The Slapdown Phase in High Acceleration Records of Large Earthquakes

This research focuses on the acceleration records of the station IWTH25, one of which recorded 4g during the 2008 Iwate-Miyagi Nairiku earthquake and high vertical acceleration records from other earthquakes. The vertical acceleration record has asymmetric amplification between the upward and downward directions. We explain this asymmetric amplification for this and other earthquakes with large vertical accelerations, in terms of a slapdown phase, which is typically seen in near-field recordings of nuclear explosions. We assume that there is a near-surface soil layer that separats from a sublayer and is flung upwards by large vertical accelerations. When this layer returns striking the separation surface, the high acceleration slapdown phase is produced.