(2)Shallow inland earthquakes (Depths of 20 km or less)


A look at the topography of the Kinki region shows the Median Tectonic Line running east to west across the central part of the Kii Peninsula. The topographical characteristics of the region differ to the north and south of this line. The area to the north of the Median Tectonic Line has large plains and basins, including the Osaka Plain (Osaka Heiya), the Kyoto Basin, and Lake Biwa (Biwa Ko). This area is ringed by mountainous areas, including the Suzuka Mountains (Suzuka Sanmyaku), and the Hira, Rokko, Ikoma, and Kongo Mountains (Kongo Santi). Most of the active faults of the Kinki region are in this area, and many run along the boundary between the plains and basins on one side and the mountains on the other. The large plains and basins of the Kinki region mentioned above are thought to have developed due to the accumulation of slip on the active faults at the edge of these areas. In contrast, vast mountainous areas and almost no active faults characterize the area to the south of the Median Tectonic Line.

A look at the active faults in the Kinki region shows one active fault zone with Tsuruga Bay (Tsuruga Wan) as the summit extending to Ise Bay (Ise Wan), and another with Tsuruga Bay as the summit extending to the Awaji Island (Awaji Sima) after passing through Kyoto and Kobe. The active faults are concentrated in the triangular area bounded by these two fault zones and the Median Tectonic Line. This has one of the highest densities of active faults of any area in Japan. Except for the Kizugawa fault zone, the active faults lying in this triangular area have a predominantly dip-slip component. The fault zones that comprise the boundary of this triangular area and the fault zones outside this area have a predominantly strike-slip component.

Most of the active faults in the Kinki region have a B class activity, and their movements repeated correspond to compression in an east-west direction. Part of the Median Tectonic Line fault zone lies west of the central area of the Kii Peninsula, however, and its activity is an A class. Their movements repeated correspond to compression in a northwest-southeast direction. Observation of the crustal movement shows significant compression in an east-west direction north of the Median Tectonic Line, in keeping with the activity of the active fault (Fig.7-6 A, Fig.7-6 B). South of the Median Tectonic Line, there is usually compression in the northwest-southeast direction as a result of the strong effect of the subducting Philippine Sea Plate (Fig.7-6B). Extension in a northwest-southeast direction is created during great earthquakes along the Nankai Trough, however (Fig.7-6 A). The activity of the Median Tectonic Line is also thought to be strongly affected by the subducting Philippine Sea Plate.

One example of a destructive earthquake in this region is the 1596 earthquake (M 7 1/2, Keicho Fushimi Earthquake), which caused damage primarily in Kyoto, Osaka, and Nara. A recent survey of active faults has shown that this earthquake may have occurred on the Arima-Takatsuki fault zone, which stretches east to west from southern Hyogo Prefecture to the southern Kyoto and Rokko-Awajishima fault zone. Two other examples can be cited. The first is the 1662 earthquake (M 7 1/4 - 7.6) that caused substantial destruction primarily on the west side of Lake Biwa, and is thought to have occurred in the northern section of the West Coast of Lake Biwa (Biwako-seigan) fault zone and the Mikata/Hanaore fault zone. The second is the 1854 earthquake (M 7 1/4, the Iga-Ueno Earthquake) that caused extensive damage primarily in Iga, and is thought to have occurred on the Kizugawa fault zone. Major destructive earthquakes that occurred after 1868 (the start of the Meiji period) include the 1925 North Tajima Earthquake (M 6.8) in northern Hyogo Prefecture, the 1927 North Tango Earthquake (M 7.3) that occurred on the Tango Peninsula (Tango Hanto) in Kyoto, and the 1995 Southern Hyogo Prefecture Earthquake (M 7.2). Research into these earthquakes indicates that the North Tango Earthquake and the Southern Hyogo Prefecture Earthquake occurred on previously known faults. [For details, refer to 7-2 (2), (3).] It is not known, however, on which fault the North Tajima Earthquake occurred, though it caused slip at the earth's surface. Therefore, it is possible for earthquakes to occur in areas where there are no known active faults. In addition, shallow earthquakes M 6 or so can cause damage locally, and are not necessarily limited to active fault zones. Since the interval of activity for most active faults is 1,000 years or longer, this does not indicate that no earthquakes can occur where there are no historical records.

There is stationary seismic activity in Wakayama City. Most of these are earthquakes of M 5 or less, but 35 of them can be felt here in an average every year. This area is one of the highest active area of felt earthquakes in Japan. [Refer to 7-3(7)]