source spectra and spectral attenuation of seismic waves from mexican earthquakes and evidence of amplification in the hill zone of mexico city it is well known that seismic waves are enormously amplified at lake bed sites with respect to hill zone sites in mexico city it has been suggested that even hill zone sites suffer amplification in this paper we use eight recent well recorded earthquakes to determine spectral attenuation of seismic waves and to quantify the suggested amplification in the hill zone results show that indeed spectral amplitudes at these sites are roughly 10 times greater than expected for sites outside mexico city at similar hypocentral distances this implies that lake bed sites are amplified by a factor of 100 to 500 at some frequencies between 0 2 to 0 7 hz the source spectra retrieved from the regression analysis agree well with those obtained from epicentral recordings alone for two of the largest earthquakes ms 8 1 and 7 6 whose seismic moments are well determined and whose source spectra can be patched in a very broad band of frequencies 0 015 to 20 hz we observe clear deviation from an 2 model the spectra at intermediate frequencies from about 0 05 to 2 hz show a sag whether the observed spectra deviate from an 2 model for events with ms 6 depends on the reliability of the reported long period seismic moments if they are considered reliable then these events also deviate from the model if however the reported seismic moments of smaller events are ignored then the observed spectra can be fitted well with the 2 model

origin of andesite in the deep crust and eruption rates in the tancitaro nueva italia region of the central mexican arc the tancitaro nueva italia region 4400 km 2 is located within the west central mexican volcanic belt and spans 80 km of arc length along the volcanic front on the basis of 65 40 ar 39 ar ages it is shown that a total volume of 326 57 km 3 of magma has erupted from 200 vents since 1 2 ma nearly a third of this volume 103 5 km 3 was erupted as crystal rich andesite from a large stratovolcano volcan tancitaro whereas the remaining two thirds 223 52 km 3 span the complete range from 51 to 63 wt sio 2 are relatively crystal poor and were erupted from peripheral vents the total proportions of erupted lava types are 6 7 basalt 18 basaltic andesite and 74 76 andesite there is no rhyolite and only a trace of dacite there is no evidence that any significant magma differentiation occurred in the upper crust along this arc segment based on phenocryst abundances and modes major and trace element data as well as phase equilibrium experiments from the literature the following mechanisms for formation of andesite in this region can be ruled out 1 crystal fractionation of basalt basaltic andesite in the upper crust 2 magma mixing of basalt basaltic andesite with dacite rhyolite and 3 assimilation of granitoid by basalt basaltic andesite instead the same body of evidence indicates that the andesites were formed primarily in the deep crust through partial melting 20 of hornblende rich 40 gabbronorite the basaltic andesites are required owing to their elevated ni concentrations to be hybrids of the andesitic partial melts with basaltic residual liquids that have undergone variable two pyroxene fractionation in the deep crust

an estimate of shear wave q of the mantle wedge in mexico we utilize earthquake recordings at two broadband seismographs located on the coast of the gulf of mexico to estimate q of the mantle wedge from the data at a station situated in the laguna verde nuclear power plant lvig about 100 km northwest of veracruz and at the eastern edge of the mexican volcanic belt we estimate an upper bound of shear wave q of the mantle wedge q f 120 f 0 75 0 1 f 10 hz as compared with q f 251 f 0 58 for the average path through subducted slab and continental lithosphere curiously the estimated q of the mantle wedge at station scig which is located on the yucatan block is about the same as the average q in southern mexico there are several possible explanations for the difference 1 very few recordings at scig 2 a site effect at scig masking the effect of low q of the mantle wedge and 3 relatively high mantle q beneath the yucatan block the third possibility is supported by surface wave tomography that reveals a thick cold mantle lithosphere below the yucatan block and near absence of mantle lithosphere in the backarc region of central mexico a higher density of seismographs along the gulf coast is needed to resolve these issues our study predicts diminished ground motions at the laguna verde nuclear power plant if the seismic waves pass through the mantle wedge or traverse below the popocatepetl or orizaba volcanoes

shear wave seismic interferometry for lithospheric imaging application to southern mexico seismic interferometry allows for the creation of new seismic traces by cross correlating existing ones with sufficient sampling of remote source positions it is possible to create a virtual source record by transforming a receiver location into a virtual source the imaging technique developed here directly retrieves reflectivity information from the subsurface other techniques namely receiver function and tomography rely on mode converted energy and perturbations in a velocity field respectively to make inferences regarding structure we select shear phases as an imaging source because of their lower propagation velocity sensitivity to melt and ability to treat vertical shear and horizontal shear wavefields independently teleseismic shear phases approximate a plane wave due to the extent of wavefront spread compared to a finite receiver array located on the free surface the teleseismic shear phase transmission responses are used as input to the seismic interferometry technique we create virtual shear source records by converting each receiver in the array into a virtual source by cross correlating the received signals the complex source character of distant earthquakes is imprinted on the virtual source records as the average autocorrelation of individual source time functions we demonstrate a technique that largely removes this imprint by filtering in the common offset domain a field data set was selected from the meso america subduction experiment despite the suboptimal remote source sampling an image of the lithosphere was produced that confirms features of the subduction zone that were previously found with the receiver function technique

