Table Speech

“Distribution of Seismic Intensity Relative to Old Terrain: Examples from Tokyo’s Urban Site and Osaka’s Kawachi Plain”

November 15, 2006

Mr. Yoshinobu Tsuji,
Associate Professor of Earthquake Research Institute, University of Tokyo

The epicenter between the“Genroku Era Earthquake,” occurred at 3 a.m. on December 23 in 1703 (Genroku Era) and the “Taisho Earthquake” on September 1, 1923 (Taisho Era) is almost overlapped. It is a relationship as if the “Genroku Earthquake” is the older brother and “Taisho Earthquake” is the younger one. About five years ago, Dr. Takemura, the authority of seismology checked, using police-generated documents, the total collapse of wooden houses or buildings by city, town and village in 1923, and analyzed the relationship between the total collapse and seismic regions. He made a distribution map with seismic intensity classifying into 7 (30% probability of total collapse), 6 upper (30 to 10%), 6 lower (10 to 1%), and 5 upper (1 to 0.1%) for destroyed areas.

Naturally, the area immediately right above the epicenter was most severely attacked, however, the intensity 6 upper was observed in another area off the epicenter. The area was where the Tone River was at that time. When the great “Kanto Region Earthquake” occurred, the source of flooding in the old Tone River was determined as being the upper 6 intensity level. The river’s flow back then was reflected well in the data.

It is interesting to know that the data of Dr. Takemura’s “Detailed Distribution Map of Seismic Intensity in Tokyo for Kanto Earthquake” and the result from the old data on the Genroku Era Earthquake were similar to a large extent. Harbors, river basins, and inland swamps in Japan’s medieval times are perfectly matched with places severely attacked by the Genroku and Taisho’s earthquakes. Although it took 300 years after the rivers or swamps had been developed, the grounds on the Kanto earthquake were still loosened.

Then, once in how many years does such a big earthquake occur?

In Chikura, Boso Peninsula, a four-step coastal terrace-elevated by massive earthquakes including the Genroku one-can be seen. As a result of our study on fossil remains such as shells and corals with the radiocarbon dating method, we found the first coastal terrace closest to inland was elevated 5,500 years ago; the second terrace 3,600 years ago; the third 2,900 years ago; and, the forth 300 years ago. The forth terrace was apparently made by the Genroku Earthquake in 1703. The costal terrace at the edge of Boso Peninsula was elevated four times in 6,000 years.

About one-meter of elevation, equivalent to the Kanto Earthquake, can be determined as an event of once between 200 and 250 years. Therefore, the elder brother, “Genroku Earthquake,” is a four-time phenomenon in 6,000 years—once every 1,500 years, whereas the younger brother, “Kanto Earthquake” is the one in 200 to 250 years.

Now, I would like to move on to the Osaka Plain.

In the Osaka’s Kawachi Plain, the Hoei Earthquake and the Ansei Nankai Earthquake occurred in 1707 and 1854 respectively. “Tokai” and “Nankai” earthquakes are a couple. The Nankai Earthquake in 1946 and the East Nankai Earthquake in 1944 apply to the case. On a day before the Ansei Nankai Earthquake of December 24, 1854, the Ansei Tokai Earthquake occurred.

In the “Kojiki” (“Record of Ancient Matters”) completed on New Year’ Day in 712, says, “Emperor Jinmu went to Naniwa (current “Osaka”) from Kibi, Okayama Prefecture and fought with Nagasunehiko. Ships were moored at Shirakata and military shields were put into the ground. The place is now called Tatetsu (literally, “putting the shields into the ground). The Emperor Jinmu went to Wakayama Prefecture from the Osaka Bay through Minamikata”. The description shows that the Tatetsu (on the foot of Ikoma Mountains and on the east edge of the Kawachi Plain), and the Minamikata (on the northern Yodogawa) were in the sea terrain. According to the study of old terrain in Osaka Prefecture, the Lake Kawachi in Yayoi Era was a big bay in Jyomon Era.

With some old documents, the detailed seismic intensity in Osaka Prefecture on the Ansei Tokai and Ansei Nankai earthquakes in 1854 was confirmed. It reported the place of Kawachi Bay had a great deal of havoc on a map in 200 to 400 B.C.

The areas most severely attacked by these large earthquakes almost matched with the area in the Kawachi Bay during the Jyomon Era. The north of West Yokobori in Osaka City was in the sea until the Heian Era. From the Kamakura Era to the Muromachi Era, the sea became land, which extended closely to the Kizu River. And in Edo Era, the land was further expanded to the today’s region. I think these facts show that big earthquakes uncover the coastlines of old times. From these results, the seismology law-on a principle that the areas once as seas and rivers are heavily damaged by the current quakes-can be empirically proven.