Dong Youp Kwak

Strong Ground Motion Attributes of the 2010 Mw 8.8 Maule, Chile, Earthquake

The Mw 8.8 Maule, Chile, earthquake produced 31 usable strong motion recordings from currently accessible arrays over a rupture distance range of 30 to 700 km. Site conditions range from firm rock to soft soil but are most often competent soil (NEHRP Category C or C/D). Most of the data were recorded on analogue instruments, which was digitized and processed with low- and high-cut filters designed to maximize the usable frequency range of the signals. The stations closest to the fault plane do not exhibit evidence of ground motion polarization from rupture directivity. Response spectra of nearby recordings on firm ground and soft soil indicate pronounced site effects, including several cases of resonance at site periods. A prior GMPE for interface subduction events captures well the distance scaling and dispersion of the data, but under-predicts the overall ground motion level, perhaps due to too-weak magnitude scaling.

Characterization of Seismic Levee Fragility Using Field Performance Data

We characterize the seismic fragility of levees along the Shinano River system in Japan using field performance data from two M 6.6 shallow crustal earthquakes. Levee damage is quantified based on crack depth, crack width, and crest subsidence for 3,318 levee segments each 50 m long. Variables considered for possible correlation to damage include peak ground velocity (PGV), geomorphology, groundwater elevation, and levee geometry. Seismic levee fragility is expressed as the probability of exceeding a damage level conditioned on PGV alone and PGV in combination with other predictive variables. The probability of damage (at any level) monotonically increases from effectively zero for PGV < 14 cm/s to approximately 0.5 for PGV ≈ 80 cm/s. Of the additional parameters considered, groundwater elevation relative to the levee base most significantly affects fragility functions, increasing and decreasing failure probabilities (relative to the PGV-only function) for shallow and deep groundwater conditions, respectively.

Prediction equations for estimating shear-wave velocity from combined geotechnical and geomorphic indexes based on Japanese data set

Author(s): Kwak, DY; Brandenberg, SJ; Mikami, A; Stewart, JP | Abstract:

Seismic Levee System Fragility Considering Spatial Correlation of Demands and Component Fragilities

Seismic levee performance is most readily computed for short segments having consistent geometry, soil conditions, and seismic demands. Spatial variations of seismic demands and of segment capacities significantly influence system risk, which is critical for flood protection because any segment failure within the system can cause inundation. We present a methodology to compute the probability of seismic levee system failure conditional on individual segment fragility and spatial correlations of demands and of capacities. Seismic demands are estimated from ground motion prediction equations; their correlation is available in the literature. Capacities and their correlation are derived from levee damage observations from a levee system in Japan shaken by two earthquakes. We find seismic capacities to exhibit positive correlations over shorter distances than for demands. System fragility is computed using Monte Carlo simulations where segment demand and capacity realizations are generated to account for spatial correlations. We find that the probability of system failure is lower than would be obtained under an assumption of no correlation and that damage probability increases as the number of components in the system increases.

Taiwan-Specific Model for VS30 Prediction Considering Between-Proxy Correlations

Ergodic site response models are generally conditional on the time-averaged shear wave velocity in the upper 30 m (VS30). Ground motion databases contain many recordings from Taiwan, and because of site characterization efforts, 56% of recording sites have VS30 derived from measurements. We develop proxy-based VS30 prediction models, one application of which is for the remaining 44% of Taiwan sites. Our approach, which can be suitable for other regions, differs from previous studies in which proxies are based on detailed geologic categories and possible within-category topographic gradient effects. Instead, we use three broad, age-based geologic categories, and for the youngest category of Holocene and Quaternary undivided sediments, we propose models conditioned on gradient and elevation. We also adapt a geomorphic terrain-based method, thus providing two VS30-prediction models. We describe a model weighting scheme that combines the models in consideration of their relative dispersions and correlation, producing a weighted mean and standard deviation natural-log VS30. Included as an electronic supplement is a profile database file and a site database with site parameters for Taiwan ground motion stations.