ALBERT

Description: This work presents an innovative strategy to enhance the resolution of surface wave tomography obtained from ambient noise cross-correlation ( C 1 ) by bridging asynchronous seismic networks through the correlation of coda of correlations ( C 3 ). Rayleigh wave group dispersion curves show consistent results between synchronous and asynchronous stations. Rayleigh wave group traveltimes are inverted to construct velocity–period maps with unprecedented resolution for a region covering Mexico and the southern United States. The resulting period maps are then used to regionalize dispersion curves in order to obtain local 1-D shear velocity models ( V S ) of the crust and uppermost mantle in every cell of a grid of 0.4°. The 1-D structures are obtained by iteratively adding layers until reaching a given misfit, and a global tomography model is considered as an input for depths below 150 km. Finally, a high-resolution 3-D V S model is obtained from these inversions. The major structures observed in the 3-D model are in agreement with the tectonic-geodynamic features and with previous regional and local studies. It also offers new insights to understand the present and past tectonic evolution of the region.

Description: We present 3-D models of the P- and S -wave velocity distributions in the crust and uppermost mantle beneath Sicily, Calabria (Southern Italy), and surrounding submerged areas, obtained by tomographic inversion of traveltimes of regional body waves phases. Our method combines double-difference tomographic inversion with a post-processing procedure [Weighted Average Model method (WAM)]. This procedure was applied to a set of models consistent with the experimental data. We tested the ability of the WAM procedure to mitigate the uncertainty associated with the arbitrary nature of the many input parameters required for each inversion. The local reliability and resolution of the obtained models have been assessed through: synthetic tests, experimental tests carried out with independent data sets and unconventional tests based on the analysis of the internal consistency of the P - and S -velocity models. The tomographic images provide a detailed sketch of P- and S- wave velocity anomalies. These clearly show the shape of the Sicilian-Maghrebian belt beneath Sicily and Calabrian Arc at different depths. Low V P and Vs bodies are imaged beneath Stromboli and Marsili volcanoes in the southern Tyrrhenian, whereas high and low seismic velocities alternate beneath the Etna giving inferences on the possible depth of the mantle melting feeding the volcano. In the upper crust, the main sedimentary basins and tectonic features are also well imaged. Finally, tomographic cross sections show the trend of the Moho in the study area, where its depth ranges between 35 and 40 km beneath the Sicilian belt and between 15 and 22 km in the southern Tyrrhenian basin and Ionian Sea.

Description: During hydraulic stimulations, a complex interaction is observed between the injected flux and pressure, number and magnitude of induced seismic events, and changes in seismic velocities. In this paper, we model formation and propagation of damage zones and seismicity patterns induced by wellbore fluid injection. The model includes the coupling of poroelastic deformation and groundwater flow with damage evolution (weakening and healing) and its effect on the elastic and hydrologic parameters of crystalline rocks. Results show that three subsequent interactions occur during stimulation. (1) Injected flux–pressure interaction: typically, after a flux increase, the wellbore pressure also rises to satisfy the flux conditions. Thereafter, the elevated pore pressure triggers damage accumulation and seismic activity, that is, accompanied by permeability increase. As a result, wellbore pressure decreases retaining the target injected flux. (2) Wellbore pressure–seismicity interaction: damage processes create an elongated damage zone in the direction close to the main principal stress. The rocks within the damage zone go through partial healing and remain in a medium damage state. Damage that originates around the injection well propagates within the damage zone away from the well, raising the damage state of the already damaged rocks, and is followed by compaction and fast partial healing back to a medium damage state. This ‘damage wave’ behaviour is associated with the injected flux changes only in early stages while fracture's height ( h ) is larger than its length ( l ). The ratio h / l controls the deformation process that is responsible for several key features of the damage zone. (3) Stress- and damage-induced variations of the seismic P -wave velocities ( V p ). V p gradually decreases as damage is accumulated and increases after rock failure as the shear stress is released and healing and compaction are dominant. Typically, V p decreases within the damage zone and increases in most regions outside the damage zone. After a ‘damage wave’ that is originated at the well, V p rises back and may exceeds its initial values. Similar transient variations of the elastic parameters and the effects of h / l are observed at the Soultz-sous-Forêts Enhanced Geothermal System (EGS) records of induced seismicity during hydraulic injection.

