multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

a