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  • Articles  (15)
  • Wiley  (15)
  • American Association for the Advancement of Science (AAAS)
  • Journal of Geophysical Research JGR - Solid Earth  (14)
  • 7527
  • Physics  (15)
  • Computer Science
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  • 1
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    The Fonualei Rift and Spreading Center: Effects of Ultraslow Spreading and Arc Proximity on Backarc Crustal Accretion (2016)
    Wiley
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    Publication Date: 2016-07-08
    Description: Backarc spreading center characteristics reflect interactions between plate-driven mantle advection and melting and slab-driven hydrous melting and buoyant upwelling in the mantle wedge. At the Fonualei Rift and Spreading Center (FRSC) in the Lau Basin spreading rates decrease from slow to ultraslow, providing an opportunity to examine crustal accretion as the plate-driven component is minimized. A new Lau Basin kinematic analysis predicts FRSC spreading rates of ~32-8 mm/yr southward, much slower than previous estimates of ~95-47 mm/yr. Here we examine FRSC morphology and geophysical characteristics as it approaches the Tofua arc volcanic front southward and spreading rates decrease, minimizing the plate-driven component of mantle advection and maximizing buoyant hydrous flux melting. Axial morphology changes abruptly from a deep, flat, faulted axis ~100 km away from the arc to a volcanic ridge that shoals and increases in relief southward. Within ~50 km of the arc at the south end, the volcanic ridge is abruptly replaced by isolated volcanic cones bisected by volcanic rift zones and surrounded by anomalously deep seafloor. These morphologic changes likely reflect along-axis focusing of mantle upwelling and melting similar to that seen at ultraslow mid-ocean ridges, causing the change in morphology from a segmented ridge to spaced axial cones. We propose that as opening rates slow and the ridge approaches the arc, more of the inherently three-dimensional pattern of hydrous flux melting and buoyant upwelling in the mantle wedge is expressed volcanically. With faster opening, two-dimensional plate-driven mantle advection dominates melt production, favoring ridges over point-source features.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 2
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    Analysis of Microseismicity Framing ML 〉 2.5 Earthquakes at The Geysers Geothermal Field, California (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Preparatory mechanisms accompanying or leading to nucleation of larger earthquakes have been observed at both laboratory and field scales, but conditions favoring the occurrence of observable preparatory processes are still largely unknown. In particular, it remains a matter of debate why some earthquakes occur spontaneously without noticeable precursors as opposed to events that are preceded by an extended failure process. In this study, we have generated new high‐resolution seismicity catalogs framing the occurrence of 20 ML 〉 2.5 earthquakes at The Geysers geothermal field in California. To this end, a seismicity catalog of the 11 days framing each large event was created. We selected 20 sequences sampling different hypocentral depths and hydraulic conditions within the field. Seismic activity and magnitude frequency distributions displayed by the different earthquake sequences are correlated with their location within the reservoir. Sequences located in the northwestern part of the reservoir show overall increased seismic activity and low b values, while the southeastern part is dominated by decreased seismic activity and higher b values. Periods of high injection coincide with high b values and vice versa. These observations potentially reflect varying differential and mean stresses and damage of the reservoir rocks across the field. About 50% of analyzed sequences exhibit no change in seismicity rate in response to the large main event. However, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
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  • 3
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    Geology and Kinematics of the Niuafo'ou Microplate in the Northern Lau Basin (2016)
    Wiley
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    Publication Date: 2016-07-12
    Description: We present a revised three-plate kinematic model for the Lau Basin, focusing on new interpretations of the geologic characteristics of the Niuafo‘ou microplate boundaries. It builds upon the three-plate model by Zellmer and Taylor [2001], while resolving some geologic discrepancies in that model. Along the southern branch of the Mangatolu Triple Junction and the Fonualei Rift and Spreading Center, which together form the Niuafo'ou-Tonga plate boundary, our model predicts slow to ultraslow opening rates (~32-8 mm/yr southward), consistent with axial morphology and magnetic profile modeling. Our interpretation of the seafloor magnetization pattern and flank morphology along the Central Lau Spreading Center (CLSC) indicates that opening rates along the Australia-Niuafo'ou boundary are higher than predicted in the previous model, thus compensating for slower rates along the Niuafo'ou-Tonga boundary. Lastly, we show geologic evidence that Peggy Ridge (PR), interpreted previously as an active transform fault and therefore a strong azimuthal constraint on Niuafo'ou plate motion, is inactive and that the active plate boundary is the adjacent Lau Extensional Transform Zone, extending from the CLSC to its intersection with the Northwest Lau Spreading Center. The removal of the PR azimuthal constraint allows us to determine an Euler pole for Australia-Niuafo'ou opening that satisfies three-plate closure criterion and develop a geologically consistent model of Lau Basin kinematics. While the final model focuses on constraining current plate velocities assuming rigid plate Eulerian rotations, the geologic analyses presented herein also provide insight into long term basin evolution, which appears to involve significant non-rigid plate kinematics.
