活断層研究
Online ISSN : 2186-5337
Print ISSN : 0918-1024
2017 巻 , 46 号
選択された号の論文の7件中1~7を表示しています
口絵
  • 林 広樹, 為広 遼
    2017 年 2017 巻 46 号 p. 1-4
    発行日: 2017/03/31
    公開日: 2018/03/29
    ジャーナル フリー
  • 宮下 由香里, 白濱 吉起, 東郷 徹宏, 吾妻 崇, 亀高 正男, 鈴木 悠爾, 酒井 亨, 杉田 匠平, 松浦 一樹
    2017 年 2017 巻 46 号 p. 5-7
    発行日: 2017/03/31
    公開日: 2018/03/29
    ジャーナル フリー

    口絵1 熊本県上益城郡甲佐町白旗山出地点の写真(北を望む.2016年4月21日撮影).降雨による水たまりの分布で,地表変状が生じていることが分かる.水たまりの分布形状は杉型雁行状を呈する.地表地震断層は,写真奥の中位段丘面上の断層崖(池田ほか,2001)へ連続する.

    口絵2 写真1と同地点の曇天時の様子(2016年12月1日撮影).地表地震断層は,地表踏査では,高野―白旗区間の北端部から約6km区間に認められた.本地点は,地表地震断層が認められた範囲のほぼ南端部に位置しており,地表変状は数cm程度の微弱なものであった.

    口絵3 山出トレンチ北壁面.トレンチは,写真1の地表地震断層を横断する方向に掘削した.グリッドは鉛直1m× 水平1m.壁面に露出した地層は,上位より,耕作土,旧耕作土,シルト,シルト/砂礫互層におおまかに区分される.各層は西(写真左手)に向かって撓み込むとともに,断層で明瞭に切られる.主要な断層は2条認められる.断層上盤側において,3層の腐植質シルト層が認められる.それぞれの腐植質シルト層の断層沿いの変位量及び変形の程度は下位層ほど大きく,累積性が認められる.最下位の腐植質シルト層からは,約1万5千年前の放射性炭素年代が得られた.

    口絵4 南部田トレンチ北壁面.トレンチは,宇城市小川町南部田地点で掘削した.グリッドは鉛直1m×水平1m.壁面に露出した地層は,上位より,盛土及び耕作土,旧耕作土,シルト/砂互層,腐植質シルト/砂礫互層におおまかに区分される.地層の変形の様子は,山出トレンチと酷似している.主要な断層は2条認められ,壁面東端(写真右手)の断層はほぼ鉛直,写真中央部の断層は見かけ逆断層の形状を示す.両断層に挟まれた部分は,複雑な変形構造を呈するが,大局的に見ると下位層ほど変形の程度が大きく,変位の累積が認められる.写真中央部に分布する腐植質シルト層からは,約1万2千年前の放射性炭素年代が得られた.

    南部田(口絵5)及び山出(口絵6)両トレンチ調査では,いずれも一般公開を行った.両トレンチの見学者数は延べ1,200名を超えたほか,テレビ・新聞等で多数報道された.

資料
  • 渡辺 満久, 中田 高, 水本 匡起
    2017 年 2017 巻 46 号 p. 9-15
    発行日: 2017/03/31
    公開日: 2018/03/29
    ジャーナル フリー

      We found several faulted landforms and an active fault outcrop around the Minobu fault, Yamanashi Pref., central Japan. The Neguma fault may be a reverse fault dislocating a fan surface (not dated) ca. 13 m vertically. Fluvial terrace surfaces at Wada are classified into W1 to W5 surfaces in descending order. It is probable that the W3 surface was formed in the period of MIS 5 to MIS 4. The Wada fault cuts the Neogene and the overlying gravel distributed in almost the same height with the W3 surface. The dip and strike of the fault plane are N5゜E and 50-60゜W, respectively. The striations are plunging to the south at an angle of ca. 20 degree and blow. The relative vertical component is upthrown on the east side. These structures are indicative of left-lateral movement. The maximum accumulated left-lateral slip since MIS 5 to MIS 4 is 100 m at least.

