Triaxial testing of marine sediments from offshore Costa Rica (IODP Expeditions 334 and 344)

There are two general types of ocean-continent subduction zones forming either accretionary or erosive continental margins. In the accretionary case, the overriding continental plate grows by the accretion of material tectonically detached from the downgoing oceanic plate. In the erosive case, however, the overriding continental plate shrinks due to tectonic erosion by the downgoing oceanic plate. Two major endeavors of the International Ocean Discovery Program (IODP), the Nankai Trough Seismogenic Zone Experimen (NanTroSEIZE) and the Costa Rica Seismogenesis Project (CRISP), investigate the processes and controlling factors of deformation and seismogenesis at accretionary and erosive margins including the incidence of large magnitude earthquakes and related tsunamis. Focussing here on the Costa Rica erosive margin, we study material properties of marine sediments promoting either distributed and continuous deformation or localized and discontinuous deformation in the forearc wedge. Our results will also be compared to similar investigations on sediments from the Nankai accretionary prism. Forearc stability and inherent tectonic failure processes at active continental margins very much depend on the strength of the composing sediments. Forearc sediments can either be prone to fracturing and more localized deformation or, alternatively, to creep and distributed deformation. Strength and deformation behavior can vary significantly depending on small differences in composition and fabric of the sediments as has been shown in a similar study on samples from the Nankai trench and forearc (Stipp et al., 2013). Whole-round core samples recovered during IODP Expeditions 334 and 344 from a depth range of 7–125m below seafloor were experimentally deformed in a triaxial cell under consolidated and undrained conditions at confining pressures of 460–1000 kPa, room temperature, axial displacement rates of 0.01–0.1 mm/min, and up to axial compressive strains of ~ 45%. First results show significant differences in the consolidation state and the mechanical behavior of between upper plate and incoming plate sediments (Fig. 1). Similar to previous findings from the Nankai trench, two “rheological groups” can be distinguished: structurally weak and structurally strong samples. One sample from the incoming plate shows a previously unrecognized transition from structurally strong to structurally weak behavior at elevated confining pressure of 1000 kPa. All samples, deformed and undeformed, are designated to texture analysis via synchrotron x-ray diffraction and anisotropy of magnetic susceptibility (AMS). The observed differences in mechanical behavior may hold a key for understanding strain localization and brittle faulting in forarc regions

Azimuthal anisotropy in the wider Vienna basin region: a proxy for the present-day stress field and deformation

International audienc

Coda-Q in the 2.5-20 s period band from seismic noise : application to the greater Alpine area

Coda-Q is used to estimate the attenuation and scattering properties of the Earth. So far focus has been on earthquake data at frequencies above 1 Hz, as the high noise level in the first and second microseismic peak, and possibly lower scattering coefficient, hinder stable measurements at lower frequencies. In this work, we measure and map coda-Q in the period bands 2.5-5 s, 5-10 s and 10-20 s in the greater Alpine region using noise cross-correlations between station pairs, based on data from permanent seismic stations and from the temporary AlpArray experiment. The observed coda-Q for short interstation distances is independent of azimuth so there is no indication of influence of the directivity of the incoming noise field on our measurements. In the 2.5-5 s and 5-10 s period bands, our measurements are self-consistent, and we observe stable geographic patterns of low and high coda-Q in the period bands 2.5-5 s and 5-10 s. In the period band 10-20 s, the dispersion of our measurements increases and geographic patterns become speculative. The coda-Q maps show that major features are observed with high resolution, with a very good geographical resolution of for example low coda-Q in the Po Plain. There is a sharp contrast between the Po Plain and the Alps and Apennines where coda-Q is high, with the exception a small area in the Swiss Alps which may be contaminated by the low coda-Q of the Po Plain. The coda of the correlations is too short to make independent measurements at different times within the coda, so we cannot distinguish between intrinsic and scattering Q. Measurements on more severely selected data sets and longer time-series result in identical geographical patterns but lower numerical values. Therefore, high coda-Q values may be overestimated, but the geographic distribution between high and low coda-Q areas is respected. Our results demonstrate that noise correlations are a promising tool for extending coda-Q measurements to frequencies lower than those analysed with earthquake data

The AlpArray Seismic Network: A Large-Scale European Experiment to Image the Alpine Orogen

The AlpArray programme is a multinational, European consortium to advance our understanding of orogenesis and its relationship to mantle dynamics, plate reorganizations, surface processes and seismic hazard in the Alps-Apennines-Carpathians-Dinarides orogenic system. The AlpArray Seismic Network has been deployed with contributions from 36 institutions from 11 countries to map physical properties of the lithosphere and asthenosphere in 3D and thus to obtain new, high-resolution geophysical images of structures from the surface down to the base of the mantle transition zone. With over 600 broadband stations operated for 2 years, this seismic experiment is one of the largest simultaneously operated seismological networks in the academic domain, employing hexagonal coverage with station spacing at less than 52 km. This dense and regularly spaced experiment is made possible by the coordinated coeval deployment of temporary stations from numerous national pools, including ocean-bottom seismometers, which were funded by different national agencies. They combine with permanent networks, which also required the cooperation of many different operators. Together these stations ultimately fill coverage gaps. Following a short overview of previous large-scale seismological experiments in the Alpine region, we here present the goals, construction, deployment, characteristics and data management of the AlpArray Seismic Network, which will provide data that is expected to be unprecedented in quality to image the complex Alpine mountains at depth