Abstract: This paper elucidates the relationship between landslides, geologic structures, and hydrothermal alteration zones based primarily on X-ray powder diffraction and uniaxial compressive strength tests on weakly weathered and hydrothermally altered rocks from the Ohekisawa-Shikerebembetsugawa landslide area in Teshikaga Town, Hokkaido, Japan. The OHS (Ohekisawa slide) occurred on a dip slope of sedimentary rocks from the Upper Miocene Shikerepe Formation within a homocline, and also on weathered and hydrothermally altered rocks within the boundary area between the hydrothermal smectite zone and smectite-bearing mordenite zone. The SHS (Shikerebembetsugawa slide) occurred on a dip slope of sedimentary rocks from the Upper Miocene Hanakushibe Formation within wavy folds and was also controlled by a cap rock of Teshikaga Volcano Somma Lava. The SHS occurred also on weathered and hydrothermally altered rocks within the boundary area between the hydrothermal smectite zone and smectite-bearing laumontite zone. The mechanical properties of smectite, smectite-bearing mordenite, and smectite-bearing laumontite zone weakly weathered rocks indicate that they are very weak, soft rocks. These landslides are regarded as HAZLs (hydrothermal alteration zone landslides). The hydrothermal alteration yielding smectite is thus closely related to these two ancient landslides, suggesting that the potential for HAZLs within a hydrothermal area can be assessed based on the swelling clay mineral-bearing hydrothermal alteration types, dip slope, and cap rock.
　　Key words: HAZL (hydrothermal alteration zone landslide), swelling clay mineral-bearing hydrothermal alteration zone, weathered and hydrothermally altered soft rock, dip slope, cap rock.
　　1. Introduction
　　The existence of swelling clay minerals in slip surface clays in landslides is one of the main causes of reactivation-type landslides [1]. Although DZLs(diagenetic zone landslides) and HAZLs(hydrothermal alteration zone landslides), based on geological landslide classification, have soft or hard bedrock, and are induced by earthquakes, they operate by the swelling of clay minerals in slip surface clays, as well as a rise in underground water pressure during heavy rains, long spells of rainy weather or the spring thaw. HAZLs are defined as landslides that occur within hydrothermal alteration zones formed by volcanic-hydrothermal systems over geologic time and historic period [2-5]. For instance, HAZLs in Japan include those at the Asamushi Hot Spring and Hakkoda in Aomori Prefecture, Onikobe and Naruko Hot Springs in Miyagi Prefecture, Manza and Kumaike in Gunma Prefecture, Mt. Sounzan and Owakudani in Kanagawa Prefecture, Myoban in Oita Prefecture, etc. [6-8]. These HAZLs occurred mainly within acid hydrothermal alteration zones formed by fumarolic gases derived from active acid hydrothermal systems. Heavy rains accompanying the approach of a typhoon led to the occurrence of the 2001 Manza Slide and Kumaike Failure-Flow within neutral and acid hydrothermal alteration zones formed by the hydrothermal systems of the active Kusatsu-Shirane volcano [8]. In contrast, HAZLs occurring on weathered and hydrothermally altered rocks, which were formed mostly by ancient neutral hydrothermal systems of the Upper Miocene Ikutawara and Yahagi Formations of the Ikutahara area in Engaru Town, and the Kanehana area in Rubeshibe Town, Kitami City, northeastern Hokkaido, Japan, from the Late Miocene to the Early Pliocene, are closely related in space to interstratified illite/smectite mineral and smectite zones that are characterized by swelling clay minerals, and swelling clay mineral-bearing zeolite zones, as well as kaolin mineral, illite, and K-feldspar zones [2, 9-13]. In addition, HAZLs took place within hydrothermal smectite zone or smectite-bearing mixed-layer mimeral zone along the Median Tectonic Line in Shikoku, Japan [14, 15].
　　The strength of fresh and altered rocks, including hydrothermally altered or weathered rocks, is generally evaluated based on their uniaxial compressive strengths. They are divided into hard rocks with uniaxial compressive strengths greater than 25 MPa and soft rocks that can bear less than 25 MPa in a forced-dry state. The uniaxial compressive strength test, in particular, for weakly weathered and hydrothermally altered rocks taken from outcrops or floats in a hydrothermal field is effective to estimate mechanical properties of landslide bedrocks and bodies in weathered and hydrothermally altered rockslides.
