Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology EVALUATION OF ROCKFALL HAZARD TO THE TOWN OF QIRYAT SHEMONA, N. ISRAEL – POSSIBLE CORRELATION TO EARTHQUAKES M. Kanari (1), O. Katz (2), N. Porat (2), R. Weinberger (2) and S. Marco(1) (1) Department of Geophysics and Planetary Sciences, Tel‐Aviv University, Tel‐Aviv 69978, ISRAEL. kanarimo@tau.ac.il (2) Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem 95501, Israel Abstract: We estimate rockfall hazard for the town of Qiryat‐Shemona, situated at the bottom of a fault controlled escarpment bounding the Dead Sea Transform at N. Israel. Aerial photos predating the town founding show boulders of 1 m 3 ‐150 m 3 within the town premises. The study determines: a. the area subject to rockfall hazard and predicted kinetic properties of falling blocks (using CRSP v4); b. ages of rockfall events and estimated recurrence interval, and possible triggering by earthquakes (using OSL dating of block‐soil interface); It is concluded that particular parts at SW of town are subject to rockfall hazard. OSL age analysis of rockfall events coincides with selected M>6.5 earthquakes and yields 850 years recurrence time and last rockfall probably triggered by the 1202 AD earthquake. Block velocities of 10–15 m/s and kinetic energy of 18,000–45,000 kJ are predicted for block volumes of ca. 125 m 3 . Key words: rockfall, hazard, rockfall simulation INTRODUCTION The study evaluates rockfall hazard for the town of Qiryat‐Shemona, which lies in the northern Hula Valley (N. Israel), part of a series of extensional basins that formed along the Dead Sea Transform (DST) active fault system (Freund et al., 1970; Garfunkel, 1981). The town is located at the foot of the fault‐controlled Naftali Mountain ridge, which rises to its west (Fig. 1). New quarters of the town are being planned and built below the ridge crawling up the slopes. These slopes are spotted Fig. 1: Location map of Qiryat‐Shemona (study area). Dead Sea Transform marked in pink 70 with large, scattered, cliff‐derived limestone boulders, which have apparently traveled there by rockfall mechanism. The 40‐m‐thick Ein‐El‐Assad Formation limestone outcrops provide a source material for rock blocks. Aerial photos from 1946–1951 show boulders of volumes of 1–150 m 3 situated within the now built town premises (Fig. 2). The study examines: (a) which are the feasible downhill trajectories of falling blocks, where do blocks stop, and what is the urban‐area subject to rockfall hazard? (b) when did rockfalls occur and what is the estimated recurrence interval? To answer these questions hundreds of rock‐blocks were EEA Fig. 2: Map of the study area (in black rectangle). Ein‐El‐ Assad Formation escarpment (EEA) is marked by blue line; town area in red dashed line
mapped on the slopes above Qiryat‐Shemona using both field surveys and aerial photos and their volume and spatial distributions are analyzed; burial ages of soil samples from beneath large fallen blocks were determined by OSL; rockfall trajectories were simulated using the commercial program CRSP v4 (Jones et al., 2000). Hazard evaluation maps for Qiryat‐Shemona were compiled from the results of rockfall simulations. Simulated analyses of block velocity and kinetic energy may be used as parameters for the design of mitigation of rockfall damage for Qiryat‐Shemona. Rockfall hazard estimation is derived from rockfall recurrence time based on OSL age determinations. DISCUSSION Results show that the block volume distribution follows an exponential function of the form ax b where a=0.4 and X is block volume (m 3 ), with b value –1.17, in agreement with worldwide rockfall inventories (Dussauge‐Peisser et al., 2002; Dussauge et al., 2003; Guzzetti et al., 2003; Malamud et al., 2004). Mapped maximal downhill block travel distances combined with slope morphological analysis were used to calibrate the simulation program variables, which were later used to simulate possible downhill rockfall block trajectories towards the town premises. Simulation results were used to compile the rockfall hazard map (Fig. 3), derived from maximal travel distance of the largest simulated blocks (D=6.2 m, V=125 m 3 ) from 25 rockfall simulation profiles performed using CRSP. Another map was compiled for location‐specific mitigation design considerations, introducing detailed locations where future rockfalls are predicted to impact town premises, at which kinetic analysis was performed. Block velocity and kinetic energy analyses at town border impact locations, provided by CRSP were used in order to obtain worst‐case hazard evaluation. At these analysis points, predicted block impact velocity varies between 10–15 m/s (mean 12.2 m/s with SD 1.7 m/s) and kinetic energy between 18,000–45,000 kJ (mean 31,000 kJ with SD=9000 kJ). These values may be used as general guidelines for rockfall damage mitigation for the entire study area. It is concluded that at the south‐westernmost part of town, life and property are at rockfall hazard in particular areas. Sensitivity analysis for block initial horizontal velocity (Vx), which represents horizontal acceleration induced during an earthquake, was performed in CRSP using velocities of up to 3 m/s. No significance was observed on simulated block travel distances compared to V x=0 m/s (no initial horizontal velocity). It is concluded that the main affect of earthquake induced acceleration on the studied rockfalls is in triggering them rather than affecting block travel distance, once detached from the cliff mass. The nine OSL age results from the current study, combined with ages of large earthquakes determined suggested by two other studies (Kagan et al., 2005; Yagoda‐Biran, 2008) demonstrate clustering around dates that coincide with selected known M>6.5 earthquakes, historic and prehistoric (Table 1; Fig. 4). The known earthquakes were selected from historical earthquake 1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology 71 Fig. 3: Rockfall hazard map of Qiryat‐Shemona. Source of rockfall (Ein‐El‐Assad formation) marked in blue line; area subject to rockfall hazard (from source escarpment to 100% of blocks stop line) dashed in yellow; town border line in red dashed line; Route 90 in orange solid line. Map compiled from maximal travel distance (100% of blocks stop line) of 25 simulation profiles performed using CRSP. catalogs (Amiran et al., 1994; Guidoboni and Comastri, 2005; Guidoboni et al., 1994) based on two considerations: (a) their estimated maximum intensity is ‘IX’ or higher (EMS local intensity scale); (b) the distance between the study area and affected localities reported in the catalogs does not exceed 100 km following Keefer's (1984) upper limit for disrupted slides or falls triggered by earthquakes. It is concluded that earthquakes of large intensities (depending on local ground acceleration) are the triggering mechanism of the studied rockfalls, yet apparently not all large earthquakes trigger rockfall. Analysis of currently available OSL ages of rockfall events yields an 850 years recurrence time and suggests that the last rockfall was triggered by the 1202 AD earthquake. The recurrence time of 850 years corresponds to recurrence time of M=6.5 earthquakes in the DST (Begin, 2005). However, since hundreds of blocks are scattered on the studied slope, nine OSL ages cannot provide a robust statistical basis of rockfall recurrence pattern. The determination of additional several dozens of OSL ages is required to obtain a reliable analysis of rockfall recurrence pattern.
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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
Page 21 and 22: continuous support. N. Shalev, G. G
Page 23 and 24: The tumulus initial morphology show
Page 25 and 26: 1 st INQUA‐IGCP‐567 Internation
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 and 54: are a valuable addition to the rout
Page 55 and 56: to 400 m) the calculated values for
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: however, the massive 1995 earthquak
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
Page 105 and 106: produced by significantly different
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
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
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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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