Geophysical Journal International

doi: 10.1111/j.1365-246X.2011.05045.xpmid: N/A

Zhang, Guohong; Qu, Chunyan; Shan, Xinjian; Song, Xiaogang; Zhang, Guifang; Wang, Chisheng; Hu, Jyr-Ching; Wang, Rongjiang

doi: 10.1111/j.1365-246X.2011.05039.xpmid: N/A

SummaryWe investigate the slip distribution of the 2008 May 12 Wenchuan Ms 7.9 earthquake using GPS data and InSAR measurements under a linear inversion scheme, with emphasis on the effect of three factors, including constraint on rake, different discretizations, and layered elastic model. Within our inversion parameterization context, we find the most influential factor would be constraint on rake. Without constraint on rake, the slip model seems physically wrong under the depth of 15 km, due to the limited depth resolution of the geodetic data used, especially the one orbit of InSAR measurements. Thus it is necessary to add a priori to the slip rake to obtain a reasonable fault source model. Different discretizations of the subfault patches have a notable impact on the slip distribution. Also, the layered elastic model predicts more slip at depth than does the half-space model, by about 15–20 per cent. The characteristics of slip distribution established through our inversions include some points as follows: (1) the most reliable results would be expected at depth range of 0–15 km; (2) three peak-slip asperities are inverted, at Yingxiu county (near epicentre), Yuejiashan county, and Beichuan county, respectively; (3) the inverted rake along the Yingxiu-Beichuan fault changes from dominantly thrusting motion at the southwest segments to dominant right-lateral or even pure right-lateral strike slip at the northeast segments, and (4) only thrust slip is occurred on the Guanxian-Jiangyou fault.

Place, Joachim; Sausse, Judith; Marthelot, Jean-Michel; Diraison, Marc; Géraud, Yves; Naville, Charles

doi: 10.1111/j.1365-246X.2011.05012.xpmid: N/A

SummaryThis paper illustrates the efficiency of vertical seismic profiling (VSP) for the investigation of dipping and hydraulically conductive structures affecting a granitic basement covered by sediments. A three-component (3C) VSP data set has been acquired in the GPK1 and EPS1 wells of the Soultz-sous-Forêts enhanced geothermal system (EGS) located within the Upper Rhine Graben (URG). Our study focuses on the isotropic processing of profiles acquired with vertical vibrator P and their subsequent interpretation. Mainly P–S converted reflections are identified from the analysis of the 3C records. These P–S conversions occur on steep permeable faults that are positioned in space by traveltime modelling. These faults cut the granite basement in several hectometric-scale blocks, and represent the main fluid paths between the boreholes. These faults are thought to be inherited from late Variscan and Alpine deformation periods, reactivated by the current stress field. When properly processed and interpreted, VSP allow the scale gap between surface and borehole data to be bridged.

Dinc, A. Nilay; Rabbel, Wolfgang; Flueh, Ernst R.; Taylor, Waldo

doi: 10.1111/j.1365-246X.2011.05041.xpmid: N/A

SummaryThe continental margin of Nicaragua and Costa Rica is characterized by significant lateral changes from north to south such as a decreasing dip of the slab, a decreasing magma production and a shift in the volcanic front. To investigate this transition, a joint on- and offshore local earthquake tomography was performed. Low P-wave velocities and high Vp/Vs ratios, indicative for hydration, were found in the upper-mantle and lowermost crust beneath the Sandino Basin. The mantle wedge hydration can be estimated to 2.5 wt. per cent beneath south Nicaragua. In contrast, the mantle wedge beneath north Costa Rica is weakly or not hydrated. The hydration leads to a local gap in the seismicity in Nicaragua. The lateral transition between the hydrated and non-hydrated areas occurs within a distance of about 10 km. This transition coincides with a change in the crustal thickness in the order of 5–10 km, thickening to the south, and in the tectonic regimes. The change in the tectonic regimes towards a stronger extension along the margin of Nicaragua could be the key for understanding the observations: the extension may support the opening of pathways for a wide zone of fluid migration and hydration through the overriding plate which are identified with areas of low Vp, high Vp/Vs and low seismicity.

Allègre, V.; Jouniaux, L.; Lehmann, F.; Sailhac, P.

doi: 10.1111/j.1365-246X.2011.05038.xpmid: N/A

SummaryRevil and Linde recently commented our paper concerning streaming potential (SP) measurements in unsaturated sand during drainage experiments. This comment suggests that the approach used to infer SP coefficients was inappropriate for unsaturated conditions, and therefore yielded wrong conclusions and ‘unphysical’ results regarding the behaviour of the relative SP coefficient. This reply argues that even if in Allègre et al. we neglected some secondary electrokinetic sources, the resulting conclusions are still representative of the behaviour of the true SP coefficient, and that the remarks of Revil & Linde arose from a misunderstanding of the drainage experiment conditions. We also find support for our results from a comparison between our observations and previous experimental studies.

