Physics · Science

Seven Years Shaking

Two years and about two weeks ago, I woke up to the feeling that a rather strong person was shaking the shit out of the bed. It was 3:20 AM, and I had just fallen asleep. There was no stranger; rather, I was in Berkeley, California about 25 miles from the epicenter of the largest earthquake to hit the San Francisco Bay Area in my lifetime (1989 Loma Prieta — the World Series earthquake — was two days before my birth).

The 2014 earthquake was measured at 6.0, which was high enough to kill a person, injure 200 more and cause approximately one billion dollars of damage. It shattered thousands of bottles of Napa’s famous wine, and millions of people were awoken to an angry stranger violently shaking their beds too.

Recently, people felt this again; although, not in California, where people expect it, but in Oklahoma where amazingly, people have come to expect this in that state as well. Oklahoma has been country’s most seismically active state over the past three years.

oklahoma_earthquakes

As of September 9th, the 2016 count is up to 489, which puts the state on pace for 707 earthquakes of at least 3.0 this year. The largest of these earthquake was a few days ago on September 3rd. It was measured at 5.8, which is the largest earthquake in state history.


Before we think about why this might be, let’s take a step back and discuss a couple of oft-quoted properties for earthquakes.

The moment magnitude is a measure of the energy released by the earthquake. It is a logarithmic scale, which means that for every increase in magnitude by one, the size of the earthquake is multiplicatively larger. You can calculate the relative energy released in two separate earthquakes: ten raised to 1.5 times the magnitude difference. Using this prescription, the 2014 California earthquake was about twice as energetic as the recent Oklahoma earthquake.

Let me put this into context. In California, we expect large earthquakes because the Pacific Plate is sliding along the North American Plate. Quite literally, the chunk of the Earth’s crust that separates the Pacific Ocean from the mantle is sliding against the chuck that separates North America from the mantle. These things are incredibly large: thousands of kilometers wide and tens of kilometers thick. It is not surprising when this interaction generates an earthquake. However, in Oklahoma, there are no plate boundaries. Oklahoma is in the middle of the North American Plate. Large earthquakes in Oklahoma are unexpected in a geological sense.

The depth of the earthquake, which is the distance underground where the Earth’s crust ruptured, gives further indication of the potential destruction. Shallow earthquakes are more damaging for any given magnitude (remember that magnitude basically tells us the size or amount of energy released). This is because the ground is able to absorb the energy like a shock absorber on a car: the larger the depth, the less shaking you get at the surface — the less destruction. The physics behind this is complicated, but it largely depends on the local soil type. Softer substrates tend to allow the energy to pass through very easily. The plains in the middle of the country are very good at transmitting an earthquake’s energy with minimal dissipation. This explains why people in seven different states felt the recent earthquake even though it originated in Oklahoma.  This also explains why homes were spared atop the hills of San Francisco while the flat marshy areas near the shoreline of the bay took the brunt of the infamous 1906 earthquake (the ensuing fire ended up destroying homes regardless of the content of the soil below their foundations).

The depth of the 2014 California earthquake was 11.3 km, and the depth of the recent Oklahoma earthquake was just 4.5 km. Although both of these are relatively shallow, the 4.5 km depth of the Oklahoma earthquake is notable. I’ll explain why soon.


What is going on in Oklahoma? Why are there large earthquakes in a state with no historical seismic activity? Oklahoma sits in the middle of the North American plate over 1000 miles from the nearest major fault system. There are small faults and cracks even in the middle of a plate, but in general, there is not enough movement to set off an earthquake.

It appears that the culprit may be the deep storage wells that oil and gas companies are using to dispose of the large amounts of wastewater generated by the hydraulic fracturing process (fracking). Keep in mind that any suggestion that fracking is anything other than a boon for the economy is typically hit hard by the oil and gas industries. Fracking is a large component of our renewed “energy independence” and coupled with newly discovered oil fields in North Dakota a boom has occurred in that state. In fact, you can thank fracking for the cheap gas that you’ve been enjoying lately. US produced petroleum caught Saudi Arabia’s attention, and the Saudis have been flooding the market in hopes to choke out the US producers with cheaper prices. Perhaps the most telling evidence that these wells play a role in Oklahoma’s new seismic reality is the fact that oil and gas companies have largely been passive as state regulators have placed tighter restrictions on the location of these wells. Geologists have found statistically significant links between these wells in several states. Here is a list of recent publications pertaining to induced seismic activity around the country.

Oklahoma’s oil and gas reserves are not newly discovered, but the advances in hydraulic fracturing techniques have opened the door to a lot of money to be generated in the state. The recent wave of economic activity relating to this process reached far and wide. For example, my family owns a few hundred acres in Northern Michigan; people approached us and many other land owners looking to purchase our mineral rights in order to survey for possible natural gas below our farm fields. That was in 2009, and if you look at the above figure again, you’ll notice something. Fracking took off in 2009 in the state of Oklahoma and so did the earthquakes.

About those deep storage wells: the wastewater is typically stored at least a couple miles below ground (recall the depth of the recent earthquake — 4.5 km or 2.8 miles). Some of you may remember the experiment that shows water is an incompressible fluid: the one where you connect two syringes containing water with a tube and try to press both plungers as once. The take home point is that you can’t; even under pressure, the density of liquid water is basically constant. You can’t make the molecules get closer together than they already are. Now imagine forcing water miles underground. At the bottom of these wells, the water finds its way into cracks and crevasses in the crust. As more water comes in, the strain builds up. These small cracks and faults in the crust typically don’t cause earthquakes because there isn’t bulk motion as there is at a plate boundary, but the wastewater pries open the cracks and serves to lubricate rearrangement of the crust.

damage1Credit: Brian Sherrod / U.S. Geological Surveydamage2Credit: David Bitton / Stillwater Press

This rearrangement is exactly what has left Oklahoma shaking for the past seven years.

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