Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology GEOMORPHOLOGY, PALEOSEISMOLOGY AND GEOLOGICAL ANALYSIS FOR SEISMIC HAZARD ESTIMATIONS 152 M. Tahir Mian (1) (1) Hostel Building, Street # 1, Sector H‐8/1, Islamabad, PAKISTAN. miantahir45@hotmail.com Abstract: Geomorphology is described as a tool for the identification of active tectonics. A discussion on paleoseismology, with a case study of Caporio site in Rieti region, central Italy and other approaches for seismic hazard estimation is presented. Evaluation of landforms and deposits are providing basic data for seismic hazard estimation. Geomorphic indices and landform assemblages are useful in regional evaluation to identify relative tectonics activities. In the recent years, appreciable progress has been made in the area of paleoseismology where documentation of ages and displacement of various young geological features has had great impact on seismic hazard estimation. Paleoseismic techniques include identification of structures related to strong earthquakes and assessment of the magnitude of the causative event. A fundamental element of any geological analysis for seismic hazard is to estimate the size of earthquake that can occur along a fault or within a region. Other than paleoseismology there are many, commonly used, approaches to earthquake size analysis. Key words: Geomorphology, Paleoseismology, Geological Analysis, Seismic Hazard INTRODUCTION Geomorphology (Seismic Landscape) Presence of well developed tectonics geomorphic features is recognised as the most reliable criteria for identifying active faulting. Reconnaissance work to identify areas where active tectonics is particularly significant generally involves the use of geomorphic indices (sensitive to rock resistance, climatic change, or tectonics process) or assemblages of landforms produced or modified by active tectonics process. Geomorphic Indices and Active Tectonics Geomorphic indices are very useful tools because they quickly provide insight concerning specific area or sites in a region that is adjusting to relatively rapid rates of active tectonics deformation. Stream‐Gradient Index (SL): Anomalously high indices in rock of low or uniform resistance are a possible indicator of active tectonics. Areas where stream‐gradient indices are low are associated with two general conditions: areas where soft sedimentary rocks are abundant and along major strike‐slip faults where horizontal movement has crushed the rocks, producing zones low in resistance to erosion. Mountain‐Front Sinuosity (Smf): Mountain front associated with active uplift are relatively straight, but if the rate of uplift is reduced or ceased, erosional process will begin to form sinuous front that becomes more irregular with time. Low values of Smf (1.1 to 1.14) suggest active tectonics. Ratio of Valley Floor Width to Valley Height: Comparison of Vf values measured from valleys emerging from different mountain fronts or different parts of the same front provides an indication whether streams are actively downcutting (forming V‐shaped valleys with low Vf) in response to active tectonics or are being eroded laterally (forming broad valley with high Vf) in response to relative stability of the front. Figure 1: Alluvial fan morphology: (A) deposition adjacent to mountain front and (B) deposition shifted down‐fan as a result of fan‐head entrenchment Tectonics Geomorphology and Landform Assemblage Alluvial Fans: When the rate of the uplift of the mountain front is high relative to rate of stream‐channel downcutting in the mountain and to fan deposition, then fan head deposition tends to occur, and the youngest fan segment is near the apex of the fan. If the rate of uplift of the mountain is less than or equal to the rate of downcutting of the stream in the mountain, then fan‐ head trenching occurs and deposition is shifted down‐fan. Younger fan segments will then be found well away from the mountain front (Fig. 1 shows the two conditions). Strike‐Slip Faulting: Active strike slip faulting produces a characteristic assemblage of landforms (Fig. 2).
Fig. 2: Assemblage of landforms associated with active strike‐slip faulting (modified from Wesson et al., 1975) PALEOSEISMOLOGY AND SEISMIC HAZARD It is not uncommon to identify the specific dates of strong earthquakes (the ones that are relevant for their impact on the society) along a fault over the past few thousand of years, permitting a fare better quantitative understanding of the local earthquake hazard than has ever been possible before. Figure 3: Sketch diagrams of cross sections of geologic relations that might result from individual paleo‐earthquakes. A tremendous advantage of paleoseismic studies is that they offer the opportunity to extend the seismicity record to an appropriate time window (i.e. the Holocene for interplate areas and the whole Quaternary for intraplate areas). Objectives of Paleoseismology are: To recognise “Seismites” by comprehensive geological investigations, including trenching (Fig. 3). To identify the seismic source producing a specific seismite through tectonics and geophysics. To quantify the “energy” (intensity or magnitude) by using displacement vs magnitude relationships (Fig. 4). A Case Study The magnitude vs displacement relationship has been used for seismic hazard measurement from paleoseismic evidence in the Rieti region, central Italy. 1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology 153 Fig. 4: Diagram showing our model of surface displacement vs. magnitude relationships for a region in crustal extension like the Central Apeninnes in Italy Rieti region is a part of the central Apeninnes, a mountain belt characterized by active extentional tectonics. This area has three paleoseismic sites along the normal fault system at the eastern boundary of Rieti graben which is the main Quaternary structures of this region. Evidence of late Pleistocene and Holocene earthquake rupture is well preserved. One of these sites called Caporio site was personally visited by the author (Photo). At Caporio site a recent Photo: CAPORIO site in Rieti Region of Central Apennines quarry excavation along the fault provides a cross section of a graben in the limestone bedrock. In excess of 20m of upper Quaternary slope deposits fill this graben. The accumulation of such a thick colluvial sequence is the result of a small valley trending perpendicularly to the graben in the mountain slope. Near the base a volcanic layer provides a marker suitable for evaluating the displacement inside the graben. This layer is offset 5m at one fault and dragged against the second fault. Located down relative to the graben and in the non faulted coluvial sequence the same layer is present near the bedrock at the bottom of the slope deposits, suggesting that the 20m of displacement occurred after its deposition. Several channel fill deposits are preserved in another fault in the upper 10m of the graben, which indicate deflection in the original course of such channels. This suggests that recurrent surface faulting events occurred at this site.
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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
Page 103 and 104: CONCLUDING REMARKS The exposed exam
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Page 107 and 108: It is possible that a portion of th
Page 109 and 110: In addition, the Database of histor
Page 111 and 112: were published by IGME (1983) and E
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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
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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 and 164: The stratigraphic sequence (Fig. 4)
Page 165 and 166: especially in the zones of hanging
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
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Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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