acceleration records are directly related to these periods of dilatancy and generation of pore pressure. These results are significant in strong-motion seismology because these spikes produce large if not the largest acceleration. They are site related, not source related. Using the in situ observations from the Kushiro Port downhole array, we have modeled the 1993 Kushiro-Oki earthquake. The synthetic accelerograms show the development of intermittent behavior—high frequency peaks—as observed in the recorded acceleration time histories. Shear modulus degradation due to pore pressure produces large strains in the soil with large amplification in the low-frequency band of the ground motion. We also modeled data from the 1987 Superstition Hills earthquake recorded at the Wildlife Refuge station. The results show the importance of better soil characterization when pore pressure may develop and the effects of dilatancy in the understanding of nonlinear site response.

Universal multifractal indices for the ocean surface at far red wavelengths

For some time, ocean wave breaking has been conceptualized as a cascade process in which the large scale wind energy flux driving the system is dissipated by wave breaking at small scales, the two seperated by the “equilibrium” scaling range. Cascades are now known to generically lead to multifractals; with special “universal” multifractals theoretically predicted. In this paper we use far red (0.95 μm) radiances at 1m resolution obtained from aircraft to test the multifractal behavior of the ocean surface and estimate the corresponding universal multifractal parameters of the radiance field.

Finite-fault site-specific acceleration time histories that include nonlinear soil response

To estimate the broadband strong ground motion one might expect at a given site we develop a method that includes heterogeneous slip on a finite-fault, full wave propagation with high frequencies, and site-specific material properties with nonlinear soil response. The faulting is simulated as a stochastic process with the spatial variation of the key parameters determined by probability distribution functions. The wave propagation from source to site is accounted for by using small earthquake recordings as empirical Green’s functions (EGF). This accounts for the regional effects of scattering, attenuation and structure while providing the basis for a broadband (0.5–10 Hz) time history. Because we are interested in sites where the ground motion is expected to be severe, we have included nonlinear wave propagation through the soil. The material properties of the soil column have been determined from laboratory tests, borehole logs and confirmed through seismological modeling of weak motion. We have computed 240 three-component acceleration time histories to represent the range of ground motion one might expect from a M 6.8 earthquake for a site that is located 10 km above the hanging wall of blind thrust (7.1 km closest distance). Based on the suite of time histories we computed a S.D. of 0.45 (natural log units) for the acceleration response spectra in the passband 0.5–10 Hz. The total S.D. (modeling plus parameterization) is 0.6 in natural log units. The mean acceleration response spectrum is near the median 10% in 50-year probabilistic seismic hazard analysis (PSHA) spectrum for the site; the 84% spectrum of the simulations is closer to the 5% in 50 years median spectrum.

UNIVERSAL MULTIFRACTAL CHARACTERIZATION AND SIMULATION OF SPEECH

In the 1970s it was found that; for low frequencies (<10 Hz), speech is scaling: it has no characteristic time scale. Now such scale invariance is associated with multiscaling statistics, and multifractal structures. Just as Gaussian noises frequently arise because they are generically produced by sums of many independent noise processes, scaling noises have an analogous universal behavior arising from nonlinear mixing of processes. We show that low frequency speech is consistent with these ideas, and use the measured parameters to produce stochastic speech simulations which are strikingly similar to real speech.