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Regional digital elevation models
A digital elevation model is a numerical representation of a landscape surface given in (X, Y, Z) format where X is northings, Y is eastings (both in degrees, kilometers or meters) and Z is the elevation (traditionally in meters above sea level). These datasets may be obtained by direct measurement in the field (high-resolution microtopography with a total station –see section 1.2.5.-, differential GPS or LIDAR), digitization of existing topographic maps, photogrammetry (use of stereoscopic pair of aerial/satellite images to extract elevation information) or Synthetic Aperture Radar (SAR) interferometry (radar stereoscopy between two satellite passes).
Global datasets such as GTOPO30, DTED0 and GLOBE (see Section World Wide Web resources) have ~1 km resolution which is sufficient for continental or national studies but not adequate for regional ones. National mapping agencies usually provide 25-m-resolution datasets well adapted to geomorphological studies. However, these datasets are often digitized maps for which the density of original (i.e. verified) data points is not known and often low. It may thus be reasonable to turn to updated satellite data such as these provided by ASTER and SRTM (Shuttle Radar Topographic Mission) missions. They consist in 30-m-spaced data points with strong internal consistency, though ASTER elevations are relative for a single shot/file (they are internally calibrated with respect to one-off atmospheric conditions) and may vary significantly between two consecutive satellite passes (users should prefer sequenced files recorded the same day with similar –preferentially dry and cloudless- atmospheric conditions or may experience significant offsets of elevations, typically of the order of 30m to 50m).
Geometry at depth and long-term vertical displacement: deep reflection seismics
In regions where no recent surface rupture has been observed and only historical earthquakes are known, a geomorphological signature is needed but not always enough to establish the presence of a fault (see Chapters 2 and 3). Reflection seismics with sufficient resolution, focusing on the first hundreds of meters below the surface may help strengthen the active fault hypothesis. Indeed, a seismic section perpendicular to the fault trend shall evidence faulted and displaced units which, on the basis of their age and amount of deformation, should document long-term vertical slip rate and, possibly, give an estimate of the beginning of activity on the fault. Indeed, abutting relationships may show that a group of units are similarly deformed, thus suggesting that deformation started, at soonest, after deposition of the last unit. Furthermore, it may help better characterize the fault over long-term and show vertical deformation on apparently strike-slip faults. The geometry itself shows the deformation style and may document more about the complexity of the fault system which may spread as a flower structure or be very concentrated. For example, it is most often uneasy to check whether a normal fault is listric at depth or not from surface observations. This is crucial point as it is directly linked to rupture width (or depth) and, consequently, to potential rupture area and pertaining maximum event magnitude.
Shallow geometry and recent displacement: sub-surface geophysics
Once a potential feature for a seismogenic source has been identified, paleoseismological investigations should focus on locating the most recent surface ruptures in order to plan excavations. High-resolution geophysics may help image the sub-surface and show recent displacements affecting shallow units that may be unearthed by trench investigations (usually, 2m to 5m-depth).
Recent cumulative displacements: micro-topography
Once a potentially active fault or scarp has been identified and located, fieldwork shall start with a microtopographic survey. Microtopography is the measurement of geomorphological features with a meter-order resolution (or better) and a centimeter-order accuracy (or better). This can be achieved through the use of a total station (or theodolite), a common tool for geometers. A total station (see Fig. 1.8) is a high-precision optical instrument used as a monocular. The station is installed at a fixed base where horizontality is set to zero and a given direction (preferably north) is set as origin. Then, an operator covers the study area with a reflector installed on top of plumb cane, which length is known. When a measurement shall be performed, the station operator targets the reflector and the station measures the horizontal and vertical angles with respect to the horizontal and origin direction (north) as well as the direct distance by means of a laser pulse. These three parameters are converted to (X, Y, Z) with respect to the station on the basis of simple geometrical relationships.
Identification of a paleoseismic event: criteria
There are numerous branches of Earth Sciences involved in paleoseismological investigations. Each of them focuses on specific aspects of the geological record to retrieve events information and needs to compare observations to standard criteria to be able to link events to earthquake occurrences. These criteria are –obviously- different for lakes (seismites), caves (damage to speleothems), rock falls (trapped soil) and active faults. We will concentrate here on criteria that should be addressed when investigating fault excavations (trenches). The unique advantage to excavate trenches across surface rupture is the primary nature of the data. While off-fault observations often remain questionable about their triggering mechanism (storms can strongly disturb recent lake sediments, slowly flowing ice or mudpacks can easily break speleothems, rock falls usually happen during huge rainfall), excavations give a direct access to the seismogenic fault.
Optimizing dates: the Bayesian statistical analysis
Calibration of radiocarbon ages to calendar dates does not yield results under scalar form but as probability distributions that correspond to 1σ (68.2 %) or 2σ (95.4 %) confidence intervals. Litton and Buck (1995) showed that a classical probabilistic approach called Bayesian statistical analysis could be applied to radiocarbon calibration in order to restrict probability distributions, with some limitations (Steier and Rom, 2000). The approach consists in building a relative a priori chronological sequence that integrates radiocarbon and thermoluminescence ages (on the basis of stratigraphic relationships between corresponding layers) as well as faulting events (following abutting relationships). The Bayesian analysis focuses on overlapping probability distributions to separate different samples and merge similar ones. Resulting dates for faulting events have a lower precision and but a narrower time range. Moreover, it is possible to compute the internal consistency of individual samples and events within the sequence model. For the Basel-Reinach fault study (see Chap. 2 and 3), the Bayesian statistical approach permitted a better correlation of events across different trenches and different sites and strengthened the events chronology during the Holocene. Additionally, it showed that our radiocarbon dataset was highly consistent, without any probable inversion, and that interpreted events chronology was reliable.
