Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

More documents

Recommendations

Info

1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology) CRITERIA TO DISTINGUISH NEOTECTONIC FROM OTHER ACTIVE FAULTS: EXAMPLES FROM THE CENTRAL PYRENEES 98 M. Ortuño (1) (1) RISKNAT group. Dept. Geodinàmica i Geofísica. Universitat de Barcelona (Spain) C/Martí i Franquès s/n. 08028‐Barcelona, SPAIN. maria.ortuno@ub.edu Abstract: In several settings, such us the high mountain environment or the karstic terrains, active faults might be the result of non‐tectonic processes, all of which can be grouped under the term “active deformation”. To characterize the seismogenic potential of a fault and, thus, its associated seismic hazard, it is necessary to determine the causes of its activity. However, the nature of the deformation along a faults is, often, not obvious. To deal with this problem, a number of criteria have been reviewed and proposed in order to constitute a working‐guide to determine the origin of faulting. Two examples of the Maladeta massif (Spanish Central Pyrenees) illustrate how very different processes are capable to generate similar scarp‐forms and how one single fault scarp might be the result of their simultaneous interaction. Key words: neotectonics, non‐tectonic faults, composite fault, active deformation INTRODUCCION Active faults can be defined as planes of fracture along which displacement takes place or has taken place in recent times. Besides the plate tectonics, other sources of forces in the upper crust must be taken into account as causes of deformation. The term “active deformation” might serve as an umbrella to refer to all processes causing deformation along faults (Ortuño, 2008). In certain settings, the magnitude of the non tectonic stresses might equal or overpass the tectonic ones. Straightforward examples are deglaciated regions where the isostatic forces are controlling the activity of pre‐ existing faults (i.e. Stewart et al. 2000) or areas where the intrusion of magmatic or salty domes are responsible for fault generation or fault reactivation. In contrast with these phenomena, that can take place up to tens of kilometres under the earth surface, surface processes might also be the cause of new generation or reactivation of faults. Such is the case of the instability phenomena (slope mass movements and subsidence) (i.e. Chighira, 1992) and the crust unloading effects produced by drainage of a lake or melting of ice masses (Hampel and Hetzel, 2006; Ustaszewsky et al., 2008). Accordingly, these forces must be interacting simultaneously in a particular area of the crust, and thus, the movement along a fault could be due to more than one process. To deal with this situation, Ustaszewsky et al. (2008) have proposed the term “composite faults” to refer active faults in the Swiss Alps in which deformation is controlled by tectonic, gravitational, and elastic rebound forces. Paleosismological studies are always preceded by the identification of active faults in the landscape through geomorphological and geophysical methods. However, in these studies, the causes of deformation are frequently obviated or not enough discussed. Even when the seismogenic nature of the faulting can be showed, the possibility of deformation being owed to non‐tectonic forces must be discussed since the misinterpretation of a fault origin could invalidate the results derived from paleosismological analysis (i.e. recurrence time, maximum magnitude earthquake, etc.). To better constrain the nature of faulting, we need to perform or consider more regional studies. In order to contribute to this task, a number of criteria are revised below. Ortuño (2008) recognize the composite nature of several cases of faulting in the Spanish Pyrenees. Two of these active structures are analyzed in this work, to illustrate how the use the discussed criteria can help to better characterize the nature of faulting, and thus, its seismogenic potential. USEFUL CRITERIA The difficulties in differentiating tectonics from non‐ tectonics faults have constantly worried geologists. McCalpin (1999) collected and discussed criteria to distinguish tectonic from gravitational faults. This work has been reviewed and extended by Ortuño (2008), who proposed a guideline for the use of different criteria to determine the origin of faulting. Criteria can be grouped into morphologic, kinetic‐structural and others (such as chronology of deformation or processes spatial distribution). Most of them have to do with the feasibility of the different processes causing deformation. By this reason, the origin of the fault is often suspected by the rejection of all other possible origins. Best results are achieved by combining the greater number of criteria. Morphologic criteria Morphologic features that should be considered to understand the nature of the faulting can be grouped in quantitative and qualitative. The most useful quantitative criteria are slope and relief, rupture length, cumulative displacement, changes in offset along the fault trace, maximum offset to trace length ratio (D/L), and depth of deformation. Some
elevant qualitative criteria are position and orientation of the fault with respect to the relief and the slope, curvature and continuity of the fault trace. Landform assemblage and landscape evolution might be definitive criteria. Kinetic and structural criteria Deformation observed both at trench and outcrop might provide important information regarding the nature of faulting. Quantitative criteria refer to cumulative displacement and slip rate as well as orientation of the fault, dip and slip with respect to the pre‐existing tectonics and the present and recent stress orientation obtained from other sources (i.e. focal mechanisms, recent local and regional tectonics, drillings, etc.). Qualitative criteria refer to textural and structural features of deformation, as well as style of faulting (strike slip, normal or reverse). Other criteria The temporal constrain of deformation might be a helpful criteria to explore a particular origin of faulting. For example, the occurrence of coseismic deformation would reinforce a seismogenic origin inferred by the use of other criteria. Faulting constrained to a deglaciation episode would suggest isostasy or elastic rebound as causes of deformation. The spatial distribution of seismicity, maximum uplift rates or enhanced depositional/erosive processes with respect to the location of the fault might as well be clue aspects to determine the fault origin. EXAMPLES FROM THE MALADETA MASSIF (CENTRAL PYRENEES) The Maladeta massif (Fig. 1) is located at the core of the Pyrenean range, in the paleo‐margin between the Iberian and the Eurasian plates. Owing to the small convergence rate between these plates (< 0,5 mm/a; Nocquet and Calais, 2004), neotectonic faults in the Pyrenees are expected to behave as slow faults (with slip rates
Page 1:
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 25 and 26:
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 37 and 38:
Page 39 and 40:
Page 41 and 42:
Page 43 and 44:
Table 1: Surface faulting extent re
Page 45 and 46:
Page 47 and 48:
The graben has a markedly asymmetri
Page 49 and 50: 1 st INQUA‐IGCP‐567 Internation
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 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: Fig. 8: Seismic ground shaking may
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
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 151 and 152:
Page 153 and 154:
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 and 164:
The stratigraphic sequence (Fig. 4)
Page 165 and 166:
especially in the zones of hanging
Page 167 and 168:
Page 169 and 170:
interpretations are based on relati
Page 171 and 172:
Page 173 and 174:
Fig. 5: Gray‐scale value laser im
Page 175 and 176:
Page 177 and 178:
(~40 cm), F‐ rotated building str
Page 179 and 180:
Index 1 st INQUA‐IGCP‐567 Inter
Page 181:
show all

Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

Create successful ePaper yourself

Delete template?

Save as template?