　　Teshikaga Town is situated in eastern Hokkaido, Japan (Fig. 1). A number of studies have examined landslides in this town, including papers by Maeda et al. [5, 16-18]. From the Middle Miocene to the Early Pleistocene, the Sattomonai-Okushunbetsu area in the western part of the town (Fig. 1) was subject to intense terrestrial volcanic-hydrothermal activity [19, 20]. Two ancient landslides have been identified, based on topography, in the southern part of the Okushunbetsu area (Figs. 1 and 2). These ancient landslides have been tentatively named OHS (Ohekisawa slide) and SHS (Shikerebembetsugawa slide) (Fig. 2). The OHS is 350 m wide and 410 m long, whereas the SHS is 175-450 m wide and 595 m long (Table 1).
　　The purpose of this study is to clarify the relationship between HAZLs, geologic structures, and hydrothermal alteration zones in the Ohekisawa-Shikerebembetsugawa landslide area.
　　2. Methods and Equipment
　　The relationship between HAZLs, geologic structures, and hydrothermal alteration zones was examined based on an aerial photograph interpretation, a 1:5,000-scale topographical and geological mapping, and an XRD (X-ray powder diffraction) and uniaxial compressive strength tests on weakly weathered and hydrothermally altered rocks from the southern part of the Okushunbetsu area.
　　Weakly weathered and hydrothermally altered rocks were collected from the ground surface. The modes of occurrence of these altered rocks were examined in the field, and the hydrothermal alteration minerals in the rocks were determined primarily by the XRD test. Opal-CT was determined based on the XRD patterns obtained by Jones et al. [21]. Clay minerals in the rocks were identified from the diffraction patterns of untreated and ethylene glycol-treated samples. XRD was performed using a Rigaku RAD-3R diffractometer (30 kV, 20 mA) equipped with a Cu tube, an Ni filter, a 0.3 mm receiving slit, and 1° divergence and scattering slits.
　　Rock samples for uniaxial compressive strength tests were taken using a drilling machine. The specimens were 50 mm in diameter and cut to 100 mm lengths using a diamond cutter. The uniaxial compressive strength tests were performed on core specimens in a forced-dry state, after drying in an electric oven at a temperature of 60 ± 3 °C for 4 days or more to achieve a constant mass [13].
　　
　　Fig. 1 Location map of the Sattomonai-Okushunbetsu area in Teshikaga Town, eastern Hokkaido, Japan.
　　
　　Fig. 2 Two ancient landslide areas in the southern part of the Okushunbetsu area. The topographical map is part of a 1:25,000 map of Pekereyama from the Geographical Survey Institute of Japan.
　　Table 1 Analysis of ancient landslides in the southern part of the Okushunbetsu area.
　　
　　3. Geologic Setting
　　The Teshikaga district is within the inner belt of the Kurile arc [22]. The geology of this district was described mainly by Maeda et al. [20, 23-25]. The geology consists mainly of Neogene and Quaternary Systems, as well as Neogene and Quaternary intrusive rocks. The Neogene System can be divided, in order of ascending stratigraphy, into the Middle Miocene Ikurushibe Formation, the Upper Miocene Oteshikaushinai, Hanakushibe, Shikerepe Formations, the Upper Pliocene Ikurushibeyama Lava and Shikerepeyama Lava [23-26]. The Quaternary System can be subdivided, in ascending stratigraphic order, into the Shikerepempetsu Formation, Pekereyama Lava, Teshikaga Volcano Somma Lava, higher fluvial terrace deposits, lower fluvial terrace deposits, talus deposits, landslide deposits, and alluvial river deposits(Table 2). Neogene and Quaternary intrusive rocks are chiefly composed of andesite dikes, with lesser basalt and rhyolite dikes [24]. These dikes are generally affected by hydrothermal alteration and mineralization. K-Ar ages of 2.43 ± 0.45 and 2.3 ± 0.8 Ma for the andesite dikes [24] are believed to indicate the time of the hydrothermal alteration and mineralization. The Neogene formations and the Lower Pleistocene Shikerepempetsu Formation underwent extensive hydrothermal alteration and mineralization during the Pliocene to Early Pleistocene epoch [19, 20]. The rock facies of the Neogene and Lower Pleistocene formations related to landslides in the Sattomonai-Okushunbetsu area are as follows:
　　The Oteshikaushinai Formation is chiefly composed of alternating andesitic and dacitic lapilli tuff, tuff breccia, and tuff, with basal tuffaceous conglomerate, sandstone, and mudstone;
　　The Hanakushibe Formation conformably overlies the Oteshikaushinai Formation and is chiefly composed of mudstone, and alternating fine sandstone and tuff with andesitic tuff breccia and lapilli tuff;
　　The Shikerepe Formation conformably overlies the Hanakushibe Formation and is chiefly composed of andesitic and dacitic volcaniclastic rocks, tuffaceous sandstone and mudstone, and dacitic welded tuff;
　　The Ikurushibeyama Lava unconformably overlies the Shikerepe Formation and is chiefly composed of andesitic lava, volcanic breccia, and tuff breccia;
　　Pekereyama Lava is chiefly composed of basaltic lava and has not suffered hydrothermal alteration [24];
　　Teshikaga Volcano Somma Lava is chiefly composed of basaltic and andesitic lavas and volcanic breccia and has also not suffered hydrothermal alteration.
