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In order to improve our understanding of the dynamics of potentially unstable steep glacier tongues we monitored during summer 2010 the micro seismicity of Triftgletscher Switzerland. Our system comprising 8 three'component seismometers coupled with the ice surface was installed upstream of the glacier's tongue which is likely to evolve toward an unstable regime. Complementary surface motion and proglacial runoff measurements allowed the icequake activity to be interpreted in terms of glacier dynamics and hydraulics. The strong contrast in seismic wave velocities due to the underlying bedrock was taken into account using a three'dimensional (3'D) velocity model implemented in a nonlinear probabilistic location procedure allowing to accurately define the hypocenter uncertainty. We located 120 icequakes with a focal depth accuracy that allowed distinguishing between shallow events (87 events) and near'bedrock icequakes (33 events). The first motions of most of the deep events argue against pure shear sources expected in case of stick'slip motion and our suggested source mechanism is a superimposed tensile crack and shear dislocation. The analysis of surface strain showed that near'surface events represent the opening and transverse extension of surface crevasses in a confluent flow regime. Key Points 120 icequakes were located close to the glacier surface and near the bedrock. Deep events come likely from superimposed tensile crack and shear dislocation Shallow seismicity reflects surface deformation in a confluent regime. ©2013. American Geophysical Union. All Rights Reserved.

We introduce the Anisotropic Clustering of Location Uncertainty Distributions (ACLUD) method to reconstruct active fault networks on the basis of both earthquake locations and their estimated individual uncertainties. After a massive search through the large solution space of possible reconstructed fault networks, we apply six different validation procedures in order to select the corresponding best fault network. Two of the validation steps (cross-validation and Bayesian Information Criterion (BIC) process the fit residuals, while the four others look for solutions that provide the best agreement with independently observed focal mechanisms. Tests on synthetic catalogs allow us to qualify the performance of the fitting method and of the various validation procedures. The ACLUD method is able to provide solutions that are close to the expected ones, especially for the BIC and focal mechanismbased techniques. The clustering method complemented by the validation step based on focal mechanisms provides good solutions even in the presence of a significant spatial background seismicity rate. Our new fault reconstruction method is then applied to the Landers area in Southern California and compared with previous clustering methods. The results stress the importance of taking into account undersampled sub-fault structures as well as of the spatially inhomogeneous location uncertainties., 36 pages, 17 figures

Geophysical Journal International, 191 (2), ISSN:0956-540X, ISSN:1365-246X

Swiss Journal of Geosciences, 100 (3), ISSN:1661-8734, ISSN:1661-8726

Dieser Beitrag präsentiert die Berechnung und Ergebnisse von drei Schadensszenarien für den Kanton Basel-Stadt. Bei der Berechnung wurden vorhandene Modelle für die Analyse der Bodenbewegungen und der Gebäudeverletzbarkeit mit einem probabilistischen Ansatz kombiniert. Aleatorische Unsicherheiten werden, sofern sie vorhanden waren, berücksichtigt. Die Schadensszenarien werden für das historische Basel-Erdbeben von 1356 (Mw=6.6) und für zwei generische Erdbeben berechnet, die in etwa den aktuellen Anforderungen der Schweizer Baunormen entsprechen (Mw=5.7 in 7.5 km und Mw=5.0 in 5 km Distanz). Die Ergebnisse liegen auf Niveau gesamter Kanton und für jedes Stadtquartier vor. Ein wichtiger Aspekt der Studie ist die Verlässlichkeit der Ergebnisse abzuschätzen. Ein Vergleich mit den Schäden und Verlusten des 2009 L’Aquila Erdbebens und mit den Schadensbildern, die man im Rahmen der EMS-98 makroseismischen Skala erwarten würde, zeigt, dass alle drei Schadensszenarien zu hohe Schäden und entsprechend auch zu hohe Verluste aufweisen. Die Gründe dafür liegen sehr wahrscheinlich in der begrenzten Auflösung und Zuverlässigkeit des Modelles zur Gebäudeverletzbarkeit. Dieses erlaubt nur eine Analyse der vorhandenen Gebäudetypen auf Niveau Stadtquartier.

During summer 2011, seismic activity in the ablation zone of the western Greenland ice sheet (GrIS) was monitored using a network of three-component seismometers. The seismic record includes a large variety of icequakes and seismic tremors that demonstrate a clear correlation with subglacial water flow. We verified the existence of well-known shallow icequakes (related to surface crevasse formation), deep icequakes (located at 100–160 m depth) and narrow-banded short-term seismic tremors (tens of seconds in duration). In addition, we present previously unreported long-term tremors lasting several hours. Using attenuation of the measured tremor amplitude, we locate the epicentre of this long-term tremor to a large moulin within our study area. Between 3 and 11 Hz, our continuous seismic record is dominated by this ‘moulin tremor’ and shows strong correlation with the water level of the generating moulin. We argue that monitoring of icequake and glacial tremor sources bears high potential for investigating glacier hydraulics and dynamics, and is thus an ideal supplement to traditional glaciological measurements.

Various techniques are utilized by the seismological community, extractive industries, energy and geoengineering companies to identify earthquake nucleation processes in close proximity to engineering operation points. These operations may comprise fluid extraction or injections, artificial water reservoir impoundments, open pit and deep mining, deep geothermal power generations or carbon sequestration. In this letter to the editor, we outline several lines of investigation that we suggest to follow to address the discrimination problem between natural seismicity and seismic events induced or triggered by geoengineering activities. These suggestions have been developed by a group of experts during several meetings and workshops, and we feel that their publication as a summary report is helpful for the geoscientific community. Specific investigation procedures and discrimination approaches, on which our recommendations are based, are also published in this Special Issue (SI) of Journal of Seismology.