influence of dehydration on the electrical conductivity of epidote and implications for high conductivity anomalies in subduction zones the anomalously high electrical conductivities 0 1 to 1 s m in deep mantle wedge regions extensively detected by magnetotelluric studies are often associated with the presence of fluids released from the progressive dehydration of subducting slabs epidote minerals are the ca al rich hydrous silicates with huge stability fields exceeding those of amphibole 70 80 km in subducting oceanic crust and they may therefore be transported to greater depth than amphibole and release water to the mantle wedge in this study the electrical conductivities of epidote were measured at 0 5 1 5 gpa and 573 1273 k by using a solartron 1260 impedance gain phase analyzer in a yj 3000t multianvil pressure within the frequency range of 0 1 106 hz the results demonstrate that the influence of pressure on electrical conductivity of epidote is relatively small compared to that of temperature the dehydration reaction of epidote is observed through the variation of electrical conductivity around 1073 k and electrical conductivity reaches up to 1 s m at 1273 k which can be attributed to aqueous fluid released from epidote dehydration after sample dehydration electrical conductivity noticeably decreases by as much as nearly a log unit compared with that before dehydration presumably due to a combination of the presence of coexisting mineral phases and aqueous fluid derived from the residual epidote taking into account the petrological and geothermal structures of subduction zones it is suggested that the aqueous fluid produced by epidote dehydration could be responsible for the anomalously high conductivities in deep mantle wedges at depths of 70 120 km particularly in hot subduction zones

an evaluation of proposed mechanisms of slab flattening in central mexico central mexico is the site of an enigmatic zone of flat subduction the general geometry of the subducting slab has been known for some time and is characterized by a horizontal zone bounded on either side by two moderately dipping sections we systematically evaluate proposed hypotheses for shallow subduction in mexico based on the spatial and temporal evidence and we find no simple or obvious explanation for the shallow subduction in mexico we are unable to locate an oceanic lithosphere impactor or the conjugate of an impactor that is most often called upon to explain shallow subduction zones as in south america japan and laramide deformation in the us the only bathymetric feature that is of the right age and in the correct position on the conjugate plate is a set of unnamed seamounts that are too small to have a significant effect on the buoyancy of the slab the only candidate that we cannot dismiss is a change in the dynamics of subduction through a change in wedge viscosity possibly caused by water brought in by the slab

structure and anisotropy of the mexico subduction zone based on rayleigh wave analysis and implications for the geometry of the trans mexican volcanic belt 1 we develop a three dimensional model of shear wave velocity and anisotropy for the mexico subduction zone using rayleigh wave phase velocity dispersion measurements this region is characterized by both steep and flat subduction and a volcanic arc that appears to be oblique to the trench we give a new interpretation of the volcanic arc obliqueness and the location of the tzitzio gap in volcanism based on the subduction morphology we employ the two station method to measure rayleigh phase velocity dispersion curves between periods of 16 s to 171 s the results are then inverted to obtain azimuthally anisotropic phase velocity maps and to model 3 d variations in upper mantle velocity and anisotropy our maps reveal lateral variations in phase velocity at all periods consistent with the presence of flat and steep subduction we also find that the data are consistent with two layers of anisotropy beneath mexico a crustal layer with the fast directions parallel to the north american absolute plate motion and a deeper layer that includes the mantle lithosphere and the asthenosphere with the fast direction interpreted in terms of toroidal mantle flow around the slab edges our combined azimuthal anisotropy and velocity model enables us to analyze the transition from flat to steep subduction and to determine whether the transition involves a tear resulting in a gap between segments or is a continuous deformation caused by a lithospheric flexure our anisotropy results favor a tear which is also consistent with the geometry of the volcanic belt

attenuation of coda waves in western mexico using local seismicity coda waves are an important instrument to study the elastic properties of the lithosphere the attenuation of coda waves in western mexico is analyzed using seismograms recorded during the mapping of the rivera subduction zone experiment the coda wave attenuation is measured by means of q within the single backscattering model and the single isotropic scattering model schemes the frequency dependence of q is analyzed using the power fit formulation q f q f the resulting q q and are then used to study the seismic attenuation properties in western mexico we compare our results with the crustal structure by means of the surface geology seismic activity crustal thickness and p wave velocity patterns in the study area there is a poor correlation of the attenuation parameters with such crustal features however an analysis of the variations of the p wave velocity shows that the seismic attenuation parameters correlate to some extent with seismic velocity variations previously noted in the study area where low attenuation and weak frequency dependence correlate with low velocity variation

teleseismic p wave spectra from usarray and implications for upper mantle attenuation and scattering teleseismic p wave amplitude spectra from deep earthquakes recorded by usarray are inverted for maps of upper mantle t for multiple frequency bands within 0 08 2 hz all frequency bands show high t regions in the southwestern u s southern rocky mountains and appalachian margin low t is more common across the cratonic interior inversions with narrower frequency bands yield similar patterns but greater t magnitudes even the two standard deviation t magnitude for the widest band is 2 7 times greater than predicted by global qs tomography or an anelastic olivine thermal model suggesting that much of the t signal is nonthermal in origin nonthermal contributions are further indicated by only a moderate correlation between t and p travel times some geographic variations such as high t in parts of the cratonic interior with high mantle velocities and low heat flow demonstrate that the influence of temperature is regionally overwhelmed transverse spectra are used to investigate the importance of scattering because they would receive no p energy in the absence of 3 d heterogeneity or anisotropy transverse to vertical t z spectral ratios for stations with high t are higher and exhibit steeper increases with frequency compared to t z spectra for low t stations the large magnitude of t estimates and the t z spectra are consistent with major contributions to t from scattering a weak positive correlation between intrinsic attenuation and apparent attenuation due to scattering may contribute to t magnitude and the moderate correlation of t with travel times

