Effects of multifocus earthquakes on seismic motion

Earthquakes may cause destruction disproportionate to ground conditions and distance from the epicenter. This paper suggests that disproportionate destruction may be caused by the long duration of the earthquakes. However, to date the mechanism leading to earthquakes of long duration was unknown. On the basis of a detailed analysis of a large number of macroseismic data from historical earthquakes in Armenia, we proposed during 1965–1992 that disproportionate destruction occurred as a result of the spatially separated multifocus character of these earthquakes. A new analysis of instrumental, geodynamic, and engineering-geological data on some typical highly destructive earthquakes that occurred in recent decades shows that the long duration of earthquakes is indeed related to their multifocus character. The destructive effect of the latter occurs irrespective of ground conditions and also at great distances from the epicenters. Seismic movements of long duration have additional impact on constructions, and on seismic instrumnents, as well. It is demonstrated which type of constructions are most vulnerable to long-duration earthquakes and which, on the contrary, are more resistant. We present some recommendations for improvement of the seismic construction code.