　　The geologic structures of the Neogene formations in the study area (Fig. 3) are as follows:
　　An NW-SE trending anticline can be mapped in the lower reaches of Oteshikaushinaisawa Creek and the Hanakushibegawa River in the northwestern part of the study area (Fig. 3). An ENE-WSW trending syncline leads to folds in the rocks in the middle reaches of the Hanakushibegawa River in the western part of the study area (Fig. 3), and an NNW-SSE trending anticline leads to folds in the rocks in the middle reaches of the Shikerebembetsugawa River in the southeastern part of the study area (Fig. 3). Folds with wavelengths of several hundred meters and ENE-WSW fold axes are common in the middle reaches of the Shikerebembetsugawa River in the southern part of the study area (Fig. 3). Comparable folds with NNE-SSW and ENE-WSW axes are mapped in the upper reaches of the Shikerebembetsugawa River in the southwestern part of the study area (Fig. 3);
　　The Shikerepe Formation in the northeastern part of the study area is characterized by an NNW-SSE to NW-SE striking homocline and with dips less than 10° toward the NE (Fig. 3);
　　The faults are aligned in NE-SW, NNE-SSW, and NW-SE directions in the study area (Fig. 3).
　　Hydrothermal alteration minerals identified in the study area include quartz, opal-CT, K-feldspar, albite, chlorite, illite, interstratified illite/smectite minerals, interstratified chlorite/smectite minerals, smectite, nacrite, dickite, kaolinite, analcite, laumontite, heulandite-clinoptilolite series minerals, mordenite, chabazite, stilbite, calcite, alunite, minamiite, natroalunite, and pyrite.
　　Hydrothermal alteration can be zoned according to the occurrence of characteristic minerals such as silica minerals, feldspars, clay minerals, zeolites, and alunite. The hydrothermal alteration zones have been identified in the study area, based on mineral assemblage, propylitic, laumontite, heulandite, analcite, mordenite, clinoptilolite, stilbite, smectite, interstratified illite/smectite mineral, illite, K-feldspar, and alunite-quartz zones (Table 3). These alteration zones are oblique to bedding planes within the formations described previously.
　　Table 2 Stratigraphy and volcanic-hydrothermal activity in the Sattomonai-Okushunbetsu area. Modified from Ref. [20].
　　
　　
　　Fig. 3 Geological map of the southern part of the Okushunbetsu area. Modified from Refs. [24, 27-29].
　　The distribution of landslide-related hydrothermal alteration zones, and the mineral composition and uniaxial compressive strengths of their alteration zone rocks documented in the study area are as follows:
　　The laumontite zone is widely distributed in the western part of the study area (Fig. 4) and its rocks consist mainly of laumontite with lesser amounts of smectite, quartz, albite, illite, chlorite, calcite, and pyrite (Table 3). The uniaxial compressive strengths of laumontite zone weakly weathered tuffaceous conglomerates from the Ikurushibe Formation in a forced-dry state range from 84.07 MPa to 91.10 MPa, with an average of 87.59 MPa, whereas the strengths of smectite-bearing laumontite zone weakly weathered lapilli tuff and fine tuff from the Hanakushibe Formation are 6.18 MPa and 12.91 MPa, respectively(Table 4).
