The 1960 Valdivia earthquake in Chile was the strongest record earthquake, but not the deadliest.
Imagine this. You're a fisherman from the city of Huayin, located in the Ming dynasty, exactly 457 years ago. It's brutally cold out, but clear and calm. The ground shakes violently, there's a sound like a tall mountain collapsing and, just a bit inland from your spot on the Yellow River, a low grey cloud rises above the city.
Following the road back, you're greeted with dust-coated groups of people, many of them wounded, while a fountain of sand erupts from a just-opened fissure in the ground. Closer, you start to see that, well, there's absolutely nothing left of Huayin. It's a pile of rocks and people and dust. You can't see this yet through the still-rising, thickening cloud, but not a single building still stands.
According to modern estimates, the Shaanxi quake hit 8.1 on the Richter scale, which is high, but not the highest (that'd be 1960's Great Chilean Earthquake at 9.5, with 6,000 dead). However, magnitude is a poor measure for what's regarded as the deadliest natural disaster in human history. The best/most-terrible measure would be the death toll: 820,000 people over 96 different counties, including over half of the population of Huayin. Some estimates go as high as one million. A 500 mile region was just wiped off the face of the Earth in a single morning.
Buildings in Huayin and the surrounding area were primitive, often just elaborated-on cliff dwellings that wouldn't stand much of a chance in the face of a good shake. Landslides also poured into the towns, along with water from just-opened cracks in the Earth. In some places, the ground sunk as much as three meters, according to a USGS summary of the disaster.
We can, of course, weather earthquakes much better now, though even with modern technology and earthquake-proofing, we're still subject to disasters like Fukushima (which became the disaster it was in some part because of technology). That said, it's important to note that the next deadliest quake after Shaanxi happened only three years ago, in Haiti. That quake killed over 300,000 people, for generally the same reason that Shaanxi was so lethal: the buildings couldn't take it. Haiti had no building codes at the time of the quake. Which, I suppose, is mostly its own comment.
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