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  • 4
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    Effects of long-term fluid injection on induced seismicity parameters and maximum magnitude at northwestern The Geysers geothermal field (2015)
    Wiley
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    Publication Date: 2015-10-03
    Description: The long-term temporal and spatial changes in statistical, source and stress characteristics of one cluster of induced seismicity recorded at The Geysers geothermal field (US) are analyzed in relation to the field operations, fluid migration and constraints on the maximum likely magnitude. Two injection wells, Prati-9 and Prati-29, located in the northwestern part of the field and their associated seismicity composed of 1,776 events recorded throughout a seven-year period were analyzed. The seismicity catalog was relocated and the source characteristics including focal mechanisms and static source parameters were refined using first-motion polarity, spectral fitting and mesh spectral ratio analysis techniques. The source characteristics together with statistical parameters ( b value) and cluster dynamics were used to investigate and understand the details of fluid migration scheme in the vicinity of injection wells. The observed temporal, spatial and source characteristics were clearly attributed to fluid injection and fluid migration towards greater depths, involving increasing pore pressure in the reservoir. The seasonal changes of injection rates were found to directly impact the shape and spatial extent of the seismic cloud. A tendency of larger seismic events to occur closer to injection wells and a correlation between the spatial extent of the seismic cloud and source sizes of the largest events was observed suggesting geometrical constraints on the maximum likely magnitude and its correlation to the average injection rate and volume of fluids present in the reservoir.
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  • 5
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    A New Kinematic Model of the Mid-Atlantic Ridge between 55∘55'N and The Bight Transform Fault for the Past 6 Ma (2016)
    Wiley
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    Publication Date: 2016-01-26
    Description: We present a new fine-scaled tectonic model of the Mid-Atlantic Ridge between 55 ∘ 55’N and the Bight Transform Fault for the past 6 Ma. We model our new marine magnetic data, providing a robust kinematic model for the area. The new model uses propagating rifts to explain observed asymmetry in high-resolution marine magnetic data. The area closest to the Bight Transform Fault has undergone two relatively large (9 km and 6 km) relocations of the spreading center within the past 1 Ma, which explains the divided central anomaly; we attribute this relocation to two very fast propagations. The amount of lithosphere transferred between the plates during each propagation event is 2-10 km and the propagators identified are short-lived (0.5-4.0 Ma). Most of the propagation in the area is to the north and all but two propagators transfer lithosphere from the Eurasian plate to the North American plate, which is consistent with the overall eastward relocation of the Reykjanes Ridge.
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  • 6
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    Constraints on the crustal structure of the internal Variscan Belt in SW Europe: A magnetotelluric transect along the eastern part of Central Iberian Zone, Iberian Massif (2011)
    Wiley
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    Publication Date: 2011-02-15
    Description: The Iberian Massif is the best exposed segment of the European Variscan Belt. It includes relatively well preserved terranes that were accreted by transpression along time and resulted in a number of geotectonic units that formed part of the Late Paleozoic assembly of the Pangaea Supercontinent. In SW Iberia, these units are the Central Iberian Zone (CIZ), Ossa Morena Zone (OMZ), and the South Portuguese Zone (SPZ). A 210 km long NE-SW magnetotelluric profile was carried out through the CIZ, from the OMZ-CIZ boundary toward the north, reaching the Tagus (Cenozoic) basin. Data dimensionality analysis resulted in a suitable 2-D electrical resistivity structure, allowing a 2-D inversion of the data set. Complementary available geophysical data (deep seismic, gravity and aeromagnetic) and a comparison with a detailed geological cross section led us to constrain the interpretation of the 2-D electrical resistivity structure of the CIZ crust. The results show, for the upper crust, the existence of diverse conductive/resistive bodies that correlate well with known geological features (sedimentary basins, faults, granitic plutons, mineralized systems). A mild but steady conductive band is located along the middle and lower crust that is interpreted as a mafic granulite basement. The upper section of this band connects with several elongated shallow conductors, providing further evidence for the existence, in the Central Iberian Zone, of a complex décollement system where the major faults are rooted. Such a crustal architecture is viewed as the northward continuation of the Variscan large-scale structures previously recognized in the southern sectors (OMZ and SPZ).
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  • 7
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    Spatiotemporal changes, faulting regimes and source-parameters of induced seismicity: A case study from The Geysers geothermal field (2014)
    Wiley
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    Publication Date: 2014-10-18
    Description: The spatiotemporal, kinematic and source characteristics of induced seismicity occurring at different fluid-injection rates are investigated to determine the predominant physical mechanisms responsible for induced seismicity at the northwestern part of The Geysers geothermal field, California. We analyze a relocated hypocenter catalog from a seismicity cluster where significant variations of the stress tensor orientation were previously observed to correlate with injection rates. We find that these stress tensor orientation changes may be related to increased pore pressure and the corresponding changes in poroelastic stresses at reservoir depth. Seismic events during peak-injections tend to occur at greater distances from the injection well, preferentially trending parallel to the maximum horizontal stress direction. In contrast, at lower injection rates the seismicity tends to align in a different direction which suggests the presence of a local fault. During peak injection intervals, the relative contribution of strike-slip faulting mechanisms increases. Furthermore, increases in fluid injection rates also coincide with a decrease in b values. Our observations suggest that regardless of the injection stage, most of the induced seismicity results from thermal fracturing of the reservoir rock. However, during peak injection intervals, the increase in pore pressure may likewise be responsible for the induced seismicity. By estimating the thermal and hydraulic diffusivities of the reservoir, we confirm that the characteristic diffusion length for pore pressure is much greater than the corresponding length scale for temperature and also more consistent with the spatial extent of seismicity observed during different injection rates.