　　The mordenite and clinoptilolite zones are widely distributed in the eastern and southern parts of the study area and occur locally in the western part (Fig. 4). The mordenite zone rocks consist mainly of mordenite with lesser amounts of opal-CT, quartz, smectite, clinoptilolite, and pyrite (Table 3). The uniaxial compressive strengths of smectite-lacking mordenite zone weakly weathered fine tuffs from the Shikerepe Formation in a forced-dry state range from 17.95 MPa to 44.64 MPa, with an average of 27.37 MPa, whereas smectite-lacking mordenite zone weakly weathered pumice tuffs from the Shikerepe Formation have strengths ranging from 7.24 MPa to 10.62 MPa, with an average of 8.93 MPa. The uniaxial compressive strengths of smectite-bearing mordenite zone weakly weathered pumice tuffs from the Shikerepe Formation range from 14.00 MPa to 19.69 MPa, with an average of 17.15 MPa (Table 4).
　　The smectite zone occurs mainly in the northeastern and southwestern parts of the study area (Fig. 4), and the rocks consist mainly of smectite with lesser amounts of opal-CT, quartz, and pyrite (Table 3). The uniaxial compressive strengths of smectite zone weakly weathered fine tuffs from the Hanakushibe Formation in a forced-dry state range from 6.28 MPa to 7.94 MPa, with an average of 7.11 MPa, and smectite zone weakly weathered fine tuff from the Shikerepe Formation has a strength of 10.67 MPa(Table 4).
　　4. Results and Considerations
　　The bedrocks of the OHS in the southern part of the Okushunbetsu area are weathered and hydrothermally altered and are composed of coarse tuff and tuffaceous medium sandstone of the Upper Miocene Shikerepe Formation. The landslide has a dip slope structure with an NNW-SSE strike and a dip direction of 7°E(Fig. 3 and Table 1). The scarp of the OHS consists of an N-S striking andesite dike that is 125 m wide and 325 m long (Fig. 3). The bedrock under the upper part of the landslide body is composed of smectite-bearing mordenite zone rocks, whereas that under the middle and lower parts of the landslide body is composed of smectite zone rocks (Fig. 4 and Table 1). As described previously, the uniaxial compressive strengths of smectite-bearing mordenite zone weakly weathered pumice tuffs from the Shikerepe Formation range from 14.00 MPa to 19.69 MPa, with an average of 17.15 MPa and smectite zone weakly weathered fine tuff from the Shikerepe Formation has a strength of 10.67 MPa (Table 4). These bedrocks, therefore, are classified as soft rocks.
　　The SHS bedrock also consists of weathered and hydrothermally altered rocks, mainly fine tuff, mudstone, and lapilli tuff of the Upper Miocene Hanakushibe Formation. The landslide has a dip slope structure as a whole because the bedrocks have a synclinal structure on an ENE-WSW axis; the bedrocks under the upper and middle parts of the landslide body dip 10° toward the SE, whereas those of the lower part dip 0-5° toward the NW (Fig. 3). The landslide scarp consists of a cap rock of Teshikaga Volcano Somma Lava. The bedrocks under the upper part of the landslide body are composed of smectite zone rocks, whereas those under the middle and lower parts are composed of smectite-bearing laumontite zone rocks (Fig. 4 and Table 1). As described previously, the uniaxial compressive strengths of smectite-bearing laumontite zone weakly weathered lapilli tuff and fine tuff from the Hanakushibe Formation in a forced-dry state are 6.18 MPa and 12.91 MPa, respectively, and those of smectite zone weakly weathered fine tuffs from the Hanakushibe Formation range from 6.28 MPa to 7.94 MPa, with an average of 7.11 MPa. These, therefore, are also classified as soft rocks.
　　Both of the landslides, therefore, occurred mechanically on very weak rocks consisting of smectite-bearing weathered and hydrothermally altered soft rocks such as smectite, smectite-bearing mordenite, and smectite-bearing laumontite zone rocks.
　　The OHS occurred on weathered and hydrothermally altered rocks within the boundary area between the smectite zone and the smectite-bearing mordenite zone, whereas the SHS occurred on weathered and hydrothermally altered rocks within the boundary area between the smectite and smectite-bearing laumontite zones. This circumstance has been reported in only a few other cases such as the Yasukuni landslide area [10] and the Kanehana landslide area [12] in northeastern Hokkaido, Japan.
　　Table 3 Mineral assemblages of hydrothermal alteration zones.
　　
　　
　　Fig. 4 Hydrothermal alteration map of the southern part of the Okushunbetsu area.
