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Earthquake triggering, and why we don't know where the next big one will strike


As I came through airport security in Connecticut, upon presentation of my California driver's license, the TSA officer asked me, "Aren't you folks worried about how that big Japan quake is going to hit you next?" I was glad to be able to tell him that we're not any more worried than we were before, and that a writer had just made that up. I didn't ask him where he got that idea, but on my mind already was Simon Winchester's column in Newsweek magazine on March 13. The article was wrong, and that fact has gotten a lot of traction in the blogosphere—and in real newspapers, if a distinction still exists.

The Newsweek article argues that the relatively small but very damaging Christchurch, New Zealand earthquake of February 22, 2011, the very large Chilean earthquake of Feburary 27, 2010 and the recent great earthquake in Japan constitute "triggering events" around the Pacific Plate, stating, "That leaves just one corner unaffected—the northeast. And the fault line in the northeast of the Pacific Plate is the San Andreas Fault, underpinning the city of San Francisco." After this geographical error, Mr. Winchester states that the stresses around the San Andreas have built to "barely tolerable levels" and that a triggering event is required to set off a great quake.

Mr. Winchester, a well-known author of several popular science books on geological topics, is much better versed in the history of geological events, and much of the science around them, than most people. However, his piece in Newsweek contains wrong information, baseless predictions and an ominous tone that is more fear-mongering than warning. We had a bit of correspondence about my objections, which wonks can read on my Facebook page. In that correspondence and a follow-up column in the Daily Beast, Mr. Winchester defends his earthquake prediction and implies that earthquake scientists are either hiding something or just plain stupid for not sharing his views.

I'm not saying Mr. Winchester is wrong about the great risk to San Francisco from the San Andreas Fault, on the contrary; I fully agree. And I appreciate the intention to grab the moment after the tragedy in Japan to point out the risks while public and media are showing so much interest in earthquakes. However, some of Mr. Winchester's "facts" are wrong, and logic is deeply flawed. I'm grateful for this chance to set the record straight, even though far fewer people will read this posting than the Newsweek column, which has already done its damage and faded into the fog of yesterday's Internet.

These are the points I’d like to make to the author and publishers of that piece:

  • Just because something appears to make sense, that does not mean it is true. Often times, more than one story appears to make sense, given the available information.
  • Just because you have not read about an idea already, that does not mean you are the first to have that idea, especially if you don't read much.
  • Natural systems are incredibly complex, and the fact that we have not yet understood and predicted every detail does not mean the scientific community is negligent or hiding something.

Do earthquakes trigger other earthquakes?

The risk of a big earthquake occurring close to a large shock, and soon after it, is well established1,2. Locally, an earthquake changes the stress on surrounding faults, shakes them up and causes aftershocks. As the seismic waves travel across the planet, they have subtle effects on faults near and far—which can directly trigger more earthquakes, close in time but not in space to the original earthquake.

For example, direct triggering was documented during a 2002 earthquake in Alaska3. A few minutes after the quake, when the seismic waves reached Yellowstone, a flurry of small earthquakes occurred there. We can say with some certainty that the Denali quake triggered the Yellowstone quakes because we understand the timing as well as the mechanism of energy transfer. So geophysicists might not be able to predict the exact location and timing of each individual small quake in Yellowstone, but we can say with confidence that the probability of those Yellowstone quakes increased when the seismic waves from Alaska arrived in Yellowstone. We can predict that future large earthquakes around the globe are also likely to increase the probability of quakes in Yellowstone4.

Does that mean that the Japanese earthquake caused increased risk on the San Andreas Fault, or any other?

We don't know. And here's why. There are two ways to approach this question:

1. If we know the direct causes of earthquakes, we can determine whether those causual factors have increased.

2. Even without knowing the causes, we can look at the numbers and see whether big quakes are more likely to follow big quakes, globally.

Unfortunately, we don't usually know what caused an individual quake to occur at the exact moment in the exact location when it occurred. In the direct-triggering scenario above, we can say that the frequency of earthquakes increased immediately after the seismic waves arrived, and infer that the seismic waves caused the increase in small earthquakes.

But what if the increase in small earthquakes was sustained for a long time after the waves had passed? We observe that after a big earthquake, the planet may "ring like a bell"5 for hours or days, but the ringing of a bell doesn't last indefinitely. So how can we look at the recent damaging earthquakes (for example, in Haiti, January 12, 2010; Chile, late February 2010; and New Zealand, February 20116) and say that they are causing each other? If it exists, this kind of subtle increase in risk is detectable with statistics, but to really distinguish between a random pattern of events and a pattern that indicates a common cause, we need numbers. Can you tell with one flip of a coin whether that coin is weighted? Can you tell with 10 flips? 100?

There have been 82 recorded earthquakes since 1900 which were Magnitude 8 or greater. Five of these were M9 or greater. These are the numbers; this is what we have to work with. We know that the frequency of these great quakes is relatively rare for any given tectonic boundary—every 800-1,100 years in the case of the Sendai area7, every ~600 years in the case of Cascadia (Oregon–Washington–British Columbia). Looking at the graph of these events, you might pick out an apparent spike in large earthquake activity around 1960 and another one at the present time.

I could look at the current high rates of large earthquakes and tell you that a big one is due somewhere next week, and you couldn't prove me wrong. I could just as well look at the same data and tell you that the apparent cluster of earthquakes is over, and the rate of large earthquakes will drop back to lower rates similar to the 1970s, and likewise, you couldn't disprove me. But you could point out that the non-uniqueness of my answer renders my predictions useless.

