Sedimentary basin effects in ground motions from empirical models and simulation platforms

Chukwuebuka Nweke


Date & Time
Online-only seminar via Microsoft Teams

Sedimentary basins are depressions in the earth's surface that consist of alluvial deposits and sedimentary rocks, which are geologically younger and have slower seismic wave velocities than the underlying basement rock, and also decrease in thickness towards their margins. An important characteristic of the seismic response of these basins is that seismic waves are often trapped and reflected due to their interactions with the deep structure of sediments and the associated variations in material properties, per Snell's Law, resulting in the generation of propagating surface waves in addition to the accompanying propagation of 'body' waves. Such basin effects are known from first-principles to depend on a complex series of factors related to three-dimensional basin geometry (shape, depth, etc.) and aspects of the source-to-site path (i.e., the manner in which incident waves approach and enter the basin). These complexities are implicitly included in three-dimensional ground motion simulations, which can predict strong basin effects, and are incorporated in a relatively simple manner in sediment depth-dependent components of semi-empirical ground motion models. Not surprisingly these two modeling frameworks provide different outcomes.

We investigate the benefits of regionalizing basin response in ergodic ground motion models. Using southern California data, we consider: (1) how should basin and non-basin site conditions be classified?; (2) how does mean site response and its variability differ for basins compared to non-basin locations?; (3) what are the variations in basin response between different basin structures, and how can this be parameterized for predicting ground motion intensity measures?; and (4) what are the similarities and differences related to the prior three questions for simulated and empirical estimates of ground motion?

We present results of research targeted towards answering these questions using a combination of earthquake ground motion data analysis and targeted, physics-based simulations. The results show distinct levels of site response in sedimentary structures having different geometries and geologic origins.

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