Evidence of significant forward rupture directivity aggravated by soil response in an Mw6 earthquake, and the effect on monuments

Summary While strong directivity effects have been mostly recognized in Mw > 6.5 earthquakes, the paper investigates the case of a strong such effect in a relatively small-magnitude event on 3 February 2014 in the island of Cephalonia, Greece. The second of two events (both of Mw ≈ 6) produced a pernicious accelerogram in the region's main town, Lixouri. The paper provides evidence from geology, interferometry, and seismology to convince that the motion was the result of constructive interference in front of the direction of rupture of the obliquely-strike-slip fault. The nature of the record is explored to demonstrate that its frequency content, its high velocity pulse, and its strong fault-normal (FN) favorable polarity are associated with directivity. Moreover, the broad spectral acceleration peak (of 1.7 g) of the FN motion, centered at a period (T ≈ 1.4 s) which almost coincides with the period of the velocity pulse, is shown to have also been affected by soil amplification, in quantitative agreement with Bray et al. (2009). Such a directivity-and-soil-affected motion explains much of the profound damage to monuments, slopes, and harbor quaywalls. In particular, toppling (as well as excessive rotation and sliding) of nearly-all the tombstones in Lixouri cemetery are shown to correlate well with characteristics of the FN component of motion. By contrast, the excellent performance of the building stock — despite the destructive shaking that prevailed — is persuasively attributed to conservatively-robust construction practices of the past and the high base shear coefficient of the strict latest (2000) seismic code. Copyright © 2017 John Wiley & Sons, Ltd.