horizontal subduction and truncation of the cocos plate beneath central mexico 1 based on analysis of data from a trans mexico temporary broadband seismic network centered on mexico city we report that the subducting cocos plate beneath central mexico is horizontal and tectonically underplates the base of the crust for a distance of 250 km from the trench it is decoupled from the crust by a very thin low viscosity zone the plate plunges into the mantle near mexico city but is truncated at a depth of 500 km probably due to an e w propagating tear in the cocos slab unlike the shallow slab subduction in peru and chile there is active volcanism along the trans mexican volcanic belt tmvb that lies much further inland than regions to either side where subduction dip is not horizontal geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

lateral heterogeneity imaged by small aperture scs retrieval from the ambient seismic field interpreting core related body wave travel times is challenging for seismologists because earth s heterogeneities are averaged over thousands of kilometers and the sparsity of earthquake measurements makes these heterogeneities difficult to localize we show how the ambient seismic wave field can be used to overcome these limitations we present a regional scale analysis of core mantle boundary reflections scs under mexico we show that body wave arrivals i e p and scs are retrieved from higher order cross correlations c3 a technique that provides a more uniform and controlled source distribution using the scattered waves of the coda of classical ambient field cross correlations c1 then we extract scs travel times along a dense linear array in mexico and find that lithospheric lateral heterogeneity such as the subducting cocos slab beneath mexico may have a strong impact on scs travel times in parallel we show that lateral heterogeneity such as a possible ultra low velocity zone ulvz near the core mantle boundary might also affect although to a lesser extent the travel time anomalies our results and interpretation are supported through numerical simulations that accounts for slab and ulvz properties

seismic attenuation tomography of the tonga fiji region using phase pair methods the anelastic structure of the region surrounding the tonga slab and lau back arc spreading center in the southwest pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs two differential attenuation methods determine t over the frequency band 0 1 to 3 5 hz for earthquakes in the tonga slab the s p method measures the difference in spectral decay between p and s waves arriving at the same station the p p method measures the difference in spectral decay for p waves with different paths through the upper mantle eight hundred sixty phase pairs are used to invert for two dimensional 1 q structure using a nonnegative least squares algorithm a grid search method determines the q q ratio most compatible with both the s p and p p differential measurements the highest attenuation q 90 is found within the upper 100 km beneath the active portions of the lau basin extending westward to the lau ridge these regions probably delineate the source region for the back arc spreading center magmas expected to be within the upper 100 km based on petrological considerations the high attenuation regions also correlate well with zones of low p wave velocity determined by regional velocity tomography somewhat lower attenuation is found beneath the fiji plateau than beneath the lau basin the entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km the slab is imaged as a region of low attenuation q 900 material a q q ratio of 1 75 provides the best fit between the s p and p p data sets upon inversion spectral stacking shows no frequency dependence within the frequency band analyzed

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

thermo mechanical model of the mantle wedge in central mexican subduction zone and a blob tracing approach for the magma transport the origin of the central mexican volcanic belt cmvb and the influence of the subducting cocos plate on the cmvb volcanism are still controversial in this study the temperature and mantle wedge flow models for the mexican subduction zone are developed using the finite element method to investigate the thermal structure below cmvb the numerical scheme solves a system of 2d stokes equations and 2d steady state heat transfer equation two models are considered for the mantle wedge the first one with an isoviscous mantle wedge and the second one with strong temperature dependent viscosity the first model reveals a maximum temperature of 830 c in the mantle wedge which is not sufficient for melting of wet peridotite also the geotherm of the subducting plate upper surface does not intersect the dehydration melting solidus for mafic minerals the second model predicts temperatures of more than 1200 c beneath the cmvb for a wide range of rheological parameters reference viscosity and activation energy up to 0 6 wt h2o can be released down to 60 km depth through metamorphic changes in the oceanic crust of the subducting slab the melting of this oceanic crust apparently occurs in a narrow depth range of 50 60 km and also melting of the mantle wedge hydrated peridotite is now expected to take place beneath cmvb considering that the melting processes on and in the vicinity of the subducting plate surface generate the most of the volcanic material a dynamic model for the blob tracers is developed using stokes flow at infinite prandtl number the blobs of 0 2 10 0 km in diameter migrate along very different trajectories only at low wrapping viscosities w 1 0 14 5 10 17 pa s the modeling results show that the fast trajectories terminate at the same focus location at the base of the continental crust while the arrival points of slow trajectories which are common for the blobs of smaller size 0 4 0 5 km are scattered away from the average focus location this observation may give us a hint on a possible mechanism of strato and mono volcanoes genesis the rise

volcaniclastic debris flows at la fossa volcano vulcano island southern italy insights for erosion behaviour of loose pyroclastic material on steep slopes abstract this paper describes a qualitative model of erosion of loose volcaniclastic material on steep slopes of la fossa cone vulcano island italy that generates small volume debris flows the loose volcaniclastic cover rests on an almost impermeable substratum which comprises a fine grained hydromagmatic tuff morphologically three sectors were distinguished 1 a water supply zone slope 32 where rain water is enriched in mud and concentrated in the initiation zone 2 an initiation zone or source area sensu stricto slope with 30 where debris flows form and 3 a transport and deposition zone where debris flows deposited levees and terminal lobes slope 30 main episodes of erosion of loose volcaniclastic cover and debris flow generation occur in response to thunderstorms the erosion of tuff from superficial water added a significant amount of fines mud fraction to the initiating flows debris flow stoppage on steep slopes was interpreted as the result of a loss of mass estimated between 35 and 53 of total mass to form levees from field and laboratory data the density of debris flows was estimated at about 1950 kg m3 morphological data matrix texture mass number nm and clay content of debris flow deposits suggest that they were deposited by saturated granular slurries

