Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology ROCK FALL HAZARD MAPPING AND RUN OUT SIMULATION ‐ A CASE STUDY FROM BOLONIA BAY, SOUTHERN SPAIN N. Höbig (1), A. Braun (2), C. Grützner (1), T. Fernández‐Steeger (2) and K. Reicherter (1) (1) Institute of Neotectonics and Natural Hazards, RWTH Aachen University, Lochnerstr. 4‐20, 52056 Aachen. GERMANY. (2) Department of Engineering Geology and Hydrogeology, RWTH Aachen University, Lochnerstr. 4‐20, 52056 Aachen, GERMANY. nicole.hoebig@rwth‐aachen.de Abstract: For a case study on rock fall simulation and hazard mapping the rock fall site around the San Bartolomé mountain ridge in Southern Spain was investigated. The major aim was to analyse the potential of rock falls and the risk of the inhabitants of the villages Betis and El Chaparral. Thus rock fall debris were mapped in detail to analyse them with regard to possible trigger mechanisms. Different empirical and numerical runout simulations were computed and resulted in three possibilities for rock movement. According to trigger mechanisms and predicted runout distances rock fall hazard maps were created and risk analysis was undertaken. Key words: rock falls, runout analysis, trigger mechanism, risk assessment INTRODUCTION A rock fall analysis for hazard mapping in the Bolonia Bay Region close to Tarifa (Spain) was undertaken (Fig. 1). The NE and SW‐side of the San Bartolomé mountain ridge were investigated and mapped in detail to identify the hazardous areas. The deposits of large‐scaled rock fall blocks and debris were mapped and runout distances were determined based on GIS analysis. Since housing areas are located beneath the source zone and large blocks may be found close to houses and settlements, there is a risk for inhabitants to be victim of rock falls and damage of houses or infrastructure. Fig. 1: Overview of the San Bartolomé with the two main investigation areas. GEOLOGICAL SETTING The Bolonia Bay is part of the Flysch zone of the Betic Cordillera. Steep rock walls, which consist of quartzitic Aljibe sandstone shape the NE‐ and SW‐flank of the San Bartolomé mountain ridge. This turbiditic Miocene rock is intensively weathered and shows a distinct, perpendicular shaped joint pattern. The joint system causes large boulders, which are tilted or rotated out of the rock wall as a consequence of gravitational mass movements or possible earthquakes (Silva et al., 2009) (Fig. 2). The deposits are widespread beneath the source zone and reach runout distances of up to 2 kilometers and sizes 52 from 2 m 3 up to 350 m 3 . For more than 300 blocks data on size, shape, type of rock and location were collected. Fig. 2: Intensely joined and weathered source zone with rotating boulders on the N‐side of the San Bartolomé. RUNOUT ANALYSIS As a first step runout analysis was performed using empirical models. Most empirical models for rock fall analysis are based on relationships between topographical factors and runout distances. Two important models are the "Fahrböschung" or "angle of reach", introduced 1932 by Heim and the "shadow angle" by Evans and Hungr (1993). The principle of both models is shown in figure 3. The identification of minimum angles can serve as an approximate runout distance prediction. In different case studies minimum values for the angle of reach and the shadow angle were postulated. Evans and Hungr (1993) observed a lower limit for the angle of reach of 28,43° and a minimum shadow angle of 27,5°. Dorren (2003) proposed a range of 22‐30° for the minimum shadow angle. The angle of reach and the shadow angle were calculated for the rock fall deposits on the west flank of the San Bartolomé mountain ridge using the geographical information system ArcGIS (ESRI). Compared to the proposed minimum angles from the case studies, the models fit quiet well for some areas close to the rock fall source zone (green background in Fig. 4). In areas with large travel distances (more than 300
to 400 m) the calculated values for angle of reach and shadow angle did not fit the usual ranges, especially on the SW‐flank of the San Bartolomé mountain ridge, see white background in Fig. 4. Fig. 3: Angle of reach (β) and shadow angle (α) according to Evans and Hungr (1993) with source zone A, talus slope B and shadow C. Furthermore, a numerical trajectory based model, the computer programme Rockfall 6.1 was used for runout analysis. Rockfall 6.1 enables the simulation of the fall of rocks down a two‐dimensional slope with certain surface conditions (Spang 1995; Spang 2001). The programme computes the rock fall path, total kinetic energy, bounce heights and runout distances. The simulation was performed for several slope profiles on both sides of the mountain under recent surface conditions (covered soil) and under extreme conditions 1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology) 53 Fig. 4: Calculated shadow angles with minimum shadow angles (bare rock). The slope profiles were identified with GIS by extracting steepest paths from a digital elevation model. The results of the simulation under recent conditions are shown in Fig. 5. The simulation fits very well the depositions on the east flank and on the NW‐flank of the San Bartolomé mountain ridge. Again, the areas with large travel distances on the SW‐flank mark an exception. Even under extreme conditions the simulation does not achieve the runout distances of the depositions. Fig. 5: The results of the numerical rockfall simulation with Rockfall 6.1 under recent conditions.
