Special Issue: Urgent research on the geological hazards caused by the 1995 Hyogoken-Nanbu Earthquake in the coastal plain and the hillsides of Kobe area
		The circumstances and a summary of urgent research on the geological hazards by the 1995 Hyogoken-Nanbu Earthquake
		Seismic reflection survey of Ashiyagawa survey line in the Ashiya city, Hyogo Pref.,Japan
		Seismic reflection survey of Wakinohara-2 survey line in Kobe city, HyogoPref., Japan
		Microgravity survey in Kobe, Ashiya and Nishinomiya cities, Kinkidistrict, Japan
		Microgravity survey in Kobe, Ashiya and Nishinomiya cities, Kiniki district, Japan (2) -Basement structure-
		Repetitious leveling of short interval points across the severely dameged zone caused by the 1995 Hyogoken-Nanbu Earthquake
		Landslides in gently sloping residential areas caused by the 1995 Hyogoken-Nanbu Earthquake

The circumstances and a summary of urgent research on the geological hazards by the 1995 Hyogoken-Nanbu Earthquake

Ichiyo Isobe

1996

vol. 47 (2/3), p. 65-66

Keyword: Hyogoken-Nanbu earthquake, fault, Rokko active fault system, Kobe, geological hazard

Seismic reflection survey of Ashiyagawa survey line in the Ashiya city, Hyogo Pref., Japan

Hidenori Endo, ShiroWatanabe, Masahiko Makino, Astushi Urabe, Hiroo Aso, Yasunori Koreishi and ToahinobuEjiri

1996

vol. 47 (2/3), p. 79-94, 8figs., 3tables, 2plates

Keyword: seismic survey, reflection survey, fault, earthquake, 1995 Hyogoken-Nanbu Earthquake, Ashiya, Hyogo

Abstract: TThe 1995 Hyogoken-Nanbu Earthquake caused very severe damage within a narrow zone in the coastal plain from Kobe city to Nishinomiya city. The seismic reflection survey of the Ashiyagawa survey line in Ashiya city was conducted to reveal the subsurface geological structure of the coastal plain. This survey was aimed at the shallow subsurface structures, from the down to a depth of several hundreds meters, in order to determine the cause of the earthquake gamage.
　The following conclusions were derived from the study. (1)The seismic profile shows that concealed faults exist in the shallow subsurface deposits below the Hanshin railway. (2) The concealed fault also exists at a greater depth to the south of the Hanshin railway. (3) The sediments within the block bounded by the faults dip steeply towards Osaka Bay, whereas the sediments outside the fault block are horizontal. (4) The discontinuity and humps of the reflection interfaces show a reverse fault dipping to the north, probably with some lateral-slip. (5) The accumulation of the fault movements can be recognized around the northern fault below the Hanshin railway. (6) The horizons near to this fault show pinch out, which indicates the presence of fault movements. (7) The comparison between the locations of the faults and the distribution map of damaged houses suggests that the fault below the Hanshin railway exists on the southern boundary of the severely damaged zone. This fault might also have destroyed the express highway. These results suggest that there is a close relationship between the location of the concealed faults and the extent of the damages.

Seismic reflection survey of Wakinohara-2 survey line in Kobe city, HyogoPref., Japan

Hidenori Endo, Shiro Watanabe, Masahiko Makino, Hiroshi Yokota, Toshikazu Noda and Toshiyuki Kagawa

1996

vol. 47 (2/3), p. 95-108, 9 figs., 2 tables, 2 plates.

Keyword: seismic survey, reflection survey, fault, earthquake, 1995 Hyogoken-Nanbu Earthquake, Wakinohama, Kobe, Hyogo

Abstract: The 1995 Hyogoken-Nanbu Earthquake caused very severe damage eithin a narrow zone in the coastal plain from Kobe city to Nishinomiya city. The seismic reflection survey of the Wakinohama 2-survey line near to the boundary between Chuo and Nada wards in Kobe city was conducted to reveal the subsurface geological structure of the coastal plain. This survey was aimed at the shallow subsurface structures, from the surface down to a depth of several hundreds meters, in order to document the cause of the earthquake damage. Two sets of 48 channels with offsets of 5-240m and 125-360m were alternately recorded in the seismic reflection survey. The interval between the geophone groups was 5m, and the common-midpoint stack was 36.
　The following conclusions were derived from the study. (1) The seismic profile shows that concealed faults exist below the shallow subsurface deposits around Japan Railway (JR) line, and at the southwestern corner of Oji Park. (2) The sediments within the block bounded by thesee faults dip steeply to the south, whereas the sediments outside the fault block are horizontal. (3) The discoutinuity and humps of the reflection interfaces below the JRline show a reverse fault, dipping to the north. (4) A wide fracture zone, probably with some lateral displacement exists at the southwestern corner of Oji Park. (5) The accumulation of the fault movements can be recognized around the southern fault below the JR line. (6) Some horizons near to this fault show pinch out,which may indicate the occurrence of periodic,large fault movements. (7) A common feature in the reflection patterns outside the distorted fault block suggests a relatively young age for the large displacements caused by the Oji Park fault. (8) The comparison between the locations of the faults and the distribution map of damaged houses suggests that the Oji Park fault exists on the northern boundary of the serverely damaged zone, to the north of which the extent of the houses damaged decreases sharply. The point at the surface extraporated from the fault below the JR line corresponds to the area of reinforced concrete buildings and artificial structures with steel frames which were severely damaged. These results suggest that there is a close relationship between the location of the concealed faults and the extent of the dameges.

