Energy flow in shallow depth based on vertical array records during recent strong earthquakes

Wave energy flow in shallow ground depth is calculated based on main shock records obtained at 30 vertical array sites throughout Japan during nine recent strong earthquakes (MJ=6.4–8.0) by assuming vertical propagation of SH waves. It is generally found that upward energy tends to decrease considerably as it goes up from the base (about 100 m deep) to the ground surface. Large energy is reflected at layer boundaries with clear impedance contrast and returns to deeper ground, so that only less than 10–30% of the upward energy at the base level arrives at the ground surface in most sites. Energy dissipation calculated from the upward and downward energies tends to increase with the increase of damping ratio of the ground back-calculated from the seismic records. It is also found that the upward energy at the base may roughly be estimated for engineering purposes using spherical energy radiation of the body wave despite strong effects of fault rupture/path mechanisms.

► Wave energy calculated, 30 vertical array sites, 9 strong earthquakes (MJ=6.4–8.0).
► Upward energy Eu decreases to <30% from base (GL.−100 m) to surface in most sites.
► “Wave energy is mostly unchanged in propagation” does not hold in shallow ground.
► Energy loss in the ground Eu<30–40% in most sites, the rest going back to the earth.
► Eu at base may be estimated from magnitude and focal distance despite large scatters.