Every earthquake that occurs extends the record, giving us a better shot at figuring out the details of the pattern. If I tell you that my coin lands on heads 80 percent of the time, and therefore the next flip will be heads, you should call me out, because I cannot really know what the next flip will bring. For now, with earthquake prediction, we still have not seen enough flips of the coin.

Mr. Winchester argued in the Daily Beast column that the earthquake on one edge of the Pacific Plate would "...more probably trigger an event on the same tectonic plate family...". Possible? Yes. But one could just as well argue that the westward motion of the Pacific Plate during the Sendai earthquake would effectively relieve stress on the San Andreas at its eastern edge, thereby making an earthquake there less likely than before. Again, both ideas may appear to make logical sense—but this does not make either or both any more correct. Logical exercises like this are a good place to start the process of hypothesis forming, followed by independent testing, which is at the core of scientific practice.

Why the scientists are not not telling you what you want to know

Contrary to Mr. Winchester's suggestions, we (scientists) really want you (the public) to know what we are doing! We want you to care about where your tax dollars are going (a very, very small portion of your tax dollars, anyway), and we desperately want you to use our discoveries to build a safer, cleaner, more sustainable and stable society. But we tend to scuttle like cockroaches from the media spotlight, for this reason:

If we are wrong, or misunderstood, the consequences can be dangerous for the public and brutal for our careers. Therefore, we tend to keep things under wraps, waiting until we are very sure, and results are vetted in numerous fora, before we communicate with the public. Even then, we are careful of members of the media, who often have different priorities than we do. This reduces the risk, but sometimes also the likelihood of publicizing the new knowledge we discover. This does not mean that we knowingly withhold information that could affect public safety!

To save lives and property, we need to be able to predict locations and times of individual earthquakes within hours or days. We currently cannot do that. To mitigate the overall risk, we need to be able to predict regions where earthquakes are likely to occur, offer a general timescale and give an idea of how large they might be. This we do very well, most of the time, although the short available record in many places means we are still tragically surprised by low-frequency events like the Sendai earthquake.

Popular writers have less apparent authority than professional scientists with Ph.D.s. This does not necessarily mean that they are less knowledgeable, and I know of many who maintain cutting-edge familiarity with the fields about which they write. However, it does mean they are not under the same obligation to be right, and they are not necessarily committed to careful fact checking. I am taking Mr. Winchester's point to heart when he lashes out at the non-communication from the science community—not, however, his assertion that our relative silence gives him permission to fill the gap with whatever he likes.

So I've made a commitment to jump into the fray more frequently, to not just make scientific information available but actively offer and promote it in public settings like schools, museums and the popular press, and to publicly challenge misinformation when I see it.

Chuck Ammon, Nicholas van der Elst, and Alex Hutko provided information or assistance for this column, but any errors are all mine. C.R.

Figure courtesy of Chuck Ammon, after Ammon et al. (2010) Great earthquakes and global seismic networks, Seismological Research Letters v. 81 n. 6 pp. 965-971.


1 Husen et al. (2003) Changes in geyser eruption behavior and remotely triggered seismicity in Yellowstone National Park produced by 2002 M 7.9 Denali fault earthquake, Alaska, Geological Society of America Bulletin v. 32, n. 6 p. 537-540.

2 van der Elst and Brodsky (2010) Connecting near-field and far-field earthquake triggering to dynamic strain, Journal of Geophysical Research v. 115 B07311

3 Parsons and Velasco (2011) Absence of remotely triggered large earthquakes beyond the mainshock region, Nature Geoscience v. 4 n. 4 p. 1-5

4 The susceptibility of the Yellowstone area to triggered earthquakes is attributed by Husen et al. (2003) to the overpressured geothermal system at depth.

5 Stein and Okal (2005) The 2004 Sumatra earthquake and Indian Ocean tsunami: What happened and why?, Visual Geosciences v. 10, n. 1 p. 21-25.

6 The degree of damage and injury caused by an earthquake depends not only on magnitude but a host of other factors including human population and construction. Of these earthquakes, suggested by Winchester to be key "triggering events", Christchurch and Haiti are were small to appear on the plot (M6.3 and M7.1, respectively) See the complete list at the USGS site to compare magnitude and total fatality.

7 Minoura et al. (2001) The 869 Jogan tsunami deposit and recurrence interval of large-scale tsunami on the Pacific coast of northeast Japan, Journal of Natural Disaster Science, v. 23 n. 2 p. 83-88.

8 Goldfinger et al. (2003) Holocene earthquake records from the Cascadia subduction zone and northern San Andreas Fault based on precise dating of offshore turbidites, Annual Review of Earth and Planetary Sciences v. 31, n. 1 p. 555-577.

9 Modified from Ammon et al. (2010) Great earthquakes and global seismic networks, Seismological Research Letters v. 81 n. 6 p. 965-971.

About the author: Dr. Christie Rowe is a researcher in Earth & Planetary Sciences at the University of California, Santa Cruz. She is part of a team of geologists, seismologists, geophysicists and experimentalists in rock mechanics who are striving toward an integrated model of how earthquakes work. Love and hate mail should go to christierowe[at] Photo: Pete Lippert

The views expressed are those of the author and are not necessarily those of Scientific American.

The views expressed are those of the author and are not necessarily those of Scientific American.

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