Weiwei Zhan

Weiwei Zhan

Assistant Professor at University of Central Florida

Geospatial Surrogate Models for Site Response Complexity Assessment

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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.


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Site Response Temporal Change Evaluation

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.

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.

Geomorphological Precursors for Landslide Early Detection

Early detection of rockslides at high-elevation and well-vegetated slopes remains challenging. This study used satellite and unmanned aerial vehicle (UAV) optical remote-sensing (ORS) images to track evidence of slope deformation and examine potential geomorphological precursors of five large rockslides in China. All the surveyed landslides had cracks or scarps and rockfalls within the landslide source area before the onset of rapid sliding. These precursors can be identified in ORS images taken several years or decades before the rapid slope failure, which provides sufficient time for the landslide early detection in practice. Local topography affects the spatial locations of cracks or scarps. Rockfalls within the landslide source area tend to locate at “key blocks” where slope mass provides forces resisting sliding. The rockfall area ratio, defined as the accumulated area of rockfalls over the landslide source area, ranged from 0.33 to 0.92 before rapid slope failure. The landslides developed on anti-dip and igneous rock slopes show a more significant rise of rockfall area ratio before the slope failure than the landslides on dip slopes. Given the broad availability of ORS data, this study could shed light on the ORS- based landslide early detection and landslide kinematics study.