It is my pleasure to introduce the interview with Lori Dengler, “Tracking the Destructive Power Of the Pacific Ocean’s Tsunamis,” conducted by Yale Environment 360 on March 22, 2011, eleven days after the March 11 Tohoku Earthquake and tsunami. Having become aware of this interview recently, I was struck by the substantial degree to which the interview complements my article, “Danger in the Lowground: Historical Context for the March 11, 2011 Tohoku Earthquake and Tsunami.” Here I will briefly discuss some points in the interview that struck me as especially noteworthy.

First I would echo the interviewer’s point that although the nuclear power plant failure is obviously a matter of great concern in both Japan and other parts of the world that rely on nuclear power, we should not lose sight of the awesome geological event that took place, which also has implications that go beyond Japan. The death toll from the recent tsunami is now approaching 25,000.

As Dengler points out, the 1923 Great Kanto Earthquake was many times more deadly than the recent tsunami, despite being of much lower magnitude. Its location and the nature of local conditions (dense urban area, just before noon, windy day, et cetera) are what made it so deadly. The magnitude of an earthquake, while obviously important, is not necessarily the crucial element in determining how deadly or damaging a seismic event will be—especially in the case of tsunami earthquakes, of course. It is mainly for this reason that in my article, I mention that the recent earthquake exceeded expert expectations in terms of magnitude, but I did not regard that point as particularly important. More significant in practical terms was the size of the tsunami, which was roughly the same as 1896.

My lack of concern that experts did not anticipate an M9-class event was, of course, also a function of my being an historian, not a seismologist. Writing in a recent issue of Nature on the general topic of “rebuilding seismology,” Takeshi Sagiya of Nagoya University struck an apologetic tone. He pointed out that while he and other experts were aware of a high probability of a magnitude 7.7–8.2 event in that area, nobody imagined an M9-class event despite two important pieces of evidence. One was the recent geological evidence of a massive tsunami inundating the Sendai Plain in 869, which Dengler also discusses. The other evidence was that GPS data shows that in the Japan Trench the ratio of cumulative fault slip to plate motion (seismic coupling coefficient) in large earthquakes has been only around 30%, and it has been impossible to account for the other 70%. That large remainder, combined with the historical/paleoseismological data, point to the potential for much larger events. Sagiya’s conclusion is that seismologists must consider all the data, historical and geological, and not overlook inconsistent data.

The evidence dating to 869 is a sand sheet. Dengler also discusses the importance of sand sheets and points out that they extended further inland than the historical record indicates. She also reports that some seismologists have discussed the possibility of M9-class events based on that evidence. I might add that historical evidence from the 1611 tsunami (almost certainly the result of a tsunami earthquake, as in 1896) clearly indicates a more extensive reach of tsunami waves than occurred either in 1896 or 2011. Also of interest, recent research presented on May 15, 2011 indicates that a massive tsunami on a par with the 869 event (now regarded as a likely M9-class earthquake) took place 2000 years ago during the Yayoi period. The article concludes that these massive events likely occur in a cycle of about once every 1000 years. Even if the 1000-year cycle is accurate, the 1611 and 1896 events were probably even more dangerous owing to the lack of severe shaking. Moreover, the 1933 Showa Sanriku Earthquake and tsunami should be a reminder that sub-M9 earthquakes and tsunamis in the region are also deadly and much more common.

Dengler’s interview is a delight to read owing to the clarity with which she explains relevant geologic phenomena. Her discussion of the area along the coast of the United States and Canada subject to the same general danger as Japan’s Tohoku region explains in some detail how Brian Atwater and others used data from submerged forests of Red cedars to match written documents in Japan to help date the 1700 earthquake and tsunami. Dengler’s discussion of factors making the North American zone less dangerous (e.g., fewer people living at the coast) again points to the importance of local conditions in either amplifying or mitigating natural hazards.

In short, the record of the past, including historical documents and paleoseismological data, is an invaluable tool for assessing earthquake and tsunami hazards. Moreover, the record of how people have reacted to major disasters is similarly valuable in devising strategies for mitigating such events in the future....