structure of central and southern mexico from velocity and attenuation tomography the 3d v p v p vs p and s wave attenuation structure of the cocos subduction zone in mexico is imaged using earthquakes recorded by two temporary seismic arrays and local stations direct p wave arrivals on vertical components and direct s wave arrivals on transverse components from local earthquakes are used for velocity imaging relative delay times for p and pkp phases from teleseismic events are also used to obtain a deeper velocity structure beneath the southern seismic array using a spectral decay method we calculate a path attenuation operator t for each p and s waveform from local events and then invert for 3d spatial variations in attenuation q p 1 and q s 1 inversion results reveal a low attenuation and high velocity cocos slab the slab dip angle increases from almost flat in central mexico near mexico city to about 30 in southern mexico near the isthmus of tehuantepec high attenuation and low velocity in the crust beneath the trans mexico volcanic belt correlate with low resistivity and are probably related to dehydration of the slab and melting processes the most pronounced high attenuation low v p and high v p v s anomaly is found in the crust beneath the veracruz basin a high velocity structure dipping into the mantle from the side of gulf of mexico coincides with a discontinuity from a receiver functions study and provides an evidence for the collision between the yucatan block and mexico in the miocene

tomography and thermal state of the cocos plate subduction beneath mexico city 1 the geometry and thermal state of the subducting cocos plate beneath mexico city has been enigmatic because of the absence of a deep wadati benioff zone we present a tomographic image of the slab based on inversion of 8869 teleseismic p wave travel times measured on a portable broadband seismic network the images combined with receiver function analysis show that the slab runs flat from the coast to near mexico city where it dives into the mantle just before the trans mexican volcanic belt with a dip of 75 it continues down to a depth of 500 km at a distance of 400 km from the trench where the tomography reveals that the dipping portion ends as well as standard block tomography we invert the travel time residuals for the parameters of a thermal slab model and find a slab thickness of 40 km that is consistent with the 15 ma age of the cocos plate the combination of a young hot plate and truncation at depth can explain the lack of deep seismicity due to high temperatures and lower negative buoyancy compared with an older thicker nontruncated plate

generation of talc in the mantle wedge and its role in subduction dynamics in central mexico geophysical evidence shows the presence of low seismic velocity material at the surface of slabs in subduction zones in the central mexican subduction zone this appears as a thin 4 km low velocity zone that absorbs nearly all of the strain the p to s velocity ratio as a function of s wave velocity distinguishes among the various candidate hydrous low strength minerals the thin layer in the flat slab region is most consistent with a layer showing enrichment in talc overlying normal morb like gabbro based on available thermodynamic data for equilibria for talc its generation at the trench is nearly impossible and hence we propose it originates from the mantle wedge during the slab flattening process coupled with trench rollback the evolution of this low strength zone has important implications for the dynamics of the slab flattening process as well as the geochemistry of the mantle wedge and arc in central mexico

evidence of a highly attenuative aseismic zone in the active collision orogen of taiwan abstract seismic attenuation across an aseismic zone in the southeastern central range of taiwan is investigated in terms of t data with inverted q models the t data are obtained by fitting the observed spectra of p and s waves with an 2 source model using broadband records of a 25 station linear array deployed across southern taiwan both tp and ts data at the eastern stations show remarkable decreasing trends with depth down to 40 km in contrast to the western stations the inverted qp and qs profiles across southern taiwan reveal a high attenuation zone qp 288 qs 219 at depth about 20 km beneath the eastern stations traversing the southeastern central range high temperature and possible presence of fluids in the high attenuation zone are derived from the q models this high attenuation zone happens to coincide with the aseismic zone suggesting that the aseismic zone may be due to high geothermal and partial melting effects of a very active collision orogeny the area is also marked by other geological and geophysical indicators in support of this interpretation

three dimensional attenuation structure beneath the northeastern japan arc estimated from spectra of small earthquakes abstract a fine scale three dimensional attenuation structure beneath the northeastern japan arc has been obtained using a joint inversion for source parameters site response and q values data used for this study are p wave spectra of microearthquakes occurring in the subducting pacific plate the subducting plate is found to have higher q values than the overlying mantle there is an along arc variation in the distribution of low q zones in the crust and the mantle wedge within the study area for the southern part of the study area low q zones are distributed in the crust beneath the volcanic front they extend into the mantle wedge and become deep toward the west that is toward the backarc region on the other hand the northern part of the study area has low q zones at two depth ranges at shallower depths just beneath the volcanic front and at the deeper part of the mantle wedge beneath the backarc region the region without active volcanoes along the volcanic front has relatively high mantle q values seismic activity in the lower plane of the double planed deep seismic zone is high beneath this relatively high q region of the mantle wedge spatial distribution of low q zones found in the present study is nearly consistent with that of the low velocity zones estimated from studies of travel times tomography partial melting zones in the mantle wedge whose existence has been suggested from the study of seismic velocity structure and laboratory experiments are located in the low q zones obtained in the present study these results suggest that spatial distribution of q values is closely related to heterogeneous distribution of temperature in the mantle wedge