　　Three ancient landslides are known to have occurred in the Sattomonai-Okushunbetsu area in the western part of Teshikaga Town [18]. Two in the Sattomonai area and one in the northern part of the Okushunbetsu area occurred within the hydrothermal interstratified illite/smectite mineral zone and the smectite zone, respectively [18]. These landslides are classified as HAZLs based on their bedrock geology. According to Ref. [18], the bedrocks of the landslide in the northern part of the Okushunbetsu area are weathered and hydrothermally altered rocks composed mainly of fine tuff of the Upper Miocene Hanakushibe Formation. This fine bedrock tuff belongs to a smectite-bearing mordenite zone having a hydrothermal alteration mineral assemblage of mordenite-smectite-quartz whereas a debris of fine tuff in the landslide body belongs to a smectite-lacking mordenite zone having an assemblage of mordenite-quartz. The landslide occurred on a dip slope of fine tuff from the Hanakushibe Formation. The scarp of the landslide consists mainly of basaltic lava of Pekereyama Lava, which is a cap rock. Therefore, the landslide is classified as a weathered and hydrothermally altered rockslide based on its landslide body [7, 30, 31] and as an HAZL on the basis of bedrock geology. In contrast, the bedrocks of the two landslides in the Sattomonai area, according to Ref. [18], are also weathered and hydrothermally altered rocks and are mainly composed of lapilli tuff of the Upper Miocene Oteshikaushinai Formation and Lower Pliocene Ikurushibeyama Lava. Debris of lapilli tuff in the landslide body belongs to the interstratified illite/smectite mineral zone having a hydrothermal alteration mineral assemblage of quartz-interstratified illite/smectite minerals-calcite. The scarps of these landslides consist mainly of andesitic lava from the Ikurushibeyama Lava, which is a cap rock. Therefore, the landslides are classified as weathered and hydrothermally altered rockslides based on their landslide body and as HAZLs based on their bedrock geology.
　　In the Sattomonai-Okushunbetsu area, five ancient landslides, including the OHS and the SHS occurred within the hydrothermal smectite, interstratified illite/smectite mineral, smectite-bearing mordenite, and smectite-bearing laumontite zones. All these landslides are classified as HAZLs on the basis of their bedrock geology, and weathered and hydrothermally altered rockslides on the basis of their landslide body types. All these landslides also occurred within weathered and hydrothermally altered soft rocks. This indicates that HAZLs occur within weathered and hydrothermally altered, very weak, soft rocks of the smectite zone, interstratified illite/smectite mineral zone, and smectite-bearing zeolite zone such as the smectite-bearing mordenite zone, smectite-bearing laumontite zone, etc.. The HAZL potential within a hydrothermal area can, therefore, be assessed by the swelling clay mineral-bearing hydrothermal alteration types, dip slope, and cap rock.
　Table 4 Uniaxial compressive strength of hydrothermal alteration zone rocks in a forced dry-state, dried in an electric oven at 60 ± 3 °C, in the southern part of the Okushunbetsu area.
　　
　　5. Conclusions
　　The relationship between two ancient landslides, the OHS and SHS, and their geological characteristics is as follows:
　　Both of the landslides occurred within the Pliocene to Early Pleistocene hydrothermal alteration zones in the tuffaceous clastic rocks and the volcaniclastic rocks of the Upper Miocene Hanakushibe and Shikerepe Formations. These landslides are classified as HAZLs based on bedrock geology;
　　Both of the landslides are classified as weathered and hydrothermally altered rockslides based on their landslide bodies.
　　The OHS occurred within the boundary area between the hydrothermal smectite zone and smectite-bearing mordenite zone.
　　The SHS occurred within the boundary area between the hydrothermal smectite zone and smectite-bearing laumontite zone.
　　The scarp of the OHS consists of a dike, whereas that of the SHS consists of a cap rock of lava.
　　The HAZLs in the Sattomonai-Okushunbetsu area of western Teshikaga Town, Hokkaido, Japan, occurred within weathered and hydrothermally altered, very weak, soft rocks such as those of the smectite, interstratified illite/smectite mineral, smectite-bearing mordenite, and smectite-bearing laumontite zones.
　　The HAZL potential within a hydrothermal field can be assessed based on the swelling clay mineral-bearing hydrothermal alteration types, dip slope, and cap rock.
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