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Abstracts Volume Archaeoseismology
Page 4 and 5: Edita e imprime: Sección de Public
Page 7 and 8: 1 st INQUA‐IGCP‐567 Internation
Page 9 and 10: the importance of performing absolu
Page 11 and 12: indicative of strike‐slip faultin
Page 13 and 14: DISCUSSION Based on detailed field
Page 15 and 16: Table 1: Source parameters used in
Page 17 and 18: elations. However, the attenuation
Page 19 and 20: mostly concentrated on the DST in I
Page 21 and 22: continuous support. N. Shalev, G. G
Page 23 and 24: The tumulus initial morphology show
Page 27 and 28: our understanding and prediction of
Page 29 and 30: spans, with a threefold increase fr
Page 31 and 32: Nevertheless the lack of research,
Page 33 and 34: earthquake. In uncertain cases, as
Page 35 and 36: The site reconnaissance successfull
Page 43 and 44: Table 1: Surface faulting extent re
Page 47 and 48: The graben has a markedly asymmetri
Page 53: are a valuable addition to the rout
Page 57 and 58: RISK ANALYSIS The created hazard ma
Page 59 and 60: 290 m water depth the basin accumul
Page 61 and 62: CONCLUSIONS From the palaeostress a
Page 63 and 64: Fig. 2: Geological map of the study
Page 71 and 72: however, the massive 1995 earthquak
Page 73 and 74: mapped on the slopes above Qiryat
Page 77 and 78: interpreted as either an expression
Page 79 and 80: The Shepran Cave is located on the
Page 87 and 88: properties. The mean value of speci
Page 91 and 92: excavated a deep trench, thus provi
Page 99 and 100: Fig. 8: Seismic ground shaking may
Page 101 and 102: elevant qualitative criteria are po
Page 103 and 104: 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
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1 st INQUA‐IGCP‐567 Internation
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The city collapsed and it was aband
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Fig. 4: Slipped lintel in a window
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to overcome shear resistance and in
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Newmark, N.M. (1965). Effects of ea
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associated with various tsunamis (<
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this method, the magnitude of the e
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etween pieces. Fragments have not f
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on the exact date of the earthquake
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THE ARCHAEOLOGICAL ZONE COLOGNE Aft
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affecting the Alcázar was about 50
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archaeoseismology could instead bec
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In the area later occupied by Vilni
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Fig. 2: Assemblage of landforms ass
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Fig. 8: Data and regression for mag
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Fig. 3: Relationships among magnitu
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CONCLUSIONS 1. Spatial distribution
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The stratigraphic sequence (Fig. 4)
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especially in the zones of hanging
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interpretations are based on relati
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Fig. 5: Gray‐scale value laser im
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(~40 cm), F‐ rotated building str
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Index 1 st INQUA‐IGCP‐567 Inter
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Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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