SIMPLIFIED METHOD IN EVALUATING LIQUEFACTION OCCURRENCE AGAINST HUGE OCEAN TRENCH EARTHQUAKE

摘要

Effectiveness of the existing simplified method for evaluating liquefaction potential under ocean trench long duration earthquakes is investigated. Effective stress analyses are made at more than 200 sites. Accuracy of the existing simplified method is evaluated by comparing the onset of liquefaction by both methods. It is found that existing method is applicable to a near field or inland earthquake but not to an ocean trench earthquake and that it overestimate liquefaction potential resulting in dangerous design although PGA is smaller in ocean trench earthquake. It comes from the difference of effective number of loading cycles; that for ocean trench earthquake is about 10 times larger than that considered in the existing method. Then a correction factor is proposed for liquefaction strength; liquefaction strength is set about a half of that used in the existing method. This method works so that both dangerous ratio (ratio of the cases where onset of liquefaction is identified by effective stress analysis but is not by existing simplified procedure) and accuracy ratio (ratio where both effective stress and simplified method show same result) keep nearly the same as for the case of the inland earthquake.