I’ve experienced several earthquakes during my years in Costa Rica. I think the strongest was the Cinchona quake. At the time it hit, I was having lunch at a restaurant. The quake came on slowly, laterally, and went on for some time. I remember looking out the trembling window and seeing parked cars bouncing back and forth on their shocks.
Even though it killed dozens of people and displaced an entire town, to me it felt like a curiosity. That quake was 6.1 on the Richter scale. I said to myself, “Not so bad.”
So compared to that experience, when another earthquake hit Costa Rica on Friday night, I thought for sure it was a 6, or at least a 5.5. It only lasted six seconds, but it felt like a truck hit the building. Everything jumped. There was a low roar. It scared the bejesus out of me.
Come to find out, it was only a measly 4.4. So why so scary?
Everyone likes to talk about the Richter scale, partly because journalists love numbers, so that’s how quakes are reported. But how an earthquake really feels depends on much more than a number.
There are all different kinds of faults, and each produces different motions. A strike-slip fault will produce side-to-side motion, which is devastating for buildings. A thrust fault produces up-and-down motion, which causes wicked tsunamis. (Incidentally, Haiti’s earthquake was caused by the former, Chile’s the latter.)
Also important is the distance you are from the source of the earthquake, both its depth and the map location of the epicenter. And there’s the rub. The epicenter of the 6.1 Cinchona quake from last year was 30km away. However, my dinky 4.4 from Friday night was basically right underneath me, and only about 7km deep.
That changed everything. And it helps explain why the Chile quake was less destructive than the Haiti quake. The Chile quake was 21 miles deep and 200 miles from Santiago. But put it 16 miles outside of town and only 8 miles underground, like the Haiti quake, and I don’t care what kind of building codes you’ve got, you’re gonna have a catastrophe.