Ansei Tokai and Ansei Nankai, 1854 (Kaei 7/Ansei 1)/11/4 and 11/5
At 9 a.m. on December 23, 1854, a M8.4 ocean trench earthquake occurred along the subducting edge of the Philippine Sea Plate between the Kumanooki (offshore from the Kii Peninsula) northeast to the Enshū-oki (Suruga Bay, Shizuoka Prefecture). Shaking could be felt as far south as northern Kyushu and as far north as the Tōhoku region. Tsunamis struck areas between the Bōsō Peninsula and Kōchi Prefecture, with wave heights ranging from 4.5 to 10 meters. The most severe ground shaking occurred along the coast between Numazu and Ise Bay. In some places, damage to structures approached 100 percent. The estimated death toll from the Ansei Tōkai earthquake is two to three thousand, and the shaking and sea waves destroyed as many as thirty thousand structures.
The next day, at approximately 4 p.m., an adjacent segment of the subducting edge of the Philippine Sea Plate ruptured off the eastern coast of Shikoku. This Ansei Nankai earthquake was essentially a continuation of the previous day's seismic event, and some areas such as the southern tip of the Kii Peninsula were shaken by both earthquakes. In the village of Koza, for example, residents spent the night after the Tōkai earthquake in the hills. They came back to what was left of their houses the next day and were shaken by the Nankai earthquake that afternoon. Seeing the sea recede, they escaped a second tsunami just in time. The Ansei Nankai earthquake shook a wide area, and ground motion was especially severe in Shikoku and adjacent coastal areas of Honshu. The death toll from the shaking and seismic sea waves amounted to several thousand. Ocean trench earthquakes often cause uplifting and subsidence, and this one was no exception. Major earthquakes during the Hōei, Ansei, and Shōwa eras raised the port of Murozu in Kōchi 1.4, 1.2, and 1.1 meters respectively. Other areas of Shikoku sank. The Tōkai earthquake caused uplift along the coast of present-day Shizuoka Prefecture, permitting construction of the Satta Pass to reconnect the Tōkaidō highway, which the earthquake had blocked. Today, the Tōmei Expressway and Tōkaidō honsen rail line both pass through this uplifted area.
Ansei Edo (Ansei Earthquake; Great Ansei Earthquake), 1855 (Ansei 2)/10/2
The Ansei Edo earthquake struck at approximately 10 p.m. on November 11, 1855. Estimates of magnitude vary slightly, but 6.9–7.0 is typical. Using the JMA seismic intensity scale (shindo), most areas of Edo were a strong or weak five. For people, a strong five would include feelings of extreme fear, the shaking impeding their movement. Modern wood-frame structures with low resistance to shaking would likely suffer damaged walls and support posts or become tilted. Generally, wooden structures in 1855 were significantly less resistant to earthquake shaking than modern buildings. Some key areas of the city suffered damage at a level of six. A weak six level of seismic intensity is characterized by difficulty standing, which would become impossible in a strong six. For wooden structures, those with poor resistance to shaking might collapse in weak six conditions, and most would collapse in strong six conditions. As a rough guide, the percentage of destruction to structures would be less than 1.5 in weak five conditions and would approach 70 in strong six conditions. A relatively modest difference of location on the night of the earthquake would have determined the difference between a rude awakening and minor damage to one's residence versus complete collapse of major structures and hundreds of people being crushed. Honjo (Kuroda-ku), Fukagawa (Kōtō-ku), “Daimyo Lane” (the area roughly between Yūryakuchō and Tōkyō Stations), Asakusa, and nearby areas experienced shaking in the six range, as did Shin-Yoshiwara. Storehouses suffered extensive damage throughout the city, even within areas in the five range. Deaths for civilian and military personnel combined were in the range of eight to ten thousand.
The Ansei Edo earthquake was an inland earthquake that caused choppy coastal waters but did not generate a tsunami. Estimates of the epicenter consistently place it between the northern part of Tokyo Bay and Etō-ku, at approximately the mouth of the Arakawa River as it empties into Tokyo Bay. The question of depth is less clear. Some estimates point to a shallow focus, while other studies have suggested a medium depth. There are nine extant accounts in historical materials that mention in some way the time difference between shaking caused by P-waves (pressure waves) and S-waves (shear waves). Seismologists have attempted to estimate the focus in part by examining these materials, but they are far from precise. The question of depth and precise location of the hypocenter remains open.
-  Okada, Jishin chizu, 119–120; Itō, Jishin to funka no Nihonshi, 91–96; Sangawa, Jishin no Nihonshi, 173; and Tsuji, “Ansei tōkai, nankai jishin,” in Chūō bōsai kaigi, 1854 Ansei tōkai jishin, 1–2. See also page 132.
-  Okada, Jishin chizu, 119; Itō, Jishin to funka no Nihonshi, 99–100; Sangawa, Jishin no Nihonshi, 180–181; Usami, Higai jishin, 151–168; and Tsuji, “Ansei tōkai, nankai jishin” and Kitahara Itoko, “Ansei tōkai, nankai jishin no higai jōhō ni tsuite: kawaraban o chūshin ni,” in Chūō bōsai kaigi, 1854 Ansei tōkai jishin, 84–85.
-  Ishibashi, Daijishinran, 25–26.
-  For a detailed table of JMA seismic intensity scale levels, along with important qualifying points, see Chūō bōsai kaigi, 1855 Ansei Edo jishin hōkokusho, 32–39. See also page 2.
-  For a comprehensive summary of the earthquake, see Usami, Higai jishin, 171–182. There are dozens of excellent modern studies of damage patterns, a topic I revisit later. An overall summary in text and in charts is Chūō bōsai kaigi, 1855 Ansei Edo jishin hōkokusho, 1–11, 25–30 (including charts 1-1 through 1-6). For a systematic collection of all relevant documents and materials, see Sayama Mamoru, Ansei Edo jishin saigaishi (hereafter SGS).
-  For a tabular summary of these nine documents, see Chūō bōsai kaigi, 1855 Ansei Edo jishin hōkokusho, Table 1-2, 23–24. Only one document permits even a rough estimate of a specific number. For further discussion of focal depth, see Okada Yoshimitsu, Saishin Nihon no jishin chizu (Tōkyō shoseki, 2006), 87; Sangawa Akira, Jishin no Nihonshi: daichi was nani o kataru ka (Chūōkōron shinsha, 2007), 191; and Nakamura Misao, Kayano Ichirō, and Matsuura Ritsuko, “Ansei Edo jishin no shubuken de no higai” Rekishi jishin, 19 (2003), 32–33, 36. Nakamura et al. estimate a focal depth of 35–70 kilometers, and many other estimates are within this range. For a detailed seismological discussion of the focal depth of the Ansei Edo earthquake that summarizes many competing theories and approaches, see William H. Bakun, “Magnitude and Location of Historical Earthquakes in Japan and Implications for the 1855 Ansei Edo Earthquake,” Journal of Geophysical Research, vol. 110, B02304 (2005), 12–22.