Description: Sicily Channel is a portion of Mediterranean Sea, between Sicily (Southern Italy) and Tunisia, representing a part of the foreland Apennine-Maghrebian thrust belt. The seismicity of the region is commonly associated with the normal faulting related to the rifting process and volcanic activity of the region. However, certain seismic patterns suggest the existence of some mechanism coexisting with the rifting process. In this work, we present the results of a statistical analysis of the instrumental seismicity and a reliable relocalization of the events recorded in the last 30 yr in the Sicily Channel and western Sicily using the Double Difference method and 3-D Vp and Vs tomographic models. Our procedure allows us to discern the seismic regime of the Sicily sea from the Tyrrhenian one and to describe the main features of an active fault zone in the study area that could not be related to the rifting process. We report that most of the events are highly clustered in the region between 12.5°–13.5°E and 35.5°–37°N with hypocentral depth of 5–40 km, and reaching 70 km depth in the southernmost sector. The alignment of the seismic clusters, the distribution of volcanic and geothermal regions and the location of some large events occurred in the last century suggest the existence of a subvertical shear zone extending for least 250 km and oriented approximately NNE–SSW. The spatial distribution of the seismic moment suggests that this transfer fault zone is seismically discontinuous showing large seismic gaps in proximity of the Ferdinandea Island, and Graham and Nameless Bank.

Description: The geothermal power plant of Soultz-sous-Forêts in Northeastern France consists of three boreholes (GPK2, GPK3, GPK4) reaching a depth of about 5 km. All the wells were stimulated through hydraulic injections. In this study, we present the results of a time-dependent (4-D) seismic tomography obtained with the P -wave arrival times of seismic events recorded in 2003 during the stimulation of the GPK3 well. The method combines double-difference tomography with the Weighted Average Model post-processing that corrects for parameter dependence effects. In light of additional processing of the continuous seismic records of 23 surface stations, some 4728 precisely located events were selected and separated into 13 subsets to examine periods defined with respect to the injection scheme. Particular attention is given to changes in injected flow rates, periods of stationary injection conditions, periods of dual stimulation with the GPK2 well and post-injection periods. Results confirm that significant structures crossing the well have controlled the evolution of the seismicity and have played a fundamental role in the distribution and amplitude of the seismic anomalies. Furthermore, the evolution of the seismic velocity field, together with the representation of the relocated seismicity, have been compared with the results of the 4-D tomography of the GPK2 well, which is located at only 600 m from the stimulated well.

Description: The aim of this work is to assess the feasibility of the noise-cross correlation tomography method for imaging and detecting potential geothermal reservoirs even in highly urbanised areas. We tested the noise correlation method to reconstruct the shape of the Soutlz-sous-Forêts enhanced geothermal system (northern Alsace, France). We inverted Rayleigh waves (RWs) reconstructed from cross-correlations of 15 months of ambient seismic noise recorded by a seismic array installed around the Soutlz geothermal power plant. By correlating noise records between 22 receivers, we reconstructed RWs with sufficient signal-to-noise ratio for 231 interstation paths. The reconstructed waveforms were used to estimate group velocity dispersion curves at periods between 1.0 and 5.0 s. The results were inverted for 2-D group velocity maps, and finally for a 3-D S -wave velocity model from 0 to 5.2 km depth. Our results clearly show the presence of low velocity bodies in the crystalline basement below the Soultz power plant at depth of 4–5 km, and at shallower depth (2.5–3.5 km) beneath the Rittershoffen and Woerth villages. These observations, in agreement with some previous studies, confirm that our procedure is suitable for geothermal exploration. Furthermore, the model presented here provides some suggestions to improve the existing geothermal power plant and inferences for further explorations in the area.

Description: In this study, we apply the double-difference tomography method to investigate the detailed 3-D structure within and around the Nový Kostel seismic zone, an area in the Czech Republic known for frequent occurrences of earthquake swarms. We use data from the extensively analyzed 2008 swarm, which has known focal mechanisms, principal faults, tectonic stress, source migration and other basic characteristics. We selected about 500 microearthquakes recorded at 22 local seismic stations of the West Bohemia Network (WEBNET). Applying double-difference tomography, combined with Weighted Average Model post-processing to correct for parameter dependence effects, we produce and interpret 3-D models of the Vp-to-Vs ratio (Vp/Vs) in and around the focal zone. The modeled Vp-to-Vs ratio shows several distinct structures, namely an area of high Vp-to-Vs ratio correlating with the microearthquakes, and a layer of low values directly above it. These structures may reflect changes in lithology and/or fluid concentration. The overlaying low Vp-to-Vs ratio layer coincides with high density metamorphic unit associated with the Fichtelgebirge (Smrčiny) granitic intrusion. It is possible that the base of the layer acts as a fluid trap, resulting in the observed periodic swarms.