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Now showing items 1 - 16 of 123930

  • The cold and relatively dry nature of mantle forearcs in subduction zones

    Abers, G. A.   van Keken, P. E.   Hacker, B. R.  

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  • The cold and relatively dry nature of mantle forearcs in subduction zones

    Abers, G. A.   van Keken, P. E.   Hacker, B. R.  

    Some of Earth's coldest mantle is found in subduction zones at the tip of the mantle wedge that lies between the subducting and overriding plates. This forearc mantle is isolated from the flow of hot material beneath the volcanic arc, and so is inferred to reach temperatures no more than 600 to 800 degrees C - conditions at which hydrous mantle minerals should be stable. The forearc mantle could therefore constitute a significant reservoir for water if sufficient water is released from the subducting slab into the mantle wedge. Such a reservoir could hydrate the plate interface and has been invoked to aid the genesis of megathrust earthquakes and slow slip events. Our synthesis of results from thermal models that simulate the conditions for subduction zones globally, however, indicates that dehydration of subducting plates is too slow over the life span of a typical subduction zone to hydrate the forearc mantle. Hot subduction zones, where slabs dehydrate rapidly, are an exception. The hottest, most buoyant forearcs are most likely to survive plate collisions and be exhumed to the surface, so probably dominate the metamorphic rock record. Analysis of global seismic data confirms the generally dry nature of mantle forearcs. We conclude that many subduction zones probably liberate insufficient water to hydrate the shallower plate boundary where great earthquakes and slow slip events nucleate. Thus, we suggest that it is solid-state processes and not hydration that leads to weakening of the plate interface in cold subduction zones.
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  • SKS Splitting Beneath Mount St. Helens:Constraints on Subslab Mantle Entrainment

    Eakin, C. M.   Wirth, E. A.   Wallace, A.   Ulberg, C. W.   Creager, K. C.   Abers, G. A.  

    yy Observations of seismic anisotropy can provide direct constraints on the character of mantle flow in subduction zones, critical for our broader understanding of subduction dynamics. Here we present over 750 new SKS splitting measurements in the vicinity of Mount St. Helens in the Cascadia subduction zone using a combination of stations from the iMUSH broadband array and Cascades Volcano Observatory network. This provides the highest density of splitting measurements yet available in Cascadia, acting as a focused "telescope" for seismic anisotropy in the subduction zone. We retrieve spatially consistent splitting parameters (mean fast direction Phi: 74 degrees, mean delay time partial derivative t: 1.0 s) with the azimuthal occurrence of nulls in agreement with the fast direction of splitting. When averaged across the array, a 90 degrees periodicity in splitting parameters as a function of back azimuth is revealed, which has not been recovered previously with single-station observations. The periodicity is characterized by a sawtooth pattern in Phi with a clearly defined 45 degrees trend. We present new equations that reproduce this behavior based upon known systematic errors when calculating shear wave splitting from data with realistic seismic noise. The corrected results suggest a single layer of anisotropy with an ENE-WSW fast axis parallel to the motion of the subducting Juan de Fuca plate; in agreement with predictions for entrained subslab mantle flow. The splitting pattern is consistent with that seen throughout Cascadia, suggesting that entrainment of the underlying asthenosphere with the subducting slab is coherent and widespread.
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  • Slip on shallow-dipping normal faults

    Abers, G. A.  

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  • Southeast Papuan crustal tectonics: Imaging extension and buoyancy of an active rift

    Abers, G. A.   Eilon, Z.   Gaherty, J. B.   Jin, G.   Kim, YH.   Obrebski, M.   Dieck, C.  

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  • Imaging the source region of Cascadia tremor and intermediate-depth earthquakes

    Abers, G. A.   MacKenzie, L. S.   Rondenay, S.   Zhang, Z.   Wech, A. G.   Creager, K. C.  

