Trampoline model of vertical earthquake ground motion
DOI: 10.1063/1.3047658
Seismic sensors at the surface of a borehole near the epicenter of a magnitude-6.9 earthquake this year in Japan revealed unpredicted asymmetry in the vertical wave amplitudes at the soil surface: The largest upward acceleration was more than twice that of the largest downward acceleration. The data also showed that the soil surface layer was tossed upward at nearly four times the gravitational acceleration—more than twice the peak horizontal acceleration. These findings run contrary to current structural engineering models, which presume that seismic waves from earthquakes shake the ground horizontally more than vertically. Shin Aoi and colleagues at Japan’s National Research Institute for Earth Science and Disaster Prevention propose what they call a trampoline model to explain the observed nonlinear bouncing behavior. In their model, the soil undergoes compression in the upward direction and behaves as a rigid mass with no intrinsic limit on acceleration, much like an acrobat rebounding from a trampoline (figures 1 and 3). In the downward direction, though, dilatational strains break up the soil and the loose particles fall freely at or below gravitational acceleration (figures 2 and 4). The observed seismographic data was simulated by combining the theoretical waveform from the trampoline model with selected borehole data that resembled elastic deformation of a deformable mass. The researchers say that other events need to be analyzed to learn how material conditions affect vertical ground response during high-magnitude earthquakes. (
