Ground-Motion and Engineering Effects in the Tohoku Earthquake

The strong ground motion data recorded in the Tohoku earthquake is a global treasure for the engineering and seismology communities, unprecedented in its completeness and data quality. The motions are the result of a complex and heterogeneous source process, and modified by highly variable site response.  The energy at higher frequencies, of primary engineering interest, is radiated most prominently from magnitude ~8-sized patches, with high stress drop on deeper parts of the fault, that are not co-located with the sources of the long-period teleseismic signals, tsunami or geodetic movements.  Ground motion amplitudes, then, while somewhat different from prior extrapolations based on smaller magnitude events, are mostly in an amplitude range that had been observed before.  As expected, the shaking lasts longer than shaking in smaller events.  The rock peak acceleration at the surface is no greater than 0.34g at stations where the near-surface geology causes the smallest site amplification. Waveforms of accelerations over 1 g generally show evidence of strong site effects, consistent also with being interspersed, geographically, among observations of smaller amplitudes.  Liquefaction was widespread, due to the long duration of shaking. Fortunately, structural damage caused by shaking is mostly found in older structures.  Building damage potential can be calculated using the nonlinear response model derived from the damage statistics during the Hyogo-ken Nanbu earthquake of 1995.  Even where peak accelerations are high because of high stress drop in the western side of the source and the large site factors, there is not generally enough energy at periods from 0.5-3 sec to cause most buildings to be heavily damaged or collapsed.