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  • 8
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    Evidence for geomagnetic excursions recorded in Brunhes- and Matuyama – Chron lavas from the Trans-Mexican Volcanic Belt (2013)
    Wiley
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    Publication Date: 2013-05-16
    Description: [1]  This study presents paleomagnetic data from 59 independent lava flows from the Trans-Mexican Volcanic Belt (TMVB) with ages from 6.4 Ma to recent, 52 being younger than 1 Ma, and 11 new 40 Ar/ 39 Ar age determinations. Most remanence carriers are Ti-poor titanomagnetite of pseudosingle-domain magnetic structure, nine lavas contain small amounts of titanomaghemite, and four lavas additional (titano-) hematite. Paleosecular variation of lava flows younger than 1.7 Ma, is consistent with latitude dependent Model G and also in agreement with other Pleistocene paleomagnetic data from the TMVB. The directional record of Brunhes- and Matuyama Chrons lavas was correlated to the geomagnetic polarity timescale and there is evidence for at least 4 geomagnetic excursions. One lava flow dated at 592 ± 20 ka has a fully reversed paleodirection and most likely erupted during the Big Lost excursion. Another fully reversed flow, dated at 671 ± 12 ka, gives new volcanic evidence for the Delta / Stage 17 excursion. This excursion is supported by a reversed-intermediate direction of another flow from a different volcanic field but of very close age of 673 ± 10 ka. From the Matuyama age lavas, one flow with normal polarity magnetization, dated at 949 ± 37 ka, could either be related to the Kamikatsura or the Santa Rosa excursion and a normal polarity flow, dated at 1628 ± 56 ka, could have been emplaced during the Gilsa excursion. The results presented here confirm in one case but disagree in four cases with results presented in two previous studies of the same lava flows and interpreted as geomagnetic excursions.
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  • 9
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    A refined methodology for stress inversions of earthquake focal mechanisms (2016)
    Wiley
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    Publication Date: 2016-11-17
    Description: We develop an improved methodology for reliable high resolution inversions of focal mechanisms to background stress field orientation and stress ratio R in 2 or 3 dimensions. The earthquake catalog is declustered to remove events likely affected strongly by local stress interactions rather than reflecting the large-scale background stress field. The declustered data are discretized with the k-means algorithm into groups containing a number of focal mechanisms between a minimum number N min and 2 N min . Synthetic tests indicate that N min  ≈ 30 provides stable inversion results under different stress regimes and noise conditions when R ≈ 0.5, while N min  ≈ 45 is needed for R near 0 or 1. Additional synthetic tests compare the performance of selecting the fault plane of each focal mechanism using a) the plane with lowest misfit angle between the slip vector from the focal mechanism and shear traction from the stress tensor, and b) the plane with highest instability coefficient representing proximity to the optimally oriented fault for given stress field and friction coefficient. The instability criterion is found to provide more accurate inversion results under all tested stress regimes, stress ratios and noise conditions. The refined inversion methodology combines selecting fault planes using the instability criterion iteratively with a damped simultaneous inversion of different focal mechanism groups. Results characterizing neighborhoods of discretized domains merged during the damped inversion provide high resolution information independent of the discretization. Some aspects of the methodology are illustrated with focal mechanism data from the San Jacinto fault zone in southern California.
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  • 10
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    Impact of fluid injection on fracture reactivation at The Geysers geothermal field (2016)
    Wiley
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    Publication Date: 2016-09-29
    Description: We analyze the spatio-temporal distribution of fault geometries from seismicity induced by fluid-injection at The Geysers geothermal field. The consistency of these faults with the local stress field is investigated using 1) the fault instability coefficient I comparing the orientation of a fault with the optimal orientation for failure in the assumed stress field and 2) the misfit angle β between slip vectors observed from focal mechanisms and predicted from stress tensor. A statistical approach is applied to calculate the most likely fault instabilities considering the uncertainties from focal mechanisms and stress inversion. We find that faults activated by fluid injection may display a broad range in orientations. About 72% of the analyzed seismicity occurs on faults with favorable orientation for failure with respect to the stress field. However, a number of events are observed either to occur on severely misoriented faults or to slip in a different orientation than predicted from stress field. These events mostly occur during periods of high injection rates and are located in proximity to the injection wells. From the stress inversion, the friction coefficient providing the largest overall instability is μ  = 0.5. About 91% of the events are activated with an estimated excess pore pressure 〈10 MPa, in agreement with previous models considering the combined effect of thermal and poroelastic stress changes from fluid injection. Furthermore, high seismic activity and largest magnitudes occur on favorably oriented faults with large instability coefficients and low slip misfit angles.
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