Legacies of Ansei Edo
Interpretations of the Ansei Edo earthquake during the modern era derived from a combination of memorial rites, earthquake lore, earthquakes elsewhere in Japan felt in Tokyo, and insights provided by seismology. It is useful at the outset to consider the state of seismological knowledge in the last decade of the nineteenth century. The powerful 1891 Nōbi earthquake shook vast areas of central Honshu, and the 1896 Meiji Sanriku earthquake and tsunami killed approximately twenty-two thousand people in precisely the same area devastated by the March 11, 2011, Great East Japan Earthquake. Although now we usually refer to the 1896 event as an earthquake followed by a tsunami, at the time it was known mainly as a tsunami. Today we know that it was a particularly deadly type of earthquake called a “tsunami earthquake.” In such an event, the fault ruptures relatively slowly, producing long-period seismic waves that cause a gentle undulation of the land. Many people experiencing such an event do not realize that it is an earthquake and are therefore less likely to seek high ground in anticipation of a tsunami. With little knowledge of faults and no knowledge of plate tectonics, explaining these events, especially the 1896 tsunami, was a challenge. A brief examination of one of the reports on the tsunami issued by the Imperial Earthquake Investigation Committee illuminates this difficulty.
The fourth chapter of the report is entitled “Discussing the Cause of the Recent Tsunami” (Konkai tsunami no genin o ronzu). It begins with two basic possibilities. Either an earthquake or undersea volcanic activity caused the tsunami. If it was an earthquake, there were two further possibilities. In the thinking of the time, earthquakes whose origin was near the coast in shallow waters usually generate tsunamis, and considerable space is devoted to summarizing past earthquakes of this type, including the Lisbon earthquake, the Ansei Tōkai earthquake, and the Jōgan earthquake of 869. The second type, an earthquake originating in deep water, was mainly a theoretical possibility, so rare that there were no good examples to cite. The report then summarizes the evidence that an earthquake caused the 1896 tsunami and lists four peculiarities of the event that made it atypical of earthquake-generated tsunamis, one of which was “extremely weak ground motion of the earthquake.” Therefore, it concludes, the more likely cause was undersea volcanic activity.
Krakatoa's eruption, after all, produced small earthquakes and large tsunami waves that traveled around the world. How is it, though, that largescale volcanic activity would be located on the sea floor off Japan's Sanriku coast? The report posits, “Indeed there exists a weak point in the earth in this area” because grooves in the earth's crust at this location are aligned in a northeast to southwest orientation. Therefore, volcanic activity could break through the crust in this weakened area and displace enough seawater to cause a tsunami. Moreover, anecdotal reports—for example, by people who were washed into the sea and rescued—indicate that the water temperature was warmer than usual, adding further support to the volcanic activity theory. Not all investigators agreed with the underwater volcanic activity hypothesis. My point in mentioning it here is simply to highlight some of the difficulties of explaining deadly seismic events in the Meiji era. Seismologists of the time could measure ground motion, wave heights, and other metrics with considerable precision. Teams of investigators carefully surveyed and documented the damage. Nevertheless, causal explanations remained highly speculative.
-  For more details, see Gregory Smits, “Danger in the Lowground: Historical Context for the March 11, 2011 Tōhoku Earthquake and Tsunami,” Asia- Pacific Journal 9, issue 20, no. 4 (May 16, 2011), at http://www.japanfocus .org/-Gregory-Smits/3531.
-  Iki Tsunenaka, “Sanriku chihō tsunami jistujō torishirabe hōkoku,” in Shinsai yobō chōsakai hōkoku, dai 7 gō (Shinsai yobō chōsakai, 1896), 30–33.