Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology ON THE PROBLEM OF MAGNITUDE CALIBRATION OF PALAEOEARTHQUAKES IN BAIKAL REGION, RUSSIA 156 R.E. Tatevossian (1) (1) Institute of Physics of the Earth, Russian Ac. Sci., Ul. B. Gruzinskaya, 10. 123995, Moscow. RUSSIA. ruben@ifz.ru Abstract: intensity of the muya earthquake is assessed based on macroseismic and geological data. Macroseismic effect distribution confirms source depth at 20‐22 km. Deep source agrees with seismic rupture length 25 km: i.e. Only a part of the source length exposed on the surface. Comparison with length of paleoseismodislocations shows that it is a regional feature. Epicentral intensity based on surface ruptures is assed x degrees in esi2007 scale. Ignoring geological effects will underestimate epicentral intensity up to two degrees. Source mechanism with three sub‐sources is in agreement with segmentation of surface ruptures. Sub‐sources are of strike‐slip type with small normal component; essential normal slip at surface is probably not representative for the source and is due to accommodation of strike‐slip movement along with a system of sub‐parallel en echelon ruptures under tension. Key words: Baikal, palaeoearthquakes, strong earthquakes INTRODUCTION Baikal is a unique zone of active continental rift (e. g. Doser, 1991). Being such, it attracts research activities of specialists in different fields of Earth sciences. Systematic studies of palaeoearthquakes started in the Baikal seismic region in early 1960s; first regional catalogue of palaeoearthquakes was published in Kondorskaya & Shebalin (1977) (Fig. 1) Fig. 1: Palaeoearthquake epicentres in Baikal region according to Kondorskaya & Shebalin (1977). Symbol size is proportional to magnitude Length and maximum offset amplitude of palaeoseismostructures (PSS) reported in the catalogue are shown in Fig. 2. Even quick look at the parameters arise suspicions: maximum offsets of 5 PSS exceed 30 m. But the exceptionally large values of maximum offsets are not the only suspicious data. Surface rupture length (SRL) and the offset along it are correlated (Wells & Coppersmith, 1994). Data on Fig. 3 is in contradiction with this observation: for PSS length less than 5 km, more than 10 m of vertical offset are reported. Possibly, errors in measurements are responsible for the inconsistency between PSS length and the offset. However, the inconsistency is so much regular that we should look for certain physical background as possible explanation. Fig. 2: PSS length (L, km) and vertical offset (A, m) according to Kondorskaya & Shebalin (1977) Magnitudes are defined based on the PSS parameters and, certainly, discrepancies between PSS parameters affect magnitude calibration. In Fig. 3 are shown magnitudes of different kinds reported in Kondorskaya & Shebalin (1977) catalogue. ML is magnitude based on PSS length, Ma – on maximum offset, Mf – final magnitude value. Magnitudes based on PSS offset are systematically larger than those derived from PSS length. The difference in average reaches 1.25 units. Mf values are somewhere in between the ML and Ma, but there is no any regularity in Mf assessment. Note also that some ML values are ascribed arbitrary (open stars).
Fig. 3: Relationships among magnitude assessments of different kinds for palaeoerathquakes: ML (dots), Ma (stars) and Mf (crosses). Blank stars are for magnitudes, which do not follow the correlation relationship. See text for details. THE MUYA, 1957, EARTHQUAKE The Muya, 1957 (M=7.6) earthquake is the largest instrumentally recorded seismic event in the region. A complex set of surface ruptures was associated with it. They were studied and reported by the special expedition of Solonenko et al. (1966). We can use detailed information on this earthquake to understand if there are some specific regional conditions, which cause the unusual relationships between PSS length and the offset. Finding of this cause is the goal of the paper. Location of the Muya earthquake and seismic source mechanisms in the region according to (Global Centroid Moment Tensor Catalog, 2009) are shown in Fig. 4. There are no focal mechanism solutions reported immediately in the epicentral area of the Muya earthquake but in its vicinities normal faulting were observed; to the east strike‐slip faulting dominates. Fig. 4: Epicentre of the Muya, 1957, earthquake and source mechanisms in the Baikal region according to (Global Moment Tensor Catalog, 2009). Mechanisms for seismic events with Mo10 24 dyncm (Mw5.3) are plotted. 1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology 157 Materials of field studies clearly show that complex set of surface faulting can be grouped in three main segments: western, central, and eastern, with different parameters of faulting (Fig. 5a, b). The segments compose en echelon system WNW trending. In the western segment surface faulting outlines small graben (3 km length and 1.3 km width). The southern boundary of the graben is controlled by a normal fault with right‐lateral slip component. Horizontal slip is 1.05 m and vertical offset is between 1.2 – 1.3 m (Solonenko et al., 1966). Left‐lateral slip (0.3 m) accompanied with some normal faulting was observed close to the northern boundary (Kurushin, 1963). In the central part of the graben 8 meter wide through was reported (Solonenko, 1965). A transition zone of NW trending links Western and Central segments. It is composed from two brunches. Northern brunch striking WNW is a normal fault (1.4 m offset) with left‐lateral slip component (0.25 m) (Kurushin, 1963). Southern brunch is a pressure zone of NNW trending. It is most probably associated with thrusting; the amplitude of the vertical offset is either 2.4 m (Kurushin, 1963) or 4 m (Solonenko, 1965). Fig. 5: Geological manifestations, instrumental epicenter, and mechanism of the Muya earthquake. a) surface faulting; epicenter locations (1) by Vvedenskaya, Balakina, 1960, (2) by Balakina et al., 1972; Sub1, Sub2, Sub3 are mechanisms of sub‐ events. Arrows show slip direction. Question mark is for doubtful part of the surface faulting. b) generalized scheme of the surface faulting.
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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
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 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: Fig. 8: Data and regression for mag
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
Page 181: 1 st INQUA‐IGCP‐567 Internation
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