7 Characteristics of seismic activity in the Kinki region


One recent destructive earthquake in the Kinki region was the well-known 1995 Southern Hyogo Prefecture Earthquake (Hyogo-ken Nanbu Earthquake) (M 7.2) that caused more than 6,000 fatalities. Other destructive earthquakes have occurred here since the Meiji period (1868) both on land and sea, including the 1925 North Tajima Earthquake (Kita -Tajima Earthquake) (M 6.8), the 1927 North Tango Earthquake (Kita-Tango Earthquake) (M 7.3), the 1944 Tonankai Earthquake (M 7.9), and the 1946 Nankai Earthquake (M 8.0). The destruction by these earthquakes was caused by seismic ground motion, a tsunami, and other factors. Thanks to historical accounts stretching back almost 1,500 years, earthquakes in the Kinki region have been documented over a longer period than any region in Japan. A long-term perspective shows that many destructive earthquakes have occurred in this region. Historical accounts record many instances of destruction, starting with the Yamato-Kawachi Earthquake in the year 416. However the location of the hypocenter is not well understood for most of these old earthquakes. It is also known that earthquakes in surrounding areas have caused damage in the Kinki region with one example being the 1891 Nobi Earthquake (M 8.0). Further, it is known that damage here has been caused by earthquakes occurring in the eastern margin of the Sea of Japan (Japan Sea), such as the 1983 Central Sea of Japan Earthquake (Nihonkai Chubu Earthquake) (M 7.7). Also, tsunami generated by earthquakes in foreign countries are known to have caused damage, such as the 1960 Chile Earthquake Tsunami. Fig.7-1 and Fig.7-2 show the primary destructive earthquakes known to have occurred in the Kinki region. Fig.7-1 shows that several destructive earthquakes are overlapped in the northeastern region of the Kyoto Basin (Kyoto Bonti). These are old earthquakes, and the reason all of them were described as occurring in that location is that damage was recorded only in Kyoto.

Seismic activity in the Kinki region can be roughly divided into two types: first, earthquakes that occur in shallow locations on land at 20 km or less; and second, those that occur near a plate boundary subducting toward land from the Nankai Trough in the Pacific Ocean. Also, earthquakes here sometimes occur in somewhat deeper locations on land. These are thought to be within the subducted Philippine Sea Plate.

The Philippine Sea Plate is approaching the Kinki region from the southeast at a speed of about 5 cm a year. This plate is subducting from the Nankai Trough in the Pacific Ocean toward the Kinki region. Seismic activity that accompanies the subduction of this plate is observed to depths of about 70-80 km in the central part of the region (Fig.7-3 and Fig.7-4). Historical data indicates that interplate earthquakes of M 8 or so recur along the Nankai Trough in roughly 100-150 year intervals. Historical accounts further indicate that seismic activity on land increases for several decades before and after great earthquakes along the Nankai Trough. Seismic activity in the Kinki region is thought to be closely related to the subduction of the Philippine Sea Plate.

A look at the topography of the Kinki region shows the Median Tectonic Line (Chuo-kozosen)running east to west across the Kii Peninsula (Kii Hanto). 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 ringed by mountains. 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 mountainous area on the other. There are many active faults known to exist in this area, which has one of the highest concentrations of active faults of any area in Japan. In contrast, vast mountainous areas characterize the area to the south of the Median Tectonic Line, where there are almost no active faults. Destructive earthquakes in shallow locations on land have occurred in areas with known active faults and areas with no known active faults. The 1995 Southern Hyogo Prefecture Earthquake (M 7.2) occurred on a known active fault, while the 1925 North Tajima Earthquake (M 6.8) occurred in an area with no known active faults. Fig.7-5 gives a bird's eye view of the topography of the Kinki region and the distribution of active faults from the southeast and the northwest.

A look at the crustal deformation in the Kinki region shows that the earth's surface is being compressed in a roughly east-west direction in the area north of the Median Tectonic Line. This deformation is harmonized with the distribution of active faults and the direction of the activity. In the area south of the Median Tectonic Line, extension in a northwest-southeast direction is occurring due to the 1944 Tonankai Earthquake and the 1946 Nankai Earthquake (Fig.7-6A). Ordinarily, however, this area experiences compression (Fig.7-6B). [Refer to 7-1 (1) 1)]

A look at recent seismic activity in the Kinki region and surrounding areas shows that destructive earthquakes have not occurred in the Pacific Ocean since the 1944 Tonankai Earthquake (M 7.9) and the 1946 Nankai Earthquake (M 8.0). The 1995 Southern Hyogo Prefecture Earthquake (M 7.2) occurred in shallow land and caused devastation concentrated in the Kobe, Osaka, and Awaji areas. Further, damage was caused by a tsunami on the coast of Wakasa Bay (Wakasa Wan) accompanying the earthquakes in the eastern margin of the Sea of Japan, including the 1983 Central Sea of Japan Earthquake (M 7.7), and the 1993 Earthquake off the Southwest Coast of Hokkaido (Hokkaido Nansei-Oki Earthquake) (M 7.8).