Chasing Earthquakes

It is a little ironic that I was on the phone with California when the 5.8 magnitude earthquake this past Tuesday hit. At first, I chalked the slight back-and-forth of my cubicle up to caffeine-induced hallucinations. Yet it did not take long for me to realize that yes, the walls and floors were indeed moving with the soothing motion of a laboratory shaker.

Now I’m no stranger to earthquakes. I was barely a day out of Christchurch, New Zealand, when the shallow 6.3 magnitude quake hit in February. Then, in March, I was calmly waiting for my mother at Narita International Airport when a sudden shaking rattled the windows and knocked loose thousands of dust bunnies from the chattering ceilings. (My mother and I did not discover until six hours later that the magnitude of the quake was a record-breaking 9.0.) All in all, I’ve had a pretty productive year chasing earthquakes.

So that got me thinking. Can one earthquake trigger another, and another, and another? Quakes are the result of the release of stored elastic strain energy between two tectonic plates. When the energy is released, one or both of the tectonic plates move. Hypothetically, then, such motion could place pressure on other points along the plates, increasing the amount of stored elastic strain energy along those faults and accelerating the possibility of another quake.

Before I could answer my own question, though, I had to learn the

science behind how earthquakes work. Most quakes occur along three main types of fault, which occur between two continental plate boundaries. Strike-slip faulting involves horizontal movement, while normal and thrust faulting involve vertical movement. Moreover, thrust faulting results in the strongest quakes – the most recent one being the 2011 Tōhoku earthquake and tsunami – but predictably, the length and width of the fault area and the shallowness of the epicenter¹ also affect the magnitude of quakes.

Why is thrust faulting more productive than normal or strike-slip? Earthquakes that occur along normal faults are usually the weakest because they involve a vertical downward motion, a dip of one plate against another. Of medium strength are strike-slip quakes: two plates sliding past one another. Thrust faulting, however, entails one plate rising above another. Unlike normal faulting where the mass of the rock pushes downward, in thrust faulting, the mass of the rock pushes upward, producing an “explosion” of energy that can travel outwards for miles and also move up gigatons of water, such as it did in the 2011 tsunami that plagued the eastern coast of Japan.

Given that background knowledge, I delved into researching whether a thrust fault earthquake could increase or decrease the likelihood of another quake somewhere else along the moving tectonic plate. Of the primary tectonic plates, there are seven, and both the February 2011 Christchurch quake and the March 2011 Tōhoku quake occurred along the western rim of the Pacific plate. Since both quakes occurred along thrust faults, maybe the movement of the Pacific plate during the Christchurch quake increased the pressure building between the Pacific plate and North American and Filipino plates, triggering the Tōhoku quake.

Turns out, it is possible. According to an article published in Nature News, earthquakes can weaken distant fault lines and make them more prone to “slip.”² What actually happens is that after large seismic events, stressed tectonic boundaries can undergo long-term changes in fault strength. For instance, after the 1992 Landers earthquake (magnitude: 7.3) and the 2004 Indian Ocean earthquake and tsunami (magnitude: 9.1), the San Andreas Fault in California weakened on an absolute scale (quantification of post-shock stress determined via hydraulic fracturing experiments).³

What does this mean in practical terms? To be honest, not much. Knowing how miniscule shifts affect the strength or weakness of distant faults does not contribute significantly to the ability of seismologists to predict when the next earthquake will strike. Instead, it is more of an academic venture than anything. Yet keeping a running tally of where pressure points along continental plates are building up or crumbling down is probably not entirely unhelpful. With an approximate list of the “wheres” of potential earthquakes in place, seismologists need only uncover a way to determine the “whens.”

Of course, that is a gross oversimplification of all the science that actually needs to be done before earthquake prediction becomes a reality. Until then, I will continue to blindly chase earthquakes. Wish me luck.

¹ The point directly above the center of the earthquake

References & Image Credits: Fault types by Wikigraphists via Wiki Commons.

²Taira, T., Silver, P., Niu, F., & Nadeau, R. (2009). Remote triggering of fault-strength changes on the San Andreas fault at Parkfield Nature, 461 (7264), 636-639 DOI: 10.1038/nature08395

³Yamashita, F. (2004). Estimation of Fault Strength: Reconstruction of Stress Before the 1995 Kobe Earthquake Science, 306 (5694), 261-263 DOI: 10.1126/science.1101771

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