    The subduction of hydrated oceanic crust releases volatiles that weaken the plate boundary interface, trigger earthquakes, and regulate transient phenomena such as episodic tremor and slip (ETS). It is not clear how dehydration can separately induce earthquakes within the subducting plate and ETS, partly because few data exist on their relationship to subduction zone structures. We present results of a seismic experiment in the Washington Cascades, United States, that images a region producing both earthquake types. Migration of scattered teleseismic waves provides images of low-velocity subducting crust at depths < 40-45 km with sharp boundaries above and below it. The sharp upper boundary indicates a layer of weak sediment or an overpressured fault zone that terminates abruptly downdip at 40-45 km depth. Regular earthquakes are at the top of the mantle within the downgoing plate everywhere the plate is < 95 km deep, but ETS only exists where the sharp upper boundary occurs. The ETS location supports models of slow slip that require near-lithostatic fluid pressure, whereas regular earthquakes nucleate closer to the origin of metamorphic dehydration. Very low shear stresses on the plate boundary may limit seismicity to ETS and similar phenomena.
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  • The causes of spatiotemporal variations in erupted fluxes and compositions along a volcanic arc

    Till, C. B.   Kent, A. J. R.   Abers, G. A.   Janiszewski, H. A.   Gaherty, J. B.   Pitcher, B. W.  

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  • The causes of spatiotemporal variations in erupted fluxes and compositions along a volcanic arc

    Till, C. B.   Kent, A. J. R.   Abers, G. A.   Janiszewski, H. A.   Gaherty, J. B.   Pitcher, B. W.  

    Decades of study on volcanic arcs have provided insight into the overarching processes that control magmatism, and how these processes manifest at individual volcanoes. However, the causes of ubiquitous and dramatic intra-arc variations in volcanic flux and composition remain largely unresolved. Investigating such arc-scale issues requires greater quantitative comparison of geophysical and geochemical data, linked through sets of common intensive variables. To work towards these goals, we use observed lava compositions to estimate the heat budget associated with Quaternary volcanism in the Cascades Arc and compare this to the heat required to produce the observed geophysical properties of the crust. Here we show that along-strike volcanic variability in the Quaternary Cascades Arc is primarily related to variations in the flux of basalt into the crust, rather than variations in their crustal storage history. This approach shows promise for studying other large-scale frontier geologic problems in volcanic arcs.
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  • Strong along-arc variations in attenuation in the mantle wedge beneath Costa Rica and Nicaragua

    Rychert, C. A.   Fischer, K. M.   Abers, G. A.   Plank, T.   Syracuse, E.   Protti, J. M.   Gonzalez, V.   Strauch, W.  

    Attenuation structure in the Central American subduction zone was imaged using local events recorded by the Tomography Under Costa Rica and Nicaragua array, a 20-month-long deployment (July 2004 until March 2006) of 48 seismometers that spanned the fore-arc, arc, and back-arc regions of Nicaragua and Costa Rica. P and S waveforms were inverted separately for the corner frequency and moment of each event and for the path-averaged attenuation operator (t*) of each event-station pair, assuming attenuation is slightly frequency-dependent (proportional to = 0.27). Then, tomographic inversions were performed for S and P attenuation (Q(S)(-1) and Q(P)(-1)). Since P wave amplitudes reflect both shear and the bulk moduli, tomographic inversions were also performed to determine shear and bulk attenuation (Q(S)(-1) and Q(kappa)(-1)), the loss of energy per cycle owing to shearing and uniform compression, respectively. Damping and other inversion tomographic parameters were systematically varied. As is typical in subduction zone attenuation studies, a less attenuating slab, upper plate, and wedge corner and a more attenuating mantle wedge were imaged. In addition, first-order differences between the mantles beneath Nicaragua and Costa Rica were observed. The slab in Nicaragua is more attenuating than the slab in Costa Rica. A larger zone of higher shear attenuation also characterizes the Nicaraguan mantle wedge. Within the wedge, maximum attenuation values at 1 Hz correspond to Qs = 38-73 beneath Nicaragua and Qs = 62-84 beneath Costa Rica, and average values are Qs = 76-78 and Qs = 84-88, respectively. Attenuation variations correlate with along-arc trends in geochemical indicators that suggest that melting beneath Nicaragua occurs at more hydrated conditions, and possibly to greater extents and depths, relative to northern Costa Rica. Shear attenuation dominates over bulk attenuation in the well-resolved regions of the wedge. The more extensive zones of greater shear attenuation observed in the Nicaraguan wedge could be explained by higher temperatures and/or greater hydration, but comparison with petrological data suggests that hydration variations play a larger role. Average wedge attenuation values are comparable to estimates for the Andes and Japan, greater than those for Alaska, and less than those for Tonga-Lau.
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  • Large accelerations and tectonic setting of the May 1993 Shumagin Islands earthquake sequence

    Abers, G. A.   Beavan, J.   Horton, S.   Jaume, S.   Triep, E.  

