Today's 7.0 earthquake in Haiti caused massive damage, and the death toll is predicted to be enormous. Because Haiti is on an island in the Caribbean, there was some initial concern about the potential for a tsunami. In fact, the U.S. Geological Society (USGS) issued a tsunami alert soon after the quake, but that alert was quickly cancelled. Why? To answer that question, we need to take a look at the relationship between earthquake and tsunami. There is no doubt that an earthquake can cause a tsunami, but in order to do so the conditions must be right.
First of all, if the quake is to directly generate a tsunami (not as a secondary effect from part of a mountain sloughing off into the water and causing a big splash), the temblor must occur beneath the water. The sudden shift of a tectonic plate in relation to its neighboring plate can send an enormous amount of energy up through the water column above the sea floor. I say CAN because not all plate shifts create an energy release in the right direction to displace the water.
The recent quake in Haiti took place on the island, not in the water, and as soon as USGS realized that they cancelled the tsunami alert.
So how bad is it in Haiti today? There is some speculation that there will be a death toll in the hundreds of thousands. The primary cause of the death and injury is poor construction of buildings, using unreinforced adobe and masonry that shatters, crumbles and collapses during a quake. By comparison, wooden structures, being more flexible, usually withstand the shaking better than rigid masonry. In Haiti, the catastrophic collapse of structures included hospitals, schools, hotels, government buildings, businesses, and residences. Victims trapped by the collapsing rubble had no time to escape, and very little chance to survive.
According to historic records, the deadliest quake occurred in 1556 when a powerful earthquake hit Shansi, China, killing as estimated 830,000.