Validation of seismic hazard curves using a calibrated 14 ka lacustrine record in the Eastern Alps, Austria.

Seismic hazard maps are crucial for earthquake mitigation and mostly rely on probabilistic seismic hazard analysis (PSHA). However, the practise and value of PSHA are under debate because objective testing procedures for seismic hazard maps are scarce. We present a lacustrine turbidite record revealing 44 earthquakes over the last ~ 14 ka and use it to test seismic hazard curves in southern Austria. We derive local seismic intensities for paleo-earthquakes by applying scaling relationships between the sedimentary imprint and seismic intensity of well-documented historical earthquakes. The last ~ 2.8 ka of the record agree with a Poissonian recurrence behaviour and therefore a constant hazard rate, which is the modelling choice for standard PSHA. The lacustrine data are consistent with the intensity-frequency relationship of the local seismic hazard curve, confirming the current PSHA approach for this part of Austria. On longer timescales, distinct phases of enhanced regional seismicity occurred, indicating a potential increase of seismic hazard after large earthquakes-a factor hitherto disregarded in the PSHA of the Eastern Alps. Our new method forms an independent procedure to test hazard maps in any setting where suitable lake systems are available

Polarimetric analysis of the human cornea measured by polarization-sensitive optical coherence tomography

Corneal polarimetry measurement has been the object of several papers. The results of techniques like polarization-sensitive optical coherence tomography PS-OCT , scanning laser polarimetry, or polarization microscopy are contradictory. Some studies propose a biaxial-like birefringence pattern, while others postulate that birefringence grows at corneal periphery. Several theoretical approaches were proposed for the interpretation of these measurements, but they usually lack accuracy and an adequate consideration of the nonnormal incidence on the tissue. We analyze corneal polarization effects measured by PS-OCT. In vivo and in vitro PS-OCT images of the human cornea are acquired. PS-OCT measurements are apparently not in agreement with the biaxial-like birefringence pattern. We present a polarimetric model of the human cornea based on the extended Jones matrix formalism applied to multilayered systems. We also apply the Poincaré equivalence theorem to extract optic axis orientation and birefringence. The results show that for a fibrils orientation pattern composed by alternating circular and radial fibrils, the birefringence is biaxial-like at the corneal center, and there is an almost circularly symmetric high-birefringence area at corneal periphery. The model could be useful for diagnosis of corneal diseases or corneal compensation in retinal polarimetric imaging

High-resolution calibration of seismically induced lacustrine deposits with historical earthquake data in the Eastern Alps (Carinthia, Austria)

Lake sediments are increasingly used to reconstruct recurrence intervals of large earthquakes - a prerequisite for the establishment of accurate seismic hazard models - because they can record strong seismic shaking as mass-transport deposits (MTDs), turbidites or sediment deformations and often reach back several thousands of years. To derive quantitative information on paleo-earthquake size, the sedimentary imprints need to be thoroughly calibrated with independent information on seismic shaking strength. A few calibration studies proposed scaling relationships between the shaking strength of historical earthquakes and the type and size of lacustrine sedimentary imprints. Due to incomprehensive lacustrine mapping or an insufficient record of documented earthquakes, however, rigorous testing of these scaling relationships is lacking. Here, we study the sedimentary infill of the past ∼800 years in Wörthersee and Millstättersee, two large lakes in the Eastern Alps (Carinthia, Austria). These lakes have experienced five well-documented historical earthquakes with local seismic intensities ranging from V – IX (EMS-98 scale). We trace the sedimentary signatures (MTDs and turbidites) of these earthquakes based on a vast dataset of multibeam bathymetry, reflection seismic profiles and numerous precisely dated sediment cores. Seismic intensities as low as V½ are recorded as turbidites originating from deltaic slopes, while hemipelagic slopes can fail from intensities of VI onwards. In Wörthersee, earthquake-recording thresholds are highly dependent on the specific core locations due to local variations in slope characteristics (composition, length, and gradient) and transport distance to the core site. This highlights the potential for establishing multi-threshold paleoseismic records based on multiple coring sites in a single basin. In both lakes, exponential size-scaling relationships are inferred between seismic intensity and i) number or volume of mass-transport deposits and ii) the cumulative thickness of turbidites. Moreover, the relative turbidite presence increases linearly with seismic intensity, confirming the results from a previous study in Chilean lakes. Application of the obtained size-scaling relationships on the first major earthquake documented for Austria (1201 CE) suggests a magnitude of ∼6.4 and an epicentre close to Millstätter See. This demonstrates that lake paleoseismology is a powerful tool to obtain quantitative information on the seismic intensity distribution of paleo-earthquakes.ISSN:0277-379

