The influence of shear‐velocity heterogeneity on ScS2/ScS amplitude ratios and estimates of Q in the mantle

Regional waveforms of deep‐focus Tonga‐Fiji earthquakes indicate anomalous traveltime differences (ScS2‐ScS) and amplitude ratios (ScS2/ScS) of the phases ScS and ScS2. The correlation between the ScS2‐ScS delay time and the ScS2/ScS amplitude ratio suggests that shear wave apparent Q in the mantle below the Tonga‐Fiji region is highest when shear wave velocities are lowest. This observation is unexpected if temperature variations were responsible for the seismic anomalies. Using spectral element method waveform simulations for four tomographic models, we demonstrate that focusing and scattering of shear waves by long‐wavelength 3‐D heterogeneity in the mantle may overwhelm the signal from intrinsic attenuation in long‐period ScS2/ScS amplitude ratios. The tomographic models reproduce the trends in recorded ScS2‐ScS difference times and ScS2/ScS amplitude ratios. Although they cannot be ruled out, variations in shear wave attenuation (i.e., the quality factor Q) are not necessary to explain the data.Key PointsThe influence of complex 3‐D wave propagation in the mantle on ScS2/ScS amplitude ratiosScS2‐ScS difference times are delayed and ScS2/ScS amplitude ratios are high on Samoa indicating low wave speeds but no attenuationBody wave amplitudes may be useful for evaluating the accuracy of tomographic models and as complementary data in tomographic inversionsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134191/1/grl54786_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134191/2/grl54786.pd

Lithospheric cooling trends and deviations in oceanic PP‐P and SS‐S differential traveltimes

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97534/1/jgrb50092.pd

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

Observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle. Several researchers have reported the presence of seismic anisotropy in the lowermost 150–250 km of the mantle (i.e., D urn:x-wiley:jgrb:media:jgrb52636:jgrb52636-math-0002 layer), based on differences in the arrival times of vertically (SV) and horizontally (SH) polarized shear waves. By computing waveforms at a period > 6 s for a wide range of 1‐D and 3‐D Earth structures, we illustrate that a time shift (i.e., apparent splitting) between SV and SH may appear in purely isotropic simulations. This may be misinterpreted as shear wave anisotropy. For near‐surface earthquakes, apparent shear wave splitting can result from the interference of S with the surface reflection sS. For deep earthquakes, apparent splitting can be due to the S wave triplication in D urn:x-wiley:jgrb:media:jgrb52636:jgrb52636-math-0003, reflections off discontinuities in the upper mantle, and 3‐D heterogeneity. The wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations

Estimate of the Rigidity of Eclogite in the Lower Mantle From Waveform Modeling of Broadband S‐to‐P Wave Conversions

Broadband USArray recordings of the 21 July 2007 western Brazil earthquake (Mw=6.0; depth = 633 km) include high‐amplitude signals about 40 s, 75 s, and 100 s after the P wave arrival. They are consistent with S wave to P wave conversions in the mantle beneath northwestern South America. The signal at 100 s, denoted as S1750P, has the highest amplitude and is formed at 1,750 km depth based on slant‐stacking and semblance analysis. Waveform modeling using axisymmetric, finite difference synthetics indicates that S1750P is generated by a 10 km thick heterogeneity, presumably a fragment of subducted mid‐ocean ridge basalt in the lower mantle. The negative polarity of S1750P is a robust observation and constrains the shear velocity anomaly δVS of the heterogeneity to be negative. The amplitude of S1750P indicates that δVS is in the range from −1.6% to −12.4%. The large uncertainty in δVS is due to the large variability in the recorded S1750P amplitude and simplifications in the modeling of S1750P waveforms. The lower end of our estimate for δVS is consistent with ab initio calculations by Tsuchiya (2011), who estimated that δVS of eclogite at lower mantle pressure is between 0 and −2% due to shear softening from the poststishovite phase transition.Key PointsBroadband recordings of S‐P conversions allow for constraining compositional properties of deep Earth materialsStishovite is present in subducted eclogite and contributes to shear velocity softeningFragments of subducted oceanic crust are entrained in mantle flow and can be preserved at depths approaching 2,000 kmPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141104/1/grl56669_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141104/2/grl56642-sup-0002-supplementary.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141104/3/grl56642-sup-0001-supplementary.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141104/4/grl56669.pd

