COMMENTARY How many great earthquakes should we expect? Gregory C. Beroza1 Department of Geophysics, Stanford University, Stanford, CA 94305-2215 reat earthquakes of magnitude 9.6 relevance far beyond earthquake occur- 8.5 or larger occur infrequently. 9.4 rence. The work by Michael (10) indepen- G For a nearly 40-y period after 9.2 dently made the same point and reached the February of 1965 M 8.7 Rat 9 the same conclusion: that the recent se- Islands, Alaska earthquake, the world did Magnitude 8.8 quence of earthquakes is consistent with not experience a single great earthquake; 8.6 the Poisson assumption. So where does the however, in the 7 y since late December of 1900 1920 1940 1960 1980 2000 perception of an anomaly arise? 2004, there have been a barrage of five Year A contributing factor to the perception great earthquakes. These earthquakes in- of an increase in earthquake rates has to clude the 2004 M 9.1 Sumatra, Indonesia 9.5 be the coverage and broadcasting of earthquake; the 2005 M 8.7 Nias, Indonesa 9 earthquake effects. For instance, 10 y ago, earthquake; the 2007 M 8.5 Bengkulu, In- 8.5 tsunami footage was exceedingly rare. donesia earthquake; the 2010 M 8.8 Maule, 8 Classic footage of the 1964 Alaska earth- Magnitude Chile earthquake; and the 2011 M 9.0 7.5 quake tsunami in Valdez harbor used to be fi Tohoku-oki, Japan earthquake. This cluster 7 shown as an example, but the lm is so 1900 1920 1940 1960 1980 2000 grainy that it is difficult to discern what is of great earthquakes seems to signal an Year anomalous increase in their frequency, but happening. That is not the case with the does it really? In their paper, “The global Fig. 1. M >7.0 earthquakes from the Prompt As- exceptionally clear and harrowing tsunami risk of big earthquakes has not recently sessment of Global Earthquakes for Response - footage from Japan. Another factor con- increased,” Shearer and Stark (1) present Catalog (PAGER-CAT) catalog (12). Upper shows tributing to the perception is that the ex- seismicity with a cutoff magnitude of 8.5. Lower a quantitative assessment of that percep- shows the same but with a cutoff magnitude of posure to earthquakes, and hence, their tion. Their analysis tests three different at- 7.0. The apparent temporal clustering in Upper is consequences, is increasing (11). The ex- tributes of the earthquake catalog against less obvious in Lower. posure has at least two aspects. The most the null hypothesis: That the observed obvious aspect is that earthquakes occur- seismicity is a plausible realization of a ring in the wrong place can have cata- century had limited bandwidth and seis- Poisson process (1). None of the three tests strophic consequences because of the mographs were sparsely deployed. Con- allows the Poisson assumption to be re- unfortunate overlay of fragile construction siderable uncertainty surrounds the size of jected with high confidence, which leads with urbanization in earthquake-prone some older events (7), and there are be- them to the conclusion that the recent spate regions. The 2010 Haiti earthquake killed of large earthquakes is not anomalous (1). haviors in earthquake catalogs (8) early in over 200,000 people, which makes it by far Seismologists know that earthquakes the 20th century that suggest biases in the deadliest natural disaster in the history cluster in space and time to some degree. magnitudes. Even if the data were perfect, of the Western hemisphere, but the mag- Omori’s Law (2) states that the frequency 100 y is a very short time to try to establish nitude of the earthquake, 7.0, was not ex- of aftershocks after a large earthquake an expected rate given the rarity of the ceptional. There have been over 1,700 decays inversely with time, and this law is largest earthquakes (9). We are left with earthquakes that size or larger since 1900 more than 1 century old. The law by only a very fuzzy idea of how frequently or about one every 4 wk. The other aspect Omori (2) has been used in epidemic-type great earthquakes should be expected, is that globalization has increased the fi aftershock sequence models to quantify which makes it more dif cult to detect an reach of earthquakes. Interruption of earthquake clustering locally (3). Earth- anomalous cluster. technology and automobile manufacturing quakes are thought to interact by static A more subtle effect is cherry picking, supply chains after the March of 2011 fi stress triggering (4), in which earthquakes which amounts to de ning retrospectively Tohoku-oki earthquake is one example of induce a stress change on nearby faults the behavior that is considered anomalous. the impact. Decisions to turn away from fi and thereby trigger or suppress other In the rst sentence of this article, I cherry nuclear power, not just in Japan but in fi earthquakes, but static stress decays rap- picked by de ning 8.5 as the threshold such far-flung corners of the world as idly with distance. Dynamic stress, affected magnitude for what should be considered Germany and Mexico, in the aftermath of through the transient seismic waves of an a great earthquake (Fig. 1). The definition the multiple meltdowns at Fukushima earthquake, is known to be capable of is arbitrary. A different definition would Daiichi are another impact. triggering small earthquakes at much have led either to a shorter quiet interval Where do we go from here? Clearly larger distances (5). For large earthquakes, in the case of a lower threshold or a less we need to develop a better understanding however, such triggering does not seem impressive recent cluster in the case of a of what the Earth is capable of and how particularly strong. A recent study found higher threshold. In an interesting twist, frequently large earthquakes occur. Sound no short-term change in earthquake the work by Shearer and Stark (1) attempts application of earthquake statistics is a part probabilities for earthquakes of magnitude to assess how common unlikely anomalies of this issue, and a deeper understanding of 5.0 or larger beyond a distance of several are found to be present in individual real- the earthquake process is another. The earthquake source dimensions (6). izations of a Poisson process if they are March 11 earthquake occurred offshore We have a little more than 1 century of defined posthoc. They find, for example, earthquake monitoring for which magni- that, when anomalous clusters are defined tudes can be instrumentally determined; retrospectively, 30% of realizations of a Author contributions: G.C.B. wrote the paper. however, early in the instrumental period, Poissonian earthquake catalog will contain The author declares no conflict of interest. the reliability of magnitudes is suspect clusters that should occur less than 1% of See companion article on page 717. because instrumentation in the early 20th the time (1). This cautionary exercise has 1E-mail: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1120744109 PNAS | January 17, 2012 | vol. 109 | no. 3 | 651–652 Downloaded by guest on September 24, 2021 one of the mostly intensely studied sub- portant endeavor. Another important en- something of a waiting game. The work by duction zones in the world, but its great deavor is additional development and Shearer and Stark (1) offers some guid- size was surprising; the results of under- widespread application of seafloor geodesy ance for what it would take to render the estimating the maximum size of potential to measure crustal strain energy accumu- recent spate of large earthquake activity earthquakes in the Japan Trench proved lation, which provides the fuel for earth- significantly anomalous. Three M ≥8.5 lethal. Earthquake scientists everywhere quakes. Such efforts will be time-consuming fi earthquakes in the next year would be are reappraising their models of earth- and expensive, but the scienti candsocie- fi quake occurrence as a result. A systematic tal payoff should be tremendous. suf cient, but it is a daunting prospect. 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