  • 小松原 琢
    2017 年 2017 巻 46 号 p. 17-25
    発行日: 2017/03/31
    公開日: 2018/03/29
    ジャーナル フリー

      The Kamogawa lowland fault zone had been recognized as a highly active fault zone in the southern Boso peninsula. Many researchers had tried to make clear the activity of this fault zone, however no one could get definite evidence of activity of this fault zone during the Quaternary era. The author tried to describe fundamental features of this fault zone, so carried out re-interpretation of fault landscapes, field geological survey and observation of minor faults in the Neogene deposits around the southern part of the Kamogawa lowland. Four topographic lineaments occur in the study area, they are characterized by series of knick points on hill slopes, and partly characterized by ill-systematic left and partly right-lateral bend of streams. The northern two major lineaments facing north are coincident with geological faults cutting the Neogene deposits with north-side upheaval displacement. The other two lineaments locate south of the Kamogawa lowland, and have left-lateral bend of streams. There are many low-dipping to horizontal slickenlines on the minor fault plain. Many minor faults accompany with hard consolidated fault gauge, but some of minor faults accompany with unconsolidated fault clay and sand. These facts suggest that the two major northern lineaments have contrary sense between topographic features and geological structure, the former suggest north-side subsidence motion in the latest geological age, and the later indicate north-side upheaval movements since the Neogene. Other southern two lineaments have harmonious sense between lateral bend of streams and low angle slickenlines on the minor faults in the Neogene deposits. The author thinks that more researches have to be done to make clear the activity of southern two lineaments during the late Quaternary.

短報
  • 高橋 直也, 石村 大輔, 遠田 晋次, 中田 高, 渡辺 満久
    2017 年 2017 巻 46 号 p. 27-32
    発行日: 2017/03/31
    公開日: 2018/03/29
    ジャーナル フリー

      The 16 April 2016 Mw=7.0 Kumamoto earthquake accompanied ~ 31-km-long surface rupture along the NE part of the Hinagu fault and the Futagawa fault (Kumahara et al, 2016). The surface rupture zone along the Futagawa fault mostly exposed right-lateral strike slip up to 2.2 m (Shirahama et al., 2016), whereas a ~ 10-km-long normal faulting surface rupture with a maximum of 2-m vertical separation mostly along the previously mapped Idenokuchi fault located 1-2 km south of and sub-paralleled to the Futagawa fault.  Here we report an outcrop at the oblique-normal faulting surface rupture at the riverbed of the Kanayama River which runs through Shimojin, Mashiki Town. The site is located 300 m south east of the Futagawa fault and a 50 cm vertical slip occurred at the 2016 Kumamoto earthquake. Although we only had a brief time to observe the outcrop due to levee wall construction, we observed a normal fault (f1) responsible for the 16 April earthquake and cuts recent gravel units. Along the f1 strand, we measured 1.5 m of cumulative vertical displacement of the gravel unit including the 2016 slip. A detrital charcoal recovered from the gravel unit yields an age of 1,990-2,300 cal BP. It enables us to estimate a vertical slip rate of 0.65~0.75mm/yr. Thus it is highly likely that the coseismic simultaneous rupture of the both fault might have occurred repeatedly during the recent events.

  • 西村 卓也
    2017 年 2017 巻 46 号 p. 33-39
    発行日: 2017/03/31
    公開日: 2018/03/29
    ジャーナル フリー

      Many previous studies have revealed distribution of strain rates in the Japanese Islands using data of continuous GNSS station installed since mid 1990’s. They discovered “strain concentration zones” including the Niigata-Kobe Tectonic Zone and the Ou backbone Range in inland and a side of Sea of Japan away from major plate boundaries including the Nankai Trough and Japan Trench. We used GNSS data after the increase of GNSS stations in 2002 and examined distribution of site velocities and strain rates during 2005-2009 with higher spatial resolution. And then, we compared it with major active faults and found that many active faults locate in regions where maximum shear strain rates were high. We also removed elastic deformation due to interplate coupling on the subducting plate interface along the Nankai Trough and compared between distribution of the corrected strain rates and shallow seismicity. The comparison suggests a tendency that the higher maximum shear strain rates, the more frequent shallow M >_ 6 earthquakes occur. We, therefore, suggest that the GNSS data is incorporated into long-term evaluation of large inland earthquakes.

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