Nojima Fault as an Earthquake Fault Associated with the 1995 Hyogoken-Nanbu (Kobe) Earthquake, Awaji Island, Southwestern Japan

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  • 兵庫県南部地震 (1995. 1. 17) で現われた野島地震断層による地表変位と災害の概要
  • 特別講演 兵庫県南部地震(1995.1.17)で現われた野島地震断層による地表変位と災害の概要
  • トクベツ コウエン ヒョウゴケン ナンブ ジシン 1995.1.17 デ アラ

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Abstract

On Jan. 17, 1995, a large and shallow earthquake (M=7.2, focal depth =14 km) occurred in Kobe and the surrounding area, resulting in a tremendous disaster. Nojima fault, one of the active faults of the Rokko Fault System that strikes NE-SW and is dominated by right-lateral offset, was activated in association with this earthquake, and is identified as a seismogenetic fault for the Kobe earthquake, although there must have been other seismogenetic fault activity to account for the severe damage in the Kobe area. Repeated activity of the Nojima Fault since ca. 20 ka BP is described by Mizuno et al. (1990).<br> The surface appearance of this earthquake fault can be traced for about 10 km on the west coast of the northern part of Awaji Island. This fault is characterized by right-lateral offset, up to 1.8 m, accompanied by veritcal displacement up to 1.3 m. The upthrown side is mostly the southeast side of the fault, indicating that this fault activity has been responsible for the formation of the hilly land of Awaji Island.<br> The fault plane dips steeply southeastward, indicating that this fault is a high-angle reverse fault. Clear striation upwards to the right, on the fault plane is a composite result of strike-and-dip slip movement. Only at Nashimoto is the northwestern side uplifted. The surface deformation appears as prominent overhanging fault scarps at most localities along the fault, cutting through the lower to middle Pleistocene Osaka Group. However, at Nashimoto, where the fault displaces unconsolidated alluvial sand and gravel, a flexural scarp was formed. In addition to the fault scarp, many secondary features related to this faulting have appeared, including en echelon cracks, a minor pull-apart basin, and tectonic bulges. These features were mapped in detail by several groups immediately after the earth-quake (e. g. Nakata et al., 1995; Lin et al., 1995; Awata et al., 1995; Ota et al., 1995). Furthermore, trenching work to understand the previous fault activity has been done. The recurrence interval is estimated to be ca. 800 years by Suzuki et al. (1995). The newly exposed fault scarp provides an opportunity for the study.of the fault retreat process which is described in a companion paper by Azuma et al. Coastal uplift, up to ca. 55 cm, also occurred on the upthrown side, .which suggests that eastern side (except at Nashimoto) was absolutely uplifted at the time of the 1995 earthquake.<br> Damage to houses on Awaji Island is rather localized only on or around the Nojima Fault. Old wooden houses with, heavy roof tiles on the fault were particularly severely damaged. However, damage to new houses built with light materials, or concrete houses was not so severe, even if they were located very close to the fault. Many houses collapsed at Toshima or Gunge, where the fault trace was rather unclear or not found at all. This may be caused by lithological factors, because these settlements are located on the coastal alluvial lowland.

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