*The contents may be subject to change without prior notice.

 

K. Z. Nanjo and K. Oike

NaDiR, Global Center for Asian and Regional Research, University of Shizuoka

 

In the Kansai region, active faults repeatedly moved and induced earthquakes on the active faults, resulting in the formation of plains and basins surrounded by mountains (Fig. 1-12), such as the Osaka Plain, the Nara Basin, the Kyoto Basin, and the Omi Basin. In particular, Kyoto took advantage of this geological feature and became one of the world’s few non-walled capitals. By surveying the active faults around the Kyoto Basin, we can determine when these faults moved and induced earthquakes. Repeated active fault movements cause the ground to rise and form mountains, while erosion of the mountains causes earth and sand to slide down and be deposited in basins. Geological surveys reveal that the deposition of earth and sand in the Kyoto Basin began around a million years ago, which coincides with the period when the block that later became the Izu Peninsula began to collide with Honshu. This finding indicates that this collision contributed to the dynamism of today’s active fault movements across Honshu.

 

Fig. 1-12: Distribution of active faults and basins/plains in the Kansai region. This figure was modified after Oike (2001)(1).

 

In the areas where active faults exist, there are many underground cracks. Water ways naturally develop along these cracks, creating a wealth of groundwater available for use. The abundance of groundwater has helped the development of Japan’s traditional food culture, including tea ceremony, sake, yuba (beancurd skin or tofu skin), and Japanese-style confectionery. We take pride in this world-renowned culture, which evolved in the deformation belt. The basins in the deformation belt attracted settlers and eventually cities were built, but these are inherently at risk of being hit by great earthquakes (Fig. 1-13).

 

Fig. 1-13: How a city is developed. ① A large earthquake occurs. ② The active fault moves, causing the rock mass to slide vertically. ③ The mountain formed from the uplift is eroded and causes earth and sand to slide down the slope and be deposited in the subsided area. ④ The area develops into a basin or plain. The area becomes a basin if there is a mountain on the opposite side or a fault was formed. ⑤ Cities develop in the basin and plain. Large cities are at high risk of huge earthquakes due to their proximity to an active fault. This figure was modified after Oike (2001)(1).

 

Old documents show that an earthquake occurred on the Arima-Takatsuki fault (Fig. 1-12) in the Kansai region about 430 years ago that was the Azuchi–Momoyama Period. As the average recurrence interval of earthquakes in this fault zone is considered to be about 1,000~2,000 years, we may reasonably conclude that this fault will not cause earthquakes for several more centuries. Not all active faults have a high risk of generating earthquakes, so undue concern is not necessary. The important thing is to survey the conditions of active faults to distinguish between dangerous and harmless ones.

Japan has many old documents on earthquakes, including diaries kept by ordinary townspeople detailing when earthquakes occurred, how strong the shaking was, and what damage was caused. Some old newssheets (Kawaraban in Japanese) published in the Edo Period carry illustrations of a giant catfish being beaten for having caused an earthquake, as the catfish was believed to cause earthquakes. One illustration sarcastically depicts a catfish being treated to delicacies by a carpenter and plasterer. They ask the catfish to cause another large earthquake, because the need for reconstruction after an earthquake will bring them lucrative business. From these old documents, it is known that an earthquake occurred at the Nankai Trough every 100~200 years (Chapter 3, TBA). Various survey results also show that in western Japan, seismicity increased from around 50 years before to 10 years after a Nankai Trough earthquake, but remained inactive during the rest of the period. Western Japan entered a seismically active period after the 1995 Kobe Earthquake that caused disaster called the Great Hanshin and Awaji Earthquake. It is considered that the active period will last for some 50 years, and eventually lead to another Nankai Trough earthquake.

 

Sources and references

1. K. Oike, 2001, Trivia and Illustration of Earthquake (Trivia and illustration series), Natsumesha CO., LTD, Tokyo (in Japanese).