Imaging a shallow salt diapir using ambient seismic vibrations beneath the densely built-up city area of Hamburg,Northern Germany

Salt diapirs are common features of sedimentary basins. If close to the surface, they can bear a significant hazard due to possible dissolution sinkholes, karst formation and collapse dolines or their influence on ground water chemistry. We investigate the potential of ambient vibration techniques to map the 3-D roof morphology of shallow salt diapirs. Horizontal-to-vertical (H/V) spectral peaks are derived at more than 900 positions above a shallow diapir beneath the city area of Hamburg, Germany, and are used to infer the depth of the first strong impedance contrast. In addition, 15 small-scale array measurements are conducted at different positions in order to compute frequency-dependent phase velocities of Rayleigh waves between 0.5 and 25 Hz. The dispersion curves are inverted together with the H/V peak frequency to obtain shear-wave velocity profiles. Additionally, we compare the morphology derived from H/V and array measurements to borehole lithology and a gravity-based 3-D model of the salt diapir. Both methods give consistent results in agreement with major features indicated by the independent data. An important result is that H/V and array measurements are better suited to identify weathered gypsum caprocks or gypsum floaters, while gravity-derived models better sample the interface between sediments and homogeneous salt. We further investigate qualitatively the influence of the 3-D subsurface topography of the salt diapir on the validity of local 1-D inversion results from ambient vibration dispersion curve inversion.     

Discrimination of induced seismicity by full moment tensor inversion and decomposition

Human activities, including operations related to mining and reservoir exploitation, may induce seismicity and pose a risk for population and infrastructures. While different observations are commonly used to assess the origin of earthquakes, there is a lack of rules and methods for the discrimination between natural and induced seismicity. The inversion and decomposition of the full moment tensor and the observation of relevant deviation from a pure double couple (DC) model may be an indicator for induced seismicity. We establish here a common procedure to analyse a set of natural and induced events of similar magnitude, which occurred in Germany and neighbouring regions. The procedure is based on an inversion method and on a consistent velocity model and recording network. Induced seismicity is recorded during different mining and/or reservoir exploitations. Moment tensors are inverted using a multi-step inversion approach. This method, which was successfully applied in previous studies at regional and teleseismic distances, is further developed here to account for full moment tensor analysis. We first find a best DC solution and then perform a full moment tensor inversion, fitting full waveforms amplitude spectra at regional distances. The moment tensor solution is decomposed into DC, compensated linear vector dipole and isotropic terms. The discrimination problem is then investigated through the evaluation of distributions of non-DC source components for natural and induced data sets. Results illustrate the potential of the inversion and discrimination approach. Additional detailed analyses are carried out for the two most significant induced earthquakes, and rupture models are compared with the full moment tensor solutions.     

A probabilistic approach for the classification of earthquakes as ��triggered�� or ��not triggered��

The occurrence time of earthquakes can be anticipated or delayed by external phenomena that induce strain energy changes on the faults. ??Anticipated?? earthquakes are generally called ??triggered??; however, it can be controversial to label a specific earthquake as such, mostly because of the stochastic nature of earthquake occurrence and of the large uncertainties usually associated to stress modelling. Here we introduce a combined statistical and physical approach to quantify the probability that a given earthquake was triggered by a given stress-inducing phenomenon. As an example, we consider an earthquake that was likely triggered by a natural event: the M?=?6.2 13 Jan 1976 Kópasker earthquake on the Grímsey lineament (Tj?rnes Fracture Zone, Iceland), which occurred about 3?weeks after a large dike injection in the nearby Krafla fissure swarm. By using Coulomb stress calculations and the rate-and-state earthquake nucleation theory, we calculate the likelihood of the earthquake in a scenario that contains only the tectonic background and excludes the dike and in a scenario that includes the dike but excludes the background. Applying the Bayes?? theorem, we obtain that the probability that the earthquake was indeed triggered by the dike, rather than purely due to the accumulation of tectonic strain, is about 60 to 90?%. This methodology allows us to assign quantitative probabilities to different scenarios and can help in classifying earthquakes as triggered or not triggered by natural or human-induced changes of stress in the crust.     

