Many of us – perhaps most – living in the far Western states know what it is to feel the power of seismic waves passing through the Earth. The sharp jolts are unmistakable, and the sense of chaos is terrifying even to those of us who like a thrill.
A minor earthquake I felt in Berkeley, Calif., still stands out in my mind some 30 years later, and not because it’s one of my favorite memories. My thoughts turned to it as soon as I heard the news from the Eureka, Calif., area, which experienced an offshore earthquake of 6.5 on the Richter scale recently. Then again, the memory hit me when I heard about Haiti, slammed by a much more serious quake of about 7.0 with major aftershocks. My heart goes out to the Haitians, both harder hit and more poorly prepared than we are for the hell that earthquakes unleash.
Prior to the modern era, veterans of earthquakes had no way of understanding what caused them, nor even (really) where they were coming from.
It didn’t help that Aristotle had launched the West down the wrong road about earthquakes by teaching that they were caused by air rushing out of caves and cavities in the Earth. I’m not mocking the great philosopher – in ancient times, there was no particularly good way to say where quakes came from or what caused them.
Aristotle was an authority in a world run by authorities. His writings mattered, both in the ancient world and then again in the Middle Ages when we rediscovered the classics. And by linking earthquakes to the weather in the Western mind, we were set back. For centuries people looked for patterns of quakes and humid and close “earthquake weather,” a project without any productive result because earthquakes and sultry weather are simply not connected.
But, even in modern times, we didn’t do so very much better for a while. Twentieth century geologists struggled for generations to really understand the basics of why earthquakes occur where they do and how energy is stored in rocks to power the seismic waves. We knew that the epicenter of a quake was the point on the ground surface above the earthquake focus within the solid Earth. And we developed the Richter scale (and other scales, too) to measure quakes – always a necessary “handle” for physical science.
But the answer about what really caused quakes came over many years when a variety of evidence showed us the surface rocks of the Earth are divided into sectors, called “plates,” that they are moving with respect to each other. The movement of these plates has several results. One of them is earthquakes.
Many earthquakes are concentrated where plates come together. If one plate is moving under the other (diving down beneath it, you could say), some of the earthquakes may occur deep in the Earth along the top of the descending plate. That’s good for us people, since we live at the surface of the planet, and the farther we are from the focus of the earthquake, the better. But note there can still be shallow quakes even in this setting, too, from shallow parts of the plates.
Western South America has many, many earthquakes, but some of them are so very deep they are hardly a problem because the oceanic plate there is diving under the one on which South America is riding. But in California, the plates are moving past one another, without the “diving down” motion. This means that the focus of an earthquake in California is likely to be shallow – and that’s a major reason the really Big Ones are uniformly terrible. Haiti, too, had the misfortune to experience a shallow quake; that fact helps to explain the devastation you saw in the news.
As a scientific theory, plate tectonics is still a work in progress (one drawback: we can’t predict when, exactly, the Big Ones will come, in California or Haiti or South America), but progress it is. Aristotle would be proud, I think, of the work we have done – but for humanity’s sake, we need to do better and soon.