Abstract
On October 1, 1987 Southern California experienced the first earthquake for more than sixteen years that had sufficient strength to cause casualties and significant structural damage. The more important results of the event, and of the major aftershock which followed three days later, are summarised with particular reference to the successes in the mitigation of seismic effect which have been achieved both by the application of more stringent code requirements, in the case of new buildings, and by strengthening procedures in the case of existing ones.
Highlights
THE EARTHQUAKE The Whittier Narrows event is one of the more moderate sized earthquakes which have occurred in Southern California this century
The California Strong Motion Instrument project and the U.S Geological Survey program have succeeded in the installation and operation of instrumentation over the last fifty years to the extent that more than five hundred stations provided records of Whittier Narrows[5] earthquake response
Most of the evidence supports the contention that the earthquake mitigation efforts of the last fifty years, as reflected in the building codes, have provided progressivelv improved protection
Summary
Of the unstrengthened and partially strengthened URM buildings 31% were significantly structurally damaged, whereas only 20% of the strengthened URM buildings suffered to this degree. 2% of those buildings which had been fully strengthened had to be vacated whereas of the total group of unstrengthened and partially strengthened buildings examined, 6% had to be vacated. Several characteristic failure patterns were confirmed in unstrengthened URM buildings and were identified in strengthened ones. One problem which clearly persists is that of weak parapets in position not covered by the earlier parapet ordinance. Such parapets on walls adjoining other buildings are liable to collapse and to thereby cause damage to other property (Fig. 2 ).
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