Cross-talk between signaling pathways leading to defense against pathogens and insects

In nature, plants interact with a wide range of organisms, some of which are harmful (e.g. pathogens, herbivorous insects), while others are beneficial (e.g. growth-promoting rhizobacteria, mycorrhizal fungi, and predatory enemies of herbivores). During the evolutionary arms race between plants and their attackers, primary and secondary immune responses evolved to recognize common or highly specialized features of microbial pathogens (Chisholm et al., 2006), resulting in sophisticated mechanisms of defense

An acceptor-substrate binding site determining glycosyl transfer emerges from mutant analysis of a plant vacuolar invertase and a fructosyltransferase

Glycoside hydrolase family 32 (GH32) harbors hydrolyzing and transglycosylating enzymes that are highly homologous in their primary structure. Eight amino acids dispersed along the sequence correlated with either hydrolase or glycosyltransferase activity. These were mutated in onion vacuolar invertase (acINV) according to the residue in festuca sucrose:sucrose 1-fructosyltransferase (saSST) and vice versa. acINV(W440Y) doubles transferase capacity. Reciprocally, saSST(C223N) and saSST(F362Y) double hydrolysis. SaSST(N425S) shows a hydrolyzing activity three to four times its transferase activity. Interestingly, modeling acINV and saSST according to the 3D structure of crystallized GH32 enzymes indicates that mutations saSST(N425S), acINV(W440Y), and the previously reported acINV(W161Y) reside very close together at the surface in the entrance of the active-site pocket. Residues in- and outside the sucrose-binding box determine hydrolase and transferase capabilities of GH32 enzymes. Modeling suggests that residues dispersed along the sequence identify a location for acceptor-substrate binding in the 3D structure of fructosyltransferases

Joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations

We present a new global whole‐mantle model of isotropic and radially anisotropic S velocity structure (SGLOBE‐rani) based on ~43,000,000 surface wave and ~420,000 body wave travel time measurements, which is expanded in spherical harmonic basis functions up to degree 35. We incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure. This is possible since we utilize short‐period group‐velocity data with a period range down to 16 s, which are strongly sensitive to the crust. The isotropic S velocity model shares common features with previous global S velocity models and shows excellent consistency with several high‐resolution upper mantle models. Our anisotropic model also agrees well with previous regional studies. Anomalous features in our anisotropic model are faster SV velocity anomalies along subduction zones at transition zone depths and faster SH velocity beneath slabs in the lower mantle. The derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts, which has been difficult to constrain due to poor access compared with continental crusts.Key PointsWe used a massive and varied data set to constrain radially anisotropic mantle structureWe include crustal thickness perturbations as model parametersWe observe faster SV velocity along subduction slabs in the transition zonePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112272/1/jgrb51168.pd

Faulting structure above the Main Himalayan Thrust as shown by relocated aftershocks of the 2015 Mw7.8 Gorkha, Nepal, earthquake

The 25 April 2015, Mw7.8 Gorkha, Nepal, earthquake ruptured a shallow section of the Indian‐Eurasian plate boundary by reverse faulting with NNE‐SSW compression, consistent with the direction of current Indian‐Eurasian continental collision. The Gorkha main shock and aftershocks were recorded by permanent global and regional arrays and by a temporary local broadband array near the China‐Nepal border deployed prior to the Gorkha main shock. We relocate 272 earthquakes with Mw>3.5 by applying a multiscale double‐difference earthquake relocation technique to arrival times of direct and depth phases recorded globally and locally. We determine a well‐constrained depth of 18.5 km for the main shock hypocenter which places it on the Main Himalayan Thrust (MHT). Many of the aftershocks at shallower depths illuminate faulting structure in the hanging wall with dip angles that are steeper than the MHT. This system of thrust faults of the Lesser Himalaya may accommodate most of the elastic strain of the Himalayan orogeny.Key PointsWe relocate the 2015 Gorkha earthquakes using teleseismic and regional waveformsThe main shock is located on the horizontal Main Himalaya Thrust (MHT) at a depth of 18.5 kmAftershocks show faulting structure in the hanging wall above the MHTPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135634/1/grl53895.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/135634/2/grl53895_am.pd