Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology THE ORIGIN OF ROCKFALLS AND THE FORMATION OF HANGING VALLEYS ALONG THE LA LAJA RANGE FRONT (TARIFA, S SPAIN) A. Vollmert (1), K. Reicherter (2) and C. Grützner (2) (1) Lehrstuhl für Ingenieur‐ und Hydrogeologie, RWTH Aachen University, Lochnerstr. 4‐20, 52056 Aachen. GERMANY. andre.vollmert@lih.rwth‐aachen.de (2) Institut für Neotektonik und Georisiken, RWTH Aachen University, Lochnerstr. 4‐20, 52056 Aachen. GERMANY. Abstract: The La Laja Range Front is part of the Sierra de la Plata (Cádiz, south Spain). The steeply inclined sedimentary sandstone strata display a close system of discontinuities such as eastwards dipping bedding planes and E‐W striking tension cracks which clearly reduce slope stability. Due to its special orientation wedge failures on two intersecting planes as well as toppling and planar failures of columns of a width of 2 m on average can be observed along the range front. The formation of three hanging valleys as a consequence of those wedge slides is one of the main topics discussed in this study. However, these landslides are also caused by water pressure from joints and bedding planes, weathering, erosion and presumably historical or even recent seismicity. Because of the steep slope below the range front falling blocks reach distances of up to 600 m and are therefore characterised as “long runout rockfalls”. Key words: rockfalls, hanging valleys, runout distances INTRODUCTION Elonging to the western foothills of the Betic Cordilleras the Sierra de la Plata Mountain Range reaches a maximum elevation of 459 m above sea level. It is located within the region of the Strait of Gibraltar, straight north of the Bay of Bolonia (Cádiz, south Spain). Along the eastern side of this Mountain Range the La Laja Range Front follows a N‐S direction for about 1 km length with a maximum height of about 80 m (Fig. 1). It shows tectonical and geomorphological anomalies, which are associated with active normal faulting processes. Triangular facets and hanging tributary valleys at about 30 m height provide evidence for this. In addition, a weakly weathered reddish ribbon at the basal knick‐point, showing no lichen growth, might verify the recent tectonic uplift of the La Laja. This study describes the kinematics leading to rockfalls and provides explanations for the genesis of hanging valleys along the escarpment. Additionally, long runout rockfalls are described. GEOLOGY OF THE STUDY AREA The bedrock geology of the Sierra de la Plata is dominated by the Campo de Gibraltar flyschs, which were formed in Fig. 1: Morphology of the La Laja Escarpment 162 lowermost Cretaceous to early Miocene times (Durand‐ Delga, 2006). These highly tectonized turbiditic sandstone deposits, which can also be found in northern parts of Africa, consist of Eocene to Aquitanian Aljibe, Algeciras and Bolonia sandstone units. Between the Burdigalium and the late Tortonian these strata were thrust over plastic Cretaceous to Eocene turbiditic sandy and clayey sediments of the Facinas and Almarchal units. The emplacement of the nappes took place due to the alpine orogenesis under a WNW‐ESE compressive stress field (Weijermars, 1991), that led to an intensive folding. Fig. 2 shows a map section of the Campo de Gibraltar region relating to the geology of the Sierra de la Plata. DISCONTINUITY SET OF THE LA LAJA RANGE FRONT The steeply inclined sandstone strata offers a close discontinuity set, which mainly consists of bedding planes dipping eastwards and E‐W striking tension cracks. Due to these failures in bedrock, the La Laja Range Front is affected by toppling, falling and sliding processes leading to rockfalls. The exact reasons for these kinematics are described in the following chapters. Along the range front tension cracks have been analysed
especially in the zones of hanging valleys. Fig. 3 indicates the orientation of discontinuities in terms of their strike direction. The dip angel is shown next to the joint diagram. Apart from some exceptions, the measured planes are striking nearly perpendicular to the escarpment from east to west. With an average angle of 70°‐80° they dip uniformly into northern and southern directions. The distinctive topographic front shown in Fig. 1 evidence a “X‐formation” of two big crossing joints, which represent the described inclination of tension cracks in a two‐dimensional way. Fig. 3: Joint diagrams presenting the orientation of tension cracks in two of the hanging valleys The La Laja slope face represents the orientation of bedding planes. Due to this fact these discontinuities also dip with approximately 70° into eastern direction. They separate abundant, plan‐parallel columns with a width of 2 m on average. This is illustrated in Fig. 4, which shows the structure of the rock formation using a 3D‐model developed by a light detection and ranging method (LIDAR). Fig. 4: 3D‐LIDAR shot of the steeply inclined sandstone strata LIDAR sends a wide‐ranged laser beam and is able to detect the scattered light. The distance to an object is determined by the measured time delay between transmission of a pulse and detection of the reflected signal. In contrast to RADAR, the wavelengths of LIDAR are much. In order to reach every gap, two shoots were taken from different angles and put together with a special software. By using this method the range front was measured with millimeter precision. KINEMATICS AND FORMATION OF HANGING VALLEYS Due to the special orientation of bedding planes and tension cracks toppling failures of columns as well as 1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology 163 wedge sliding failures on two intersecting planes can be observed along the range front. According to Dikau (1996), mathematical constraints shown in Table 1 depict the criteria whether a toppling or a sliding process occurs. Table 1: Stability criteria of a cubic block < b/h > tan A block is stable > b/h > tan < b/h < tan > b/h < tan A block will slide but not topple A block will not slide but topple A block will both slide and topple The geometrical data is based on a single block model, where is the slope angle, is the friction angle, and b/h is the ration width versus height (Fig. 5). Fig. 5: Marginal case of a block between a stable position, a toppling and a sliding process Toppling Apart from these basics joint and bedding plane water pressures can trigger or accelerate rock movement. Such influences can be observed along the northern part of the range front, where potholes give evidences for water circulation between sandstone strata. Hence the columns lose contact to in‐situ formations and topple eastwards. Fig. 6 clarifies this process and points out the sedimentation of lose masses between displaced strata. The sediments weight acts like a wedge and accelerates the toppling. The progression of weathering leads to rockfalls with its source zone at the peak of the exposed columns. Fig. 6: E‐Toppling of thick columns along the escarpment
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Abstracts Volume Archaeoseismology
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Edita e imprime: Sección de Public
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1 st INQUA‐IGCP‐567 Internation
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the importance of performing absolu
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indicative of strike‐slip faultin
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DISCUSSION Based on detailed field
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Table 1: Source parameters used in
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elations. However, the attenuation
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mostly concentrated on the DST in I
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continuous support. N. Shalev, G. G
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The tumulus initial morphology show
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our understanding and prediction of
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spans, with a threefold increase fr
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Nevertheless the lack of research,
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earthquake. In uncertain cases, as
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The site reconnaissance successfull
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Table 1: Surface faulting extent re
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The graben has a markedly asymmetri
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are a valuable addition to the rout
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to 400 m) the calculated values for
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RISK ANALYSIS The created hazard ma
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290 m water depth the basin accumul
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CONCLUSIONS From the palaeostress a
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Fig. 2: Geological map of the study
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however, the massive 1995 earthquak
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mapped on the slopes above Qiryat
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interpreted as either an expression
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The Shepran Cave is located on the
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properties. The mean value of speci
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excavated a deep trench, thus provi
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Fig. 8: Seismic ground shaking may
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elevant qualitative criteria are po
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CONCLUDING REMARKS The exposed exam
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produced by significantly different
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It is possible that a portion of th
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In addition, the Database of histor
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were published by IGME (1983) and E
Page 113 and 114: 1 st INQUA‐IGCP‐567 Internation
Page 115 and 116: The city collapsed and it was aband
Page 119 and 120: Fig. 4: Slipped lintel in a window
Page 121 and 122: to overcome shear resistance and in
Page 123 and 124: Newmark, N.M. (1965). Effects of ea
Page 125 and 126: associated with various tsunamis (<
Page 129 and 130: this method, the magnitude of the e
Page 133 and 134: etween pieces. Fragments have not f
Page 137 and 138: on the exact date of the earthquake
Page 139 and 140: THE ARCHAEOLOGICAL ZONE COLOGNE Aft
Page 143 and 144: affecting the Alcázar was about 50
Page 147 and 148: archaeoseismology could instead bec
Page 149 and 150: In the area later occupied by Vilni
Page 155 and 156: Fig. 2: Assemblage of landforms ass
Page 157 and 158: Fig. 8: Data and regression for mag
Page 159 and 160: Fig. 3: Relationships among magnitu
Page 161 and 162: CONCLUSIONS 1. Spatial distribution
Page 163: The stratigraphic sequence (Fig. 4)
Page 169 and 170: interpretations are based on relati
Page 173 and 174: Fig. 5: Gray‐scale value laser im
Page 177 and 178: (~40 cm), F‐ rotated building str
Page 179 and 180: Index 1 st INQUA‐IGCP‐567 Inter
Page 181: 1 st INQUA‐IGCP‐567 Internation
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