seismic imaging of the cocos plate subduction zone system in central mexico broadband data from the meso america subduction experiment mase line in central mexico were used to image the subducted cocos plate and the overriding continental lithosphere beneath central mexico using a generalized radon transform based migration our images provide insight into the process of subducting relatively young oceanic lithosphere and its complex geometry beneath continental north america the converted and reverberated phase image shows complete horizontal tectonic underplating of the cocos oceanic lithosphere beneath the north american continental lithosphere with a clear image of a very thin low velocity oceanic crust 7 8 km which dips at 15 20 degrees at acapulco then flattens approximately 300 km from the middle america trench farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the trans mexican volcanic belt tmvb where the slab bends underneath the tmvb the migrated image depicts the transition from subducted oceanic moho to continental moho at 230 km from the coast neither of which were clearly resolved in previous seismic images the deeper seismic structure beneath the tmvb shows a prominent negative discontinuity fast to slow at 65 75 km within the upper mantle this feature which spans horizontally beneath the arc 100 km may delineate the top of a layer of ponded partial melt

imaging fluid related subduction processes beneath central java indonesia using seismic attenuation tomography abstract we use local earthquake data observed by the amphibious temporary seismic meramex array to derive spatial variations of seismic attenuation qp in the crust and upper mantle beneath central java the path averaged attenuation values t of a high quality subset of 84 local earthquakes were calculated by a spectral inversion technique these 1929 t values inverted by a least squares tomographic inversion yield the 3d distribution of the specific attenuation qp analysis of the model resolution matrix and synthetic recovery tests were used to investigate the confidence of the qp model we notice a prominent zone of increased attenuation beneath and north of the modern volcanic arc at depths down to 15 km most of this anomaly seems to be related to the eocene miocene kendeng basin mainly in the eastern part of the study area enhanced attenuation is also found in the upper crust in the direct vicinity of recent volcanoes pointing towards zones of partial melts presence of fluids and increased temperatures in the middle to upper crust the middle and lower crust seems not to be associated with strong heating and the presence of melts throughout the arc enhanced attenuation above the subducting slab beneath the marine forearc seems to be due to the presence of fluids

three dimensional attenuation structure of the hikurangi subduction zone in the central north island new zealand summary we use waveform data from 2708 earthquakes recorded by dense seismograph deployments in the central north island of new zealand to image the 3 d attenuation structure of the hikurangi subduction zone down to ca 300 km depth attenuation images are obtained by determining the quality factor of p waves qp using a t inversion with a previously determined 3 d seismic velocity model we have included limited frequency dependence for qp with qp being frequency independent above 10 hz and having a frequency dependence of f 10 0 5 below 10 hz the qp images provide further constraint on the large scale features associated with subduction and magmatism beneath the central north island and serve to refine interpretations of crust and upper mantle structure from vp and vp vs the subducted plate is the most prominent feature in the qp images its high qp 900 1200 is consistent with the ca 120 myr old slab being relatively cold qp is better at resolving the slab than vp the coincidence of a strong gradient in qp with the upper plane of the dipping seismic zone indicates that even below 75 km depth the upper envelope of seismicity provides a good estimate of the location of the plate interface the mantle wedge is generally imaged as a relatively low qp 400 feature below 50 km depth however there are significant changes evident in the wedge along the strike of the subduction zone the most pronounced low qp in the mantle wedge occurs from ca 50 to 85 km depth beneath the productive rhyolite dominated central segment of the taupo volcanic zone suggesting a close link between volcanism and low qp in the shallow mantle wedge there is a strong correlation between low qp l owvp and high vp vs in this part of the mantle wedge suggesting that high temperature is the controlling influence on qp there within 30 km of the surface of the slab there is a large change in qp but only a modest change in vp our results are consistent with a fluid rich viscous blanket being entrained with the motion of the subducted slab down to ca 140 km depth where hydrous melting of mantle peridotite initiates trenchward of the volcanic front a strong horizontal gradient to higher qp is imaged in the mantle wedge we interpret this as a stagnant mantle nose not involved in the corner flow of the adjacent mantle the distribution of qp within both the slab and the overlying plate provides support for dehydration embrittlement being important in promoting seismicity in the mantle of the slab where qp is high at shallow depth in the forearc it is usually underlain by low qp near the plate interface this suggests that competent terranes in the forearc may act as aquicludes increasing fluid content in the rocks below and hence might control the distribution of coupling at the plate interface

seismic wave dispersion and attenuation in aheim dunite an experimental study summary a machine has been developed which provides for the study of both shear modulus dispersion and internal friction in geological materials through the observation of forced torsional oscillations of low frequency 10 1000 mhz and strain amplitude 1000 c the eventual goal is an understanding of the dispersion and attenuation of seismic shear waves in the earth s upper mantle measurements have been made on cylindrical specimens of an olivine rich rock from aheim norway which contains in addition to olivine about 10 per cent pyroxene and 5 10 per cent of hydrous silicate phases dominantly clinochlore serpentine and talc in order to separate aspects of the mechanical behaviour associated with the olivine aggregate from those attributable to the hydrous phases and or their dehydration products the specimens were either previously fired under controlled oxygen fugacity at 1200 c for 24 hours in order to effect complete dehydration within the olivine stability field or simply oven dried at 110 c linearity of the mechanical behaviour has been demonstrated by the amplitude independence of the results for the strain amplitude range 10 8 10 6 and by the quantitative consistency between the observed modulus dispersion and that calculated from the measured internal friction through the kramers kronig relations of linear theory both the pressure dependence at room temperature and the temperature dependence at 300 mpa of the shear modulus and internal friction have been investigated much of the variation of the shear modulus is attributed to changes of crack porosity associated with changes in pressure and temperature and with in situ dehydration temporal evolution of q 1 towards a lower asymptotic value over periods of hours of exposure to given conditions of high pressure and temperature is tentatively attributed to the gradual diminution of enhanced anelastic relaxation associated with regions of decaying stress concentration at asperities on cracks and or grain boundaries at the highest pressures and temperatures achieved in this study 300 mpa and 1000 c marked dispersion of the shear modulus amounting to 5 per cent between periods of 3 and 100 s and concomitant strong internal friction varying with oscillation period to approximately as 0 011to1 6 are observed the energy dissipation appears to be concentrated within rather than at the boundaries between the olivine grains these results provide the clearest indication yet that solid state probably intragranular anelastic relaxation in ultramafic rocks gives rise to losses comparable with those observed seismologically in the earth s upper mantle although the mechanistic basis for the observed anelasticity remains to be established

