Abstract
Shear-waves are the most energetic body-waves radiated from an earthquake, and are responsible for the destruction of engineered structures. In both short-term emergency response and long-term risk forecasting of disaster-resilient built environment, it is critical to predict spatially accurate distribution of shear-wave amplitudes. Although decades’ old theory proposes a deterministic, highly anisotropic, four-lobed shear-wave radiation pattern, from lack of convincing evidence, most empirical ground-shaking prediction models settled for an oversimplified stochastic radiation pattern that is isotropic on average. Today, using the large datasets of uniformly processed seismograms from several strike, normal, reverse, and oblique-slip earthquakes across the globe, compiled specifically for engineering applications, we could reveal, quantify, and calibrate the frequency-, distance-, and style-of-faulting dependent transition of shear-wave radiation between a stochastic-isotropic and a deterministic-anisotropic phenomenon. Consequent recalibration of empirical ground-shaking models dramatically improved their predictions: with isodistant anisotropic variations of ±40%, and 8% reduction in uncertainty. The outcomes presented here can potentially trigger a reappraisal of several practical issues in engineering seismology, particularly in seismic ground-shaking studies and seismic hazard and risk assessment.
Highlights
The theoretical formulation[6] of S-wave radiation patterns is anisotropic yet deterministic, and depends on the rupture geometry and focal mechanism
Empirically[7,8,9,10] though, S-wave radiation patterns transition between deterministic and stochastic depending on the heterogeneity of propagation medium, which makes it difficult to calibrate and include in prediction models
Numerical simulations[17] of wave propagation in randomized three-dimensional crustal velocity models have been able to reproduce this transitional range, but the results are sensitive to the parametric description of crustal scattering properties
Summary
Shear-waves are the most energetic body-waves radiated from an earthquake, and are responsible for the destruction of engineered structures. We tackle these limitations using the recently published large datasets compiled of several thousand seismograms, from a variety of earthquake focal-mechanisms, occurring across the globe, and uniformly processed for engineering applications i.e., code[23,24] based design of earthquake resistant structures Even with such large datasets derived from spatially dense strong-motion networks in Japan[25] and southern California, USA26, the imprint of S-wave radiation pattern remained untraceable and unquantifiable within the mixed-effects regression based empirical ground-shaking models[11]. Radiation pattern is a secondary physical effect masked by other dominant physical processes, including: geometric spreading and anelastic attenuation with distance, scaling with event magnitudes and event-to-event variability of rupture dynamics, and the strong influence of local soil conditions at a recording surface site These difficulties explain why the well-known four-lobed S-wave radiation pattern has never been taken into account in the development of PSA predicting Ground-Motion Prediction Equations[11]. Despite using data from scores of earthquakes across Japan and southern California, where crustal structures are known to be highly complex and heterogeneous, the theoretical idealizations appear to work surprisingly well – at least for low S-wave frequencies
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