. >> going us right now on the science of earthquake is geophysicist justin reubenstein. liquefaction it was a major factor in 1989. >> liquefaction is a situation where you looking at soft soils. what happens is it's subjected to strong shaking it loses some of the cohesive strength and begins to act more like a liquid as opposed to a solid. what that means for construction purposes, in extreme cases this structures can fall over. the marina district in particular, most of the damage was related to liquefaction in the '89 earthquake. >> are there other problems in the bay area? >> pretty much anywhere along the bay front, is built on landfill. when you got reclaimed land you are building on soft soil. so the real risk in those areas. >> and in japan, horrible quake and tsunami there, what did we learn from that defect? >> certainly japanese earthquake was a great tragedy with hundreds of thousands of lives lost but in some respects it was a greater success. it really showed that proper preparation for earthquakes can . of lives. most of the damage, most of deaths really