three dimensional attenuation model of the shallow hikurangi subduction zone in the raukumara peninsula new zealand 1 the material properties of the shallow subduction zone are imaged with data from a passive seismic deployment in the raukumara peninsula attenuation images are obtained using t values measured from 2848 spectra of p wave arrivals the t values are inverted for frequency independent three dimensional 3 d qp using a 3 d velocity model linked nodes in a preliminary 2 d inversion are used to obtain the most reliable initial model for 3 d inversions the results show the benefits of obtaining 3 d q in addition to 3 d vp and vp vs q is related to localized permeability and heterogeneity and thus 3 d q is an important tool for interpreting active tectonic regions qp in the shallow overlying plate generally corresponds to the mapped geology two high qp features 600 represent subducted seamounts or cretaceous volcanics within the overlying plate the gradient above the subducting plate is the strongest feature in the qp model with the slab having high qp 600 900 below 20 km depth and above the plate interface there is a zone of 50 decrease in qp and high vp vs about 10 km thick which represents actively subducting sediment and has distributed microearthquake activity associated with active underplating

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

distribution of hydrous minerals in the subduction system beneath mexico abstract teleseismic converted phases are used to probe the composition of the downgoing oceanic crust as a function of depth along the cocos slab in central and southern mexico previously modeling of the receiver function rf conversion amplitude of the flat cocos slab beneath central mexico at 45 km depth revealed a thin low velocity upper oceanic crust of a thickness of 4 1 km which has much lower seismic velocities 20 30 reduction in shear wave velocities than normal lower crust high vp vs ratio 2 0 also suggested a large concentration of hydrous minerals such as talc in combination of high pore fluid pressure in the horizontal segment we extend this previous effort to examine seismic properties of both the steeply subducting cocos oceanic crust beneath the trans mexican volcanic belt tmvb in central mexico and the shallowly dipping crust beneath southern mexico inverted seismic velocities using the converted amplitudes at the top and bottom of the dipping oceanic crust are compared with experimentally constrained seismic velocities of candidate mineral phases in a range of likely pressures and temperatures the composition of the oceanic crust downdip in the steep part of slab beneath the tmvb includes the minerals such as lawsonite and zoisite at 60 100 km depth and the eclogitization occurs around 100 km depth this is related to arc volcanism in the tmvb directly above the slab as well as the slab rollback in contrast the dominant mineral phase in the upper oceanic crust of southern mexico beneath the isthmus of tehauntepec is amphibole on top of unaltered gabbroic oceanic crust the difference in mineral assemblages of the subducted oceanic crust may help explain the difference in slab geometries between central and southern mexico

velocity and stress fields in grounded glaciers a simple algorithm for including deviatoric stress gradients a new and efficient algorithm for computing the three dimensional stress and velocity fields in grounded glaciers includes the role of deviatoric stress gradients a consistent approximation of first order in the aspect of ratio of the ice mass gives a set of eight field equations for the five stress and three velocity components and the corresponding boundary conditions a coordinate transformation mapping the local ice thickness on to unity and approximating the derivatives in the horizontal direction by centred finite differences yields five ordinary differential and three algebraic equations this allows use of the method of lines starting the integration with prescribed stress and velocity components at the base and a simple iteration procedure converges rapidly the algorithm can lie used for a wide range of stress strain rate relations as long as strain only depends on deviatoric and shear stresses and on temperature sensitivity tests using synthetic and realistic ice geometries show the relevance of normal deviatoric stresses in the solutions for the velocity components even for ice sheets stress and velocity fields may deviate substantially from the widely used shallow ice approximation

new boundary conditions for the west antarctic ice sheet subglacial topography of the thwaites and smith glacier catchments airborne radar sounding over the thwaites glacier tg catchment and its surroundings provides the first comprehensive view of subglacial topography in this dynamic part of the west antarctic ice sheet wais and reveals that tg is underlain by a single broad basin fed by a dendritic pattern of valleys while smith glacier lies within an extremely deep narrow trench subglacial topography in the tg catchment slopes inland from a broad low relief coastal sill to the thickest ice of the wais and makes deep connections to both pine island glacier and the ross sea embayment enabling dynamic interactions across the wais during deglaciation simple isostatic rebound modeling shows that most of this landscape would be submarine after deglaciation aside from an island chain near the present day ross amundsen ice divide the lack of topographic confinement along tg s eastern margin implies that it may continue to widen in response to grounding line retreat

q of lg waves in the central mexican volcanic belt from seismograms of nine shallow coastal earthquakes recorded at a pair of broadband stations we estimate q of lg waves in the part of central mexican volcanic belt mvb that includes the valley of mexico the two stations straddle the central mvb and are located on cretaceous limestone a weighted least square fit to the q 1 f data in the frequency range 0 25 to 8 hz yields q f 98 f 0 72 this estimate of q is lower than the corresponding q in the forearc region that is given by q f 273 f 0 66 note that our estimate of q f corresponds to a 200 km wide zone of the mvb the result of this study sheds light on the characteristics of seismic waves as they traverse through the mvb where they undergo dramatic amplification in the valley of mexico it also provides one of the critical elements needed in the estimation of expected ground motions at sites to the north of the mvb from future coastal earthquakes the lower q of lg waves in the mvb as compared with the forearc region seems correlated with lower resistivity reported in the mvb relative to the forearc region