Levy, Clara; Jongmans, Denis; Baillet, Laurent

doi: 10.1111/j.1365-246X.2011.05046.xpmid: N/A

SummaryA small-aperture (40 m) short-period seismic array was installed during four months on the Vercors massif (Western French Alps) at the top of a limestone column which collapsed one month and a half later. During this monitoring period, 193 seismic events were recorded by the seven seismometers of the array. Signal analysis yielded three main types of nearby seismic events to be identified from temporal and spectral characteristics: microearthquakes (single or multiple events), individual block falls and rock falls. It turned out that 60 per cent of the 193 events were classified as microearthquakes, exhibiting distinct P and S waves, while 17 per cent of these events remained unclassified. Out of the microearthquakes, 40 events with a good signal-to-noise ratio were selected and processed. P- and S-wave traveltimes were picked on the records and the inferred hypocentral distances agree with the two zones of the scarp exhibiting fresh ruptures after the fall. Polarization analysis of the 3-C records, along with numerical simulations, allowed discriminating between the two possible rupture mechanisms (toppling and sliding). Shear rupture (sliding) was the predominant mode in the lower part of the column whereas traction rupture (toppling) affected the upper part. Finally, the comparison between the ground motions recorded on the column and on the rock mass showed a systematic amplification on the column. Signal processing and numerical modelling both suggest that this amplification resulted from the excitation of the natural frequencies of the column and is particularly high (>3) for microearthquakes occurring at the column-to-mass interface.

Revil, A.; Linde, N.

doi: 10.1111/j.1365-246X.2010.04850.xpmid: N/A

SummaryAllègre et al. recently presented new experimental data regarding the dependence of the streaming potential coupling coefficient with the saturation of the water phase. Such experiments are important to model the self-potential response associated with the flow of water in the vadose zone and the electroseismic/seismoelectric conversions in unsaturated porous media. However, the approach used to interpret the data is questionable and the conclusions reached by Allègre et al. likely incorrect

Hier-Majumder, S.

doi: 10.1111/j.1365-246X.2011.05024.xpmid: N/A

SummaryThis paper investigates the development of anisotropic frictional resistance and mobility as a function of an applied stress in a partially molten aggregate. Shapes of initially spherical melt pockets and cylindrical melt tubules are calculated as a function of the applied stress using a perturbation analysis. The applied stress excites a local flow within the melt units (tubules or pockets) rendering their initially circular average cross-section elliptical. Average aspect ratio of tubule cross-sections predicted by the perturbation analysis is compared with the results from laboratory experiments. The increase in the average aspect ratio is related to an increase in the volume fraction of melt films. A derivation for the effective resistance and mobility tensors as a function of the applied stress is presented. The anisotropy of resistance and mobility increases continuously with the applied stress in a non-linear fashion. In the upper limit, the anisotropy of the mobility tensor can increase by a factor of 10 under an applied stress of 14 MPa.

Fournier, Alexandre; Aubert, Julien; Thébault, Erwan

doi: 10.1111/j.1365-246X.2011.05037.xpmid: N/A

SummaryWe show how a 3-D, self-consistent numerical model of the geodynamo can be used as the subjective prior information for the determination of Earth's core surface flows from the geomagnetic field and its secular variation. This is achieved by estimating those parts of the numerical model state vector hidden from the observations, through a standard Kalman filtering (or stochastic inverse) procedure, where the Kalman gain matrix is based on the multivariate statistics of the geodynamo model. To allow for a direct comparison with observations, the field variables entering these statistics are scaled following two of the scaling laws that have recently come to the fore in numerical dynamo modelling, which express the dependency of the secular variation timescale and the magnetic energy density on their respective control parameters. We perform test experiments with noisy synthetic data, showing good to excellent recovery of the hidden parts of the state vector. A geomagnetic field model parent to a candidate model to the 2010 release of IGRF is then used for a core surface flow estimation. The estimated flow confirms the presence of convective columns underneath America, whereas exhibiting a high level of equatorial symmetry. We suggest that the discrete state estimation problem considered here (in connection with the classical core flow problem) could be used generically as a means to assess the degree of geophysical realism of a given geodynamo model. More generally, this study opens the way to using scaling laws and multivariate statistics from numerical models in the broader context of geomagnetic data assimilation.

Hauksson, Egill

doi: 10.1111/j.1365-246X.2011.05042.xpmid: N/A

SummaryThe geographical distribution of the (1981–2005) seismicity in southern California forms a ±150 km broad zone adjacent to the Pacific–North America plate boundary, ranging from depths of ∼1–∼30 km, with the bulk of the focal depths in the range of 2–12 km. The distribution of the seismicity that includes both mainshock–aftershock sequences and background events is affected by both static and kinematic geophysical parameters of the crust. The static parameters include heat flow, topography, crustal density, Vp/Vs ratio, hypocentral fault-distance and crustal thickness from receiver functions. The tectonic loading is represented by kinematic parameters such as the crustal shear strain rate field, and the dilatational strain rate field. In our analysis, we normalize the seismicity relative to the areal density of the range of values of each of the parameters. Most of the seismicity occurs in areas of average heat flow, low to intermediate topography, average Vp/Vs and high late Quaternary fault density, and forms seismogenic zones that extend through the brittle crust. The location of late Quaternary faults, often described as zones of weakness, influences the geographical distribution of seismicity more than any other parameter. Although above or below average crustal properties such as high heat flow, thin crust or very low Vp/Vs values exist, these properties only influence the spatial distribution of seismicity in a minor way. As an example, the Salton Trough area of low topography, high heat flow, high Vp/Vs, high shear strain rate and thin crust has distributed seismicity within a thin seismogenic zone. Also, somewhat surprisingly, areas of high topography, low heat flow, low Vp/Vs, low shear strain rate and thick crust have low seismicity rates but a thin seismogenic zone. We determine an empirical relationship between heat flow and crustal thickness to show how the ∼400 °C temperature isotherm gradually deepens with crustal thickness and forms the base of the seismogenic zone for crustal thicknesses from 22 to 37 km. For crustal thickness ranging from 37 to 43 km, the ∼250 °C isotherm forms the base of the seismogenic zone, suggesting that seismic faulting in these regions is confined to the top of the upper crust (12–14 km), and thus does not accommodate plate motion.