Title : Energy Demand in Surface Soils for Earthquake Engineering based on Vertical Array Strong Motion Records
In earthquake engineering, acceleration has been playing a major role while wave energy has rarely been considered as a demand in design. In order to understand earthquake damage in terms of energy, the demand of wave energy in surface soils is studied here assuming one-dimensional SH-wave propagation, by using a number of vertical array records during 9 strong earthquakes in Japan in the last two decades. A drastic decreasing trend of the energy demand has been found in general with decreasing ground depth to be almost 1/10 from the bottom to the surface. A simple formula has also been developed to evaluate upward energy at a given soil from the bedrock using the corresponding S-wave impedance ratio. Incident energies extrapolated at seismological bedrocks using the depth-dependent variations have been found roughly compatible with a well-known empirical formula despite that individual fault mechanisms are utterly neglected. Also indicated quite unexpectedly is that the softer the surface soil is, the more the energy demand tends to decrease. However, it may not be contradictory with a widely accepted perception that softer soil sites tend to suffer heavier damage as far as geotechnical influence is concerned, because induced soil strain tends to be larger due to decreasing soil stiffness despite the decreasing trend of energy demand there.