shape of the subducted rivera and cocos plates in southern mexico seismic and tectonic implications the geometry of the subducted rivera and cocos plates beneath the north american plate in southern mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes the hypocenters of the teleseisms were relocated and the focal depths of 21 events were constrained using a body wave inversion scheme the suduction in southern mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the cocos plate beneath the caribbean plate to the east and of the rivera plate beneath north america to the west the dip of the interplate contact geometry is constant to a depth of 30 km and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate on the basis of the seismicity the focal mechanisms and the geometry of the downgoing slab southern mexico may be segmented into four regions 1 the jalisco region to the west where the rivera plate subducts at a steep angle that resembles the geometry of the cocos plate beneath the caribbean plate in central america 2 the michoacan region where the dip angle of the cocos plate decreases gradually toward the southeast 3 the guerrero oaxaca region bounded approximately by the onshore projection of the orozco and o gorman fracture zones where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km and 4 the southern oaxaca and chiapas region in southeastern mexico where the dip of the subduction gradually increases to a steeper subduction in central america these drastic changes in dip do not appear to take place on tear faults suggesting that smooth contortions accommodate these changes in geometry the inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the trans mexican volcanic belt suggesting it is directly related to the subduction thus the observed nonparallelism with the middle american trench is apparently due to the changing geometry of the rivera and cocos plates beneath the north american plate in southern mexico and not to zones of weakness in the crust of the north american plate as some authors have suggested

seismic attenuation and mantle wedge temperatures in the alaska subduction zone 1 anelastic loss of seismic wave energy or seismic attenuation 1 q provides a proxy for temperature under certain conditions the q structure of the upper mantle beneath central alaska is imaged here at high resolution an active subduction zone where arc volcanism is absent to investigate mantle thermal structure the recent broadband experiment across the alaska range beaar provides the first dense broadband seismic coverage of this region the spectra of p and sh waves for regional earthquakes are inverted for path averaged attenuation operators between 0 5 and 20 hz along with earthquake source parameters these measurements fit waveforms significantly better when the frequency dependence of q is taken into account and in the mantle frequency dependence lies close to laboratory values inverting these measurements for spatial variations in q reveals a highly attenuating wedge with q 500 assuming frequency dependence comparison with p results shows that attenuation in bulk modulus is negligible within the low q wedge as expected for thermally activated attenuation mechanisms bulk attenuation is significant in the overlying crust and subducting plate indicating that q must be controlled by other processes the shallowest part of the wedge shows little attenuation as expected for a cold viscous nose that is not involved in wedge corner flow overall the spatial pattern of q beneath alaska is qualitatively similar to other subduction zones although the highest wedge attenuation is about a factor of 2 lower the q values imply that temperatures exceed 1200 c in the wedge on the basis of recent laboratory based calibrations for dry peridotite these temperatures are 100 150 c colder than we infer beneath japan or the andes possibly explaining the absence of arc volcanism in central alaska

site response attenuation and source spectra of s waves along the guerrero mexico subduction zone acceleration spectra from nine earthquakes and up to 11 stations per earthquake are inverted to find the source spectra the site response and the quality factor q of s waves this method is applied to moderate earthquakes 4 0 0 8 hz does not follow a simple linear relation as observed in previous studies of attenuation in other regions when the source spectra are interpreted in terms of brune s model the stress drops obtained are less than 20 bars for the four larger events analyzed the site functions obtained indicate that although all the stations used are on rock the effect of the near surface geology caused important amplifications at some stations

s wave velocity structure below central mexico using high resolution surface wave tomography shear wave velocity of the crust below central mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense 500 km long linear seismic network vertical components of regional records from 90 well located earthquakes were used to compute rayleigh wave group velocity dispersion curves a tomographic inversion with high resolution in a zone close to the array obtained for periods between 5 and 50 s reveals significant differences relative to a reference model especially at larger periods 30 s a 2 d s wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions the results show large differences especially in the lower crust among back arc volcanic arc and fore arc regions they also show a well resolved low velocity zone just below the active part of the trans mexican volcanic belt tmvb suggesting the presence of a mantle wedge low densities in the back arc inferred from the low shear wave velocities can provide isostatic support for the tmvb

velocity structure under the trans mexican volcanic belt preliminary results using correlation of noise summary prediction of ground motion in central mexico is of great consequence not only for mexico city but also for several other cities in the region however the geometry and velocity distribution of the geological structures in central mexico is unknown in this paper the velocity structure in central mexico is investigated using seismic noise records from three different arrays one permanent and two temporary distances between stations span 3 300 km seismic noise is analysed using time domain cross correlation to estimate the green s function between stations only the fundamental mode of rayleigh waves is recovered in the cross correlation of vertical component records between 5 and 10 s period at smaller periods we obtain no correlation even between closely spaced stations this suggests that the heterogeneity of the trans mexican volcanic belt tmvb hampers the propagation of rayleigh waves smaller than 9 km we observe a large velocity contrast between the guerrero terrain and the tmvb this velocity contrast measured here for the first time at this scale explains the appearance of regional amplification in central mexico and the origin of the diffracted waves behind the long duration of ground motion observed in mexico city in the large scale average rayleigh wave group velocity at 5 s period is about 2 5 km s 1 in the tmvb and larger than 3 6 km s 1 outside it significant lateral heterogeneity is evident in the stations of the dense meso american subduction experiment array we analysed we observe in addition a significant increase in velocity from north to south in the central portion of the tmvb our results are part of ongoing studies with the final objective of building a model of tmvb that may be used to simulate ground motion in central mexico from subduction zone earthquakes