Neotectonic and Seismotectonic Framework
The Upper Rhine graben is part of a continental rift affecting intraplate Europe during late Cenozoic time. According to seismic profiles and borehole data, it is filled with more than 3.300 m of Tertiary and Quaternary sediments (Doebl and Olbrecht, 1974). The southern region of the Upper Rhine graben (Fig. 3) is limited to the east by the Tabular Jura and to the south by the folded Jura. Early extensional movements took place during the late Eocene and the late Oligocene, and according to Laubscher (2001), the Tabular Jura is related to the extensional movements of the graben. In fact, along with a well-developed bounding fault system, the large-scale N-S trending Rhine Valley flexure formed and a normal fault system propagated eastwards over more than 20 km, as far as Zeiningen (Fig. 3). This region is interpreted as the faulted upper Muschelkalk limestone forming individualized blocks tilted to the East and that slipped westwards on an underlying evaporite layer. The faulting episode presumably stopped during the late Oligocene and became a major west-dipping normal fault zone affecting the eastern rim of the tabular Jura. No mention exists in the literature on the Neogene and Quaternary tectonic episodes of this region.
Table of contents :
Chapter 1. Linking Past and Present, Depth and Surface: an introduction to the Paleoseismological approach
1.1. Unveiling past earthquake parameters: the paleoseismological approach
1.1.1. Seismic risk assessment
1.1.2. The physics of earthquakes
1.1.2.1. Rupture process
1.1.2.2. Earthquake recurrence
1.1.2.3. Characteristic earthquakes
1.1.3. What paleoseismology brings
1.2. A multidisciplinary approach: standard tools
1.2.1. Regions at risk: historical and instrumental seismicity
1.2.2. Fault identification and long-term history: geomorphology
1.2.2.1. Satellite imagery
1.2.2.2. Regional digital elevation model
1.2.2.3. Aerial photography
1.2.3. Geometry at depth and long-term vertical displacement: reflection seismics
1.2.4. Shallow geometry and recent displacement: sub-surface geophysics
1.2.4.1. High-resolution seismics
1.2.4.2. Electrical tomography
1.2.4.3. Ground-penetrating radar
1.2.5. Recent cumulative displacement: micro-topography
1.2.6. Recent history of sedimentation vs. faulting: trenching
1.2.6.1. Making the most of a trench site: site selection
1.2.6.2. Optimizing excavations: trench layout
1.2.6.3. Acquisition of trench data: logging and sampling
1.2.6.3.1. Logging
1.2.6.3.2. Photo-mosaicing
1.2.6.3.3. Dating
1.2.6.3.3.1. Radiocarbon
1.2.6.3.3.2. Thermoluminescence
1.2.6.3.3.3. Cosmogenic nuclides
1.2.6.3.3.4. Palynochronology
1.2.6.4. Identification of a paleoseismic event: criteria
1.2.6.5. Improving data quality: cross-correlating events
1.2.6.6. Optimizing dates: the Bayesian statistical analysis
Chapter 2. Active Normal Faulting in the Rhine Graben and Paleoseismic Identification of the 1356, I0=IX-X, Basel Earthquake (Central Europe)
Summary
Introduction
2.1.The Basel earthquake
2.2. Geological Setting and Seismotectonic Background
2.3. Paleoseismic results and surface ruptures of the 1356 earthquake
2.4. Discussion and conclusions
2.4.1. A major seismic source in the Basel area
2.4.2. Seismic history of the Basel area
2.4.3. Active tectonics in intraplate Europe and related seismic hazard implications
Chapter 3. Evidence for Holocene Paleoseismicity along the Basel-Reinach Active Normal Fault (Switzerland): A Seismic Source for the 1356 Earthquake in the Upper Rhine Graben
Summary
3.1. Introduction
3.2. The 1356 Basel Earthquake
3.3. Neotectonic and Seismotectonic Framework
3.4. Fault Scarp Geomorphology
3.5. Geophysical Investigations
3.5.1. Methods
3.5.2. Results
3.6. Sample Datations and Age Determination
3.7. Paleoseismic Trench Analysis
3.7.1. Site 1
3.7.2. Site 2
3.7.2.1. Trenches 1 and 4
3.7.2.2. Trench 3
3.7.2.3. Trench 5
3.7.3. Evidence of Recent Faulting From the 1962 Trench
3.8. Summary
3.8.1. Paleoseismic Events Inferred From Trenching
3.8.2. Events Identified Within Other Geological Archives
3.9. Discussion
3.9.1. Faulting Geometry and Length Extension
3.9.2. Seismotectonic Characteristics and Deformation Rate
3.9.3. Implications for the Seismic Hazard
Chapter 4. Ground-penetrating radar investigations along the North Anatolian fault near Izmit (Turkey): Constraints on the right-lateral movement and slip history
Abstract
4.1. Introduction
4.2. GPR Method And Site Selection
4.3. Profile Analyses
4.3.1. Köseköy Site
4.3.2. The A.D. 1591 Ottoman Canal Site
4.4. Discussion And Conclusions
Chapter 5. Paleoearthquake History of the western portion of the North Anatolian Fault in Western Turkey from paleoseismic data, and implications to the seismic hazard of Istanbul
Abstract
5.1 Introduction
5.2. The Saros Site
5.2.1. Site Stratigraphy
5.2.2. Evidence for Earthquakes
5.2.3. Determination of Lateral Slip
5.3. Izmit to Sapanca Segment
5.3.1. The Ottoman Canal Site
5.3.2. The Köseköy Site
5.3.2.1. Site Stratigraphy
5.3.2.2. Interpretation of Past Earthquakes
5.3.3. Discussion of Results
5.3.3.1. Hazard to Istanbul
Conclusions and Outlook
References
