Weiwei Zhan

Weiwei Zhan

Assistant Professor at University of Central Florida

Site Response Temporal Change Evaluation

less than 1 minute read

To examine if the large interevent variability of seismic site responses at liquefiable sites is reducible, I investigated the temporal variability of seismic site responses at a liquefiable site using the sliding window spectral ratio method. I used stacked images of surface-to-borehole spectral ratios to track changes in seismic site response characteristics at temporal scales varying from seconds to decades. I observed that resonance frequencies dropped by ~70% during the mainshock of a liquefaction-triggering strong earthquake (indicating significant soil softening behaviors) and had an ephemeral increase by ~40% due to the strong soil dilatant behavior. In addition, I observed that the resonance frequency recovered after a strong earthquake at two phases with different recovery speeds and fluctuated along the seasonal changes of hydrologic conditions in the shallow subsurface.


You May Also Enjoy

Evaluating the Accelerogram-based Liquefaction Detection Method Using Continuous Earthquake Recordings

I further evaluate the performance of the new method using continuous earthquake recordings at four seismic stations. The results suggest the method performs better at the soft-soil sites than at the stiff-soil sites and tends to make false negative predictions for very strong earthquakes. This study highlights the applicability of the new accelerogram-based method to regions with continuous earthquake monitoring.

Site Response Nonlinearity Assessment

Site response nonlinearity describes the change of site responses from weak to strong events, shedding light on nonlinear soil dynamic behaviors. I studied the site response nonlinearity at four liquefiable vertical array sites (with liquefaction surface manifestation during at least one earthquake; the site has at least two ground motion sensors, one at the surface and one at the depth). I processed thousands of ground motion waveforms and computed surface-to-borehole spectral ratios (frequency-wise site amplification curves). I used shape parameters to quantify site response nonlinearity and developed site-specific regression models to evaluate the site vulnerability to nonlinear site responses. I found that the liquefaction could enhance the amplification of low-frequency and very-high-frequency ground motions, and these liquefaction effects vary from site to site.

Geospatial Surrogate Models for Site Response Complexity Assessment

The term site response complexity describes the discrepancy between observed site response (i.e., empirical transfer function ETF) and theoretical site response (i.e., theoretical transfer function TTF) predicted using the widely-used one-dimensional site response methods (i.e., SH1D). I derived TTF for ~700 vertical array sites using SH1D simulations and compared them with ETF derived from a large ground motion dataset. I observed noticeable trends between site response complexity and topography: Good-matching sites are mostly located in flat sedimentary basins (where the SH1D assumptions are most valid), and poor-matching sites are often located near mountain/basin edges, and high-interevent-variability sites are located within mountainous areas.