flat slab subduction topography and mantle dynamics in southwestern mexico topography above subduction zones arises from the isostatic contribution of crustal and lithospheric buoyancy as well as the dynamic contribution from slab driven mantle flow we evaluate those effects in southwestern mexico where a segment of the cocos slab subducts horizontally the eastern part of the volcanic arc the trans mexican volcanic belt stands at an average elevation of 2 3 km nearly 1 3 km above the fore arc lateral changes in bulk crustal density are relatively small and seismic imaging shows that there is little variation in crustal thickness between these two regions thus the elevation difference between the arc and the fore arc should arise from differences in mantle properties we present finite element models of flat slab subduction that provide a simultaneous match to topography plate velocities and stress state in the overriding plate we find that the dynamic effects are primarily controlled by the amount of coupling at the subduction interface and in the mantle wedge the lack of slab anchoring into the lower mantle and the absence of continental mantle lithosphere with a mantle wedge and a subduction interface that are respectively 2 and 4 orders of magnitude weaker than the asthenosphere the flat slab exerts a downward pull that can explain most of the elevation difference between the fore arc and the arc we infer that lateral viscosity variations play a significant role in shaping dynamic topography in complex tectonic settings and that sublithospheric dynamics can influence the topography at wavelengths that are significantly shorter than previously recognized

crust and upper mantle seismic anisotropy variations from the coast to inland in central and southern mexico subduction zones are among the most dynamic tectonic environments on earth deformation mechanisms of various scales produce networks of oriented structures and faulting systems that result in a highly anisotropic medium for seismic wave propagation in this study we combine shear wave splitting inferred from receiver functions and the results from a previous sks wave study to quantify and constrain the vertically averaged shear wave splitting at different depths along the 100 station mesoamerican subduction experiment array this produces a transect that runs perpendicular to the trench across the flat slab portion of the subduction zone below central and southern mexico strong anisotropy in the continental crust is found below the trans mexican volcanic belt tmvb and above the source region of slow slip events we interpret this as the result of fluid melt ascent the upper oceanic crust and the overlying low velocity zone exhibit highly complex anisotropy while the oceanic lower crust is relatively homogeneous regions of strong oceanic crust anisotropy correlate with previously found low v p v s regions indicating that the relatively high v s is an anisotropic effect upper mantle anisotropy in the southern part of the array is in trench perpendicular direction consistent with the alignment of type a olivine and with entrained subslab flow the fast polarization direction of mantle anisotropy changes to n s in the north likely reflecting mantle wedge corner flow perpendicular to the tmvb

seismic attenuation structure in central mexico image of a focused high attenuation zone in the mantle wedge velocity spectra from moderate sized earthquakes were used to investigate the p wave attenuation structure in central mexico in particular we included regional events with magnitudes in the range of 4 5 to 6 1 recorded from 2005 to 2007 on the middle american subduction experiment mase array which consists of 100 broadband sensors across central mexico from acapulco to tempoal near the gulf of mexico by assuming a brune type source a frequency independent t value was obtained for each seismogram in the frequency band 1 to 30 hz these measurements were then inverted for two dimensional spatial variations in q p in the cross section along the mase array perpendicular to the trench the model has uniform 20 km vertical grid spacing down to a depth of 200 km and 50 km or 100 km horizontal grid spacing depending on ray coverage the inversion results show low attenuation in the subducting slab and high attenuation in the mantle wedge and the crust below and to the north of the volcanic belt the focused high attenuation zone q p 200 in the mantle wedge lies away from the top of the slab between depths of 80 km and 120 km beneath the volcanic belt and is likely to be related to relatively high temperature fluids and partial melts produced in subduction process the high attenuation region in the lower crust correlates with the low resistivity and low velocity region and could be caused by partial melts and fluids from dehydration and magmatic processes

complex patterns of fluid and melt transport in the central andean subduction zone revealed by attenuation tomography abstract we present a high resolution 3 d model of p wave attenuation q p 1 for the central andean subduction zone data from 1500 mostly intermediate depth 60 250 km earthquakes recorded at three temporary seismic networks covering the forearc arc and backarc around 23 s were used for tomographic inversion the forearc is characterised by uniformly high q p values indicating low temperature rocks in accordance with low surface heat flow values prominent low q p anomalies are found beneath the magmatic arc and the backarc in the crust and mantle continuous regions of low q p connect earthquake clusters at 100 km and 200 km depth with zones of active volcanism in the arc and backarc fluids fluxed from the subducted oceanic lithosphere into the overlying mantle wedge where they induce melting explain our observations we propose that low q p regions indicate source and ascent pathways of metamorphic fluids and partial melts ascent of fluids and melts as imaged by seismic q p are not vertical as is often implicitely assumed instead sources of fluids are located at different depth levels and ascent paths are complex and exhibit significant variation within the study area the largest quaternary backarc volcano cerro tuzgle is fed by mantle melts which are imaged as a plume of low q p material that reaches to the strong earthquake cluster at 200 km depth

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle

strong along arc variations in attenuation in the mantle wedge beneath costa rica and nicaragua 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 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 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