Late Glacial and Holocene sedimentary infill of Lake Mondsee (Eastern Alps, Austria) and historical rockfall activity revealed by reflection seismics and sediment core analysis

Glacigenic perialpine lakes can constitute continuous post-last glacial maximum (LGM) geological archives which allow reconstruction of both lake-specific sedimentological processes and the paleoenvironmental setting of lakes. Lake Mondsee is one among several perialpine lakes in the Salzkammergut, Upper Austria, and has been previously studied in terms of paleoclimate, paleolimnology and (paleo)ecology. However, the full extent and environment of Late Glacial to Holocene sediment deposition had remained unknown, and it was not clear whether previously studied core sections were fully representative of 3D sediment accumulation patterns. In this study, the sedimentary infill of Lake Mondsee was examined via high-resolution seismic reflection survey over a 57-km extent (3.5 kHz pinger source) and a sediment core extracted from the deepest part of the lake, with a continuous length of 13.76 m. In the northern basin, seismic penetration is strongly limited in most areas because of abundant shallow gas (causing acoustic blanking). In the deeper areas, the acoustic signal reaches depths of up to 80 ms TWT (two-way travel time), representing a postglacial sedimentary sequence of at least 60-m thickness. Holocene deposits constitute only the uppermost 11.5 m of the sedimentary succession. Postglacial seismic stratigraphy of Lake Mondsee closely resembles those of well-studied French and Swiss perialpine lakes, with our data showing that most of Lake Mondsee’s sedimentary basin infill was deposited within a short time period (between 19,000 BP and 14,500 BP) after the Traun Glacier retreated from the Mondsee area, indicating an average sedimentation rate of about 1.4 cm/yr. Compared to other perialpine lakes, the seismic data from Lake Mondsee reveal little indication of mass movement activities during the Holocene. One exception, however, is rockfalls that originate from a steep cliff, the Kienbergwand, situated on the southern shore of Lake Mondsee, where, in the adjacent part of the lake, seismic profiles show mass transport deposits (MTDs), which extend approximately 450 m from the shore and are mappable over an area of about 45,300 m2. Sediment cores targeting the MTDs show two separate rockfall events. The older event consists of clast-supported angular dolomitic gravels and sands, showing high amounts of fine fraction. The younger event exhibits dolomitic clasts of up to 1.5 cm in diameter, which is mixed within a lacustrine muddy matrix. Radiocarbon dating and correlations with varve-dated sediment cores hint at respective ages of AD 1484 ± 7 for Event 1 and AD 1639 ± 5 for Event 2. As our data show no evidence of larger-scale mass movements affecting Lake Mondsee and its surroundings, we infer that the current-day morphology of the Kienbergwand is the result of infrequent medium-scale rockfalls.© 2018 Christoph Daxer et al

Beiträge zum 29. Darmstädter Geotechnik-Kolloquium am 7. März 2024

Themenschwerpunkte: 1. Klimawandelfolgen und Nachhaltigkeit 2. Digitalisierung und künstliche Intelligenz 3. Planung und Bemessung von Infrastrukturprojekten 4. Großprojekt

Polarization sensitive optical coherence tomography in the human eye

Optical coherence tomography (OCT) has become a well established imaging tool in ophthalmology. The unprecedented depth resolution that is provided by this technique yields valuable information on different ocular tissues ranging from the anterior to the posterior eye segment. Polarization sensitive OCT (PS-OCT) extends the concept of OCT and utilizes the information that is carried by polarized light to obtain additional information on the tissue. Several structures in the eye (e.g. cornea, retinal nerve fiber layer, retinal pigment epithelium) alter the polarization state of the light and show therefore a tissue specific contrast in PS-OCT images. First this review outlines the basic concepts of polarization changing light–tissue interactions and gives a short introduction in PS-OCT instruments for ophthalmic imaging. In a second part a variety of different applications of this technique are presented in ocular imaging that are ranging from the anterior to the posterior eye segment. Finally the benefits of the method for imaging different diseases as, e.g., age related macula degeneration (AMD) or glaucoma is demonstrated