    Variability of earthquake rupture can lead to large disparities between expected and observed ground motion. To better understand the range of ground motions generated by subduction zone earthquakes, the authors examine teleseismic and near-source strong-motion records from an interplate thrust earthquake sequence that occurred near the Shumagin Islands, Alaska Peninsula, in May 1993. It is shown that the unusually high accelerations demonstrate that deep and moderate-sized earthquakes can occasionally produce significant ground motions in subduction zones.
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  • Reconciling mantle attenuation-temperature relationships from seismology, petrology, and laboratory measurements

    Abers, G. A.   Fischer, K. M.   Hirth, G.   Wiens, D. A.   Plank, T.   Holtzman, B. K.   McCarthy, C.   Gazel, E.  

    Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt-free rocks, but require similar to 10(2)-10(3) extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation (Q(S)(-1)), affording an opportunity for calibration. We reanalyze seismic data sets that sample arc and back-arc mantle in Central America, the Marianas, and the Lau Basin, confirming very high attenuation (Q(S) similar to 25-80) at 1 Hz and depths of 50-100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300-1450 degrees C. The Q(S) measurements correlate inversely with the petrologically inferred temperatures, as expected. However, dry attenuation models predict Q(S) too high by a factor of 1.5-5. Modifying models to include effects of H2O and rheology-dependent grain size shows that the effects of water-enhanced dissipation and water-enhanced grain growth nearly cancel, so H2O effects are modest. Therefore, high H2O in the arc source region cannot explain the low Q(S), nor in the back arc where lavas show modest water content. Most likely, the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body wave Q(S) can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs.
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  • Reconciling mantle attenuation-temperature relationships from seismology, petrology, and laboratory measurements

    Abers, G. A.   Fischer, K. M.   Hirth, G.   Wiens, D. A.   Plank, T.   Holtzman, B. K.   McCarthy, C.   Gazel, E.  

    Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt-free rocks, but require similar to 10(2)-10(3) extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation (Q(S)(-1)), affording an opportunity for calibration. We reanalyze seismic data sets that sample arc and back-arc mantle in Central America, the Marianas, and the Lau Basin, confirming very high attenuation (Q(S) similar to 25-80) at 1 Hz and depths of 50-100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300-1450 degrees C. The Q(S) measurements correlate inversely with the petrologically inferred temperatures, as expected. However, dry attenuation models predict Q(S) too high by a factor of 1.5-5. Modifying models to include effects of H2O and rheology-dependent grain size shows that the effects of water-enhanced dissipation and water-enhanced grain growth nearly cancel, so H2O effects are modest. Therefore, high H2O in the arc source region cannot explain the low Q(S), nor in the back arc where lavas show modest water content. Most likely, the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body wave Q(S) can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs.
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  • 5-Azadibenzo[a,g]corannulene

    Tsefrikas, V. M.   Greene, A. K.   Scott, L. T.  

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  • 5-Azadibenzo[a,g]corannulene

    Tsefrikas, V. M.   Greene, A. K.   Scott, L. T.  

    5-Azadibenzo[a,g] corannulene, the first azacorannulene with a nitrogen on the rim, has been synthesized in seven steps from 4-bromoisoquinoline. The strained pyridine ring opens thermally to a polycyclic aromatic nitrile and hydrolytically to an amino aldehyde with the same polycyclic aromatic hydrocarbon framework.
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  • Total Character of a Group $G$ with $(G,Z(G))$ as a Generalized Camina Pair

    Prajapati, S. K.; Sarma, R.  

    We investigate whether the total character of a finite group G is a polynomial in a suitable irreducible character of G. When (G, Z(G)) is a generalized Camina pair, we show that the total character is a polynomial in a faithful irreducible character of G if and only if Z(G) is cyclic.
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  • An Application of a new Version of (G '/G)- Expansion Method

    Yokus, Asif   Tuz, Munevver  

    In this study, we develop extended (G'/G)-expansion method which is suggested in [1]. By this method, we find traveling wave solutions of nonlinear KdV and Hirota-Satsuma equations.
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