Numerical modelling of dyke interaction and its influence on oceanic crust formation

Dykes are an essential element in building oceanic crust, most prominent in sheeted dyke complexes in the upper crust. Since dykes alter the magnitude and orientation of the local stress field, they cannot be treated as passive infillings of extensional fractures.We use a quasi-static, iterative 2-D boundary element method allowing for a wholesale movement of fluid-filled fractures. Effects of stress and pressure gradients, buoyancy and enclosed fluid mass are considered. The implications of the dyke-induced stress field are analysed combining the simulation of fracture propagation with computation of dyke interaction. Dyke interaction occurs by the adaptation of ascending dykes to the stress field caused by previous fractures arrested in the crust and leads to focussing and crossing of dykes. Examples for applications are introduced, concerning e.g. the generation of a magma chamber and the formation of the sheeted dyke complex. Our main results are that the interaction between dykes can be considerable and that the most important controlling factor is stress. The interaction is small when the horizontal tensional stress is large compared to the pressure in the dyke head. Otherwise, dykes tend to attract each other and to form centres of high dyke density or sill layers.     

Effects of topography and crustal heterogeneities on the source estimation of LP event at Kilauea volcano

The main goal of this study is to improve the modelling of the source mechanism associated with the generation of long period (LP) signals in volcanic areas. Our intent is to evaluate the effects that detailed structural features of the volcanic models play in the generation of LP signal and the consequent retrieval of LP source characteristics. In particular, effects associated with the presence of topography and crustal heterogeneities are here studied in detail. We focus our study on a LP event observed at Kilauea volcano, Hawaii, in 2001 May. A detailed analysis of this event and its source modelling is accompanied by a set of synthetic tests, which aim to evaluate the effects of topography and the presence of low velocity shallow layers in the source region. The forward problem of Green's function generation is solved numerically following a pseudo-spectral approach, assuming different 3-D models. The inversion is done in the frequency domain and the resulting source mechanism is represented by the sum of two time-dependent terms: a full moment tensor and a single force. Synthetic tests show how characteristic velocity structures, associated with shallow sources, may be partially responsible for the generation of the observed long-lasting ringing waveforms. When applying the inversion technique to Kilauea LP data set, inversions carried out for different crustal models led to very similar source geometries, indicating a subhorizontal cracks. On the other hand, the source time function and its duration are significantly different for different models. These results support the indication of a strong influence of crustal layering on the generation of the LP signal, while the assumption of homogeneous velocity model may bring to misleading results.     

Nutrient dynamics in an alpine headwater stream: use of continuous water quality sensors to examine responses to wildfire and precipitation events

Stream water quality can change substantively during diurnal cycles, discrete flow events, and seasonal time scales. In this study, we assessed event responses in surface water nutrient concentrations and biogeochemical parameters through the deployment of continuous water quality sensors from March to October 2011 in the East Fork Jemez River, located in northern New Mexico, USA. Events included two pre‐fire non‐monsoonal precipitation events in April, four post‐fire precipitation events in August and September (associated with monsoonal thunderstorms), and two post‐fire non‐monsoonal precipitation events in October. The six post‐fire events occurred after the Las Conchas wildfire burned a significant portion of the contributing watershed (36%) beginning in June 2011. Surface water nitrate (NO₃? N) concentrations increased by an average of 50% after pre‐fire and post‐fire non‐monsoonal precipitation events and were associated with small increases in turbidity (up to 15 NTU). Beginning 1 month after the start of the large regional wildfire, monsoonal precipitation events resulted in large multi‐day increases in dissolved NO₃? N (6 × background levels), dissolved phosphate (100 × background levels), specific conductance (5 × background levels), and turbidity (>100 × background levels). These periods also corresponded with substantial sags in dissolved oxygen (<4 mg l^?1) and pH (<6.5). The short duration and rapid rates of change during many of these flow events, particularly following wildfire, highlight the importance of continuous water quality monitoring to quantify the timing and magnitude of event responses in streams and to examine large water quality excursions linked to catchment disturbance. Copyright © 2015 John Wiley & Sons, Ltd.     

Moment tensors of microearthquakes from the Eyjafjallajökull volcano in South Iceland

Analyses of relative P - and S -wave amplitudes of 15 selected earthquakes ( M _L <2.3) from a seismic swarm, which occurred in May and June 1994 at the Eyjafjallajökull volcano in South Iceland, reveal similar radiation patterns, a thrust-type double-couple with an additional source component. All focal solutions have nearly vertical T -axes and horizontally oriented P -axes, with E-W-oriented nodal planes. The volume increase corresponding to an isotropic source component is estimated to be in the range of 24 m³. The temporal and spatial seismic pattern, small magnitude range, focal mechanisms and depth range of the Eyjafjallaökull earthquakes indicate vertical intrusion of magma into a confined region at the northern flank of the volcano.