Man-made earthquakes

Many places in the world get earthquakes. The very low magnitude ones are surprisingly common. But the high magnitude ones are uncommon. But when the number of earthquakes increases dramatically, it’s time to get worried about what’s going on.

The New Yorker has an article on the number of earthquakes increasing in Oklahoma. In 2008 there were one or two magnitude 3.0+ earthquakes each year. Now, it’s one or two magnitude 3.0+ earthquakes each day. What’s changed? What could be causing this? The answer is fracking. Pumping a slurry of water, sand, and chemicals into the earth at high pressure helps release natural gas.

William Ellsworth, a research geologist at the United States Geological Survey, told me, “We can say with virtual certainty that the increased seismicity in Oklahoma has to do with recent changes in the way that oil and gas are being produced.” Many of the larger earthquakes are caused by disposal wells, where the billions of barrels of brackish water brought up by drilling for oil and gas are pumped back into the ground. (Hydraulic fracturing, or fracking—in which chemically treated water is injected into the earth to fracture rocks in order to access oil and gas reserves—causes smaller earthquakes, almost always less than 3.0.) Disposal wells trigger earthquakes when they are dug too deep, near or into basement rock, or when the wells impinge on a fault line. Ellsworth said, “Scientifically, it’s really quite clear.”

Humanity is causing more and stronger earthquakes. What are we going to do about it?

More water than in the oceans in the Earth’s mantle

Where did the oceans come from? One hypothesis is that our ocean water came from comets that collided with the early Earth. Another is that the water came from underground and either seeped to the surface, or came out as water vapor in volcanoes. The second is looking more likely as scientists have found that rocks 700 km below the surface, in the Earth’s mantle, has water bound up in it.

Using seismic waves, scientists have found that ringwoodite has water in it, essentially soggy rock. This is happening around 700 km down, around the transition between the upper and lower mantles.

First forecast earthquake

On 5 September 2013 a magnitude 7.9 earthquake rocked Costa Rica. The unusual thing is that scientists were able to forecast this one. They knew that a 7.7 to 7.8 quake would occur in this area in 2000 ± 20 years. It turns out that large (> 7 magnitude) earthquakes happen on this peninsula about every 50 years. The subduction zone also is on land instead of in the ocean, so it’s much easier to study.

They started studying in 1990 using GPS stations to measure the Earth’s movement. This let geologists identify two places where the plates were locked and building up stress. Knowing this, they were able to make civil changes to improve building codes and warn the populace on what to do when a quake struck.

But forecasting is different than predicting. Predicting the actual date of an earthquake is pretty much impossible.