Microgravity survey in Kobe, Ashiya and Nishinomiya cities, Kinkidistrict, Japan(1)-Bougure anomaly and concesled faults

Yasuaki Murata, Masahiko Makino, Hidenori Endo, Kazuaki Watanabe, Shiro Watanabe, and Atsushi Urabe

1996

vol. 47 (2/3), p. 109-132, 10 figs., 1 table, 2 plates

Keywords: 1995 Hyogoken-Nanbu Earthquake, Kobe city, Ashiya city, Nishinomiya city, microgravity survey, Bouguer anomaly, concealed fault

Abstract : A microgravity survey was carried out in Kobe, Ashiya and Nishinomiya cities, which were damaged by the 1995 Hyogoken-Nanbu Earthquake,to estimate subaurface structures, such as concealed faults and basement structere. 1,824 gravity stations were observed within the area. The terrain effect in the gravity values was precisely corrected using the 50 metermesh topographic data provided by theGeographical Survey Institute. The Bouguer anomaly map and its horizontal gradient map were constructed to estimate the location of the concealed faults. The results indicate that, (1) the Bouguer anomalies suddenly change along thefault, (2) some faults, such as the Koyo fault, can be extended under the plain, and (3) the basement structure may subside in the coastal plain within the area,because the Bouguer anomalies decrease towards the sea in the plain.

Microgravity survey in Kobe, Ashiya and Nishinomiya cities, Kiniki district, Japan (2) -Basement structure-

Masahiko Makino, Yasuaki Murata, Hidenori Endo, Kazuaki Watanabe, Shiro Watanabe and Atsushi Urabe

1996

vol. 47 (2/3), p. 133-164, 6 figs, 1 table.

Keyword : 1995 Hyogoken-Nanbu Earthquake, microgravity, gravity survey, Bouguer anomaly, basement structure, two-dimentional analysis, fault, Rokko mountains, Osaka Bay

Abstract : A microgravity survey in Kobe, Ashiya and Nishinomiya cities was conducted to investigate the subsurface geological structure of the severely damaged zone caused by the 1995 Hyogoken-Nanbu Earthquake. Murata et al . (1996) found that the steepest gradients of Bouguer anomalies exist along the disaster zone. In this report, we analyze the gravity data alng survey lines to obtain a depth section of the basement structure and concealed faults.
　As thegravity basement rises up to 1 km above sea level in the Rokko mountains and sinks down to more than 1 km below sea level under Osaka Bay, it is not possible in this study area to use the usual methods of analysis, for example, Fourier series method or Fourier integral method. Therefore, we propose a new method to analyze very steep basement structure with a two-dimentional structure.
　The results show that the interface between the basement and the overlaying sediments exists at a depth of 1.5 km below Osala Bay. The basement dips steeply towards Osaka Bay near to the northern edge of the severly damaged zone. This basement structure seems to be one of factors to cause the severe damage.

Repetitious leveling of short interval points across the severely dameged zone caused by the 1995 Hyogoken-Nanbu Earthquake

Kazuki Watanabe, Hidenori Endo, ShiroWatanabe, Masahiko Makino, Yasuki Murata and Atsushi Urabe

1996

vol. 47 (2/3), p. 165-173, 7f igs., 2 tables.

Keywords : 1995 Hyogoken-Nanbu Earthquake, first order leveling, repetitious leveling, vertical movement, meansurement

Abstract : The first order leveling in an area between Suma ward in Kobe city and Nishinomiya city has been conducted to investigate the vertical movement of the crust in the severely damaged zone caused by the 1995 Hyogoken-Nanbu Earthquake. The main lines of the leveling nets were established in a north-south direction across the damaged zone, and measurement points were placed at short intervals of abou 25m. Each leveling net was connected with the nearest point of the first order benchmarks through the east-west line. Most of the measurements were conducted in March, 1995. Measurements of part of the nets were repeated in July, 1995.
　The results show vertical movements occurred at many points. These movemwnts show short and local snatches of variation, medium continuous variations among several points, and a long range of uplift or subsidence.
　There are only a few examples of repetitious leveling at short intervals of about 25m, with a line length of several km. Our results show that the leveling of short interval points is useful in the study of vertical movements in the crust.

Landslides in gently sloping residential areas caused by the 1995 Hyogoken-Nanbu Earthquake

Tositaka Kamai, Kiyohumi Suzuki and Itiyo Isobe

1996

vol. 47 (2/3), p. 175-200, 23 figs.

Keywords : 1995 Hyogoken-Nanbu Earthquake, landslide, urban development

Abstract : The 1995 Hyogoken-Nanbu Earthquake triggered numerous landslides on the gentle slopes in the hillslides between Nishinomiya and Kobe cities, which are the most important and densely populated residential areas in Japan. More than two hundred landslides in the residential areas were classified into four types, as follows:
　 a) Slides associated with high speed flow slide (0.9%)
　 b) Creep and slides in artificial valley fill (53.3%)
　 c) Creepand slides caused by liquefaction in alluvial deposit (10.7%)
　 d) Slides on steep slopes (35.1%)
　The landslides were distributed mainly in two separate regions which are composed of semi-consolidated clay, sand and gravel of the Plio-Pleistocene age, Lower Osaka Group. The original landscape of the hillslides has been fully changed by landform transformation due to urban development; such as, housing, road contraction and lifeline constructions. Landslides mainly occurred on artificial valley fill and slop cuttings. Thus, these landslides were typical of landslide disasters in an urban region; the landslides potential was created by human activity, and the landslides were triggered by the earthquake. Many cities in Japan have similar ground conditions as those observed in the Hanshin district, therefore, there is the potential for landslide disasters to occur within the urban areas of these cities. Both the disclosure of ground condition information and new urban planning methods in consideration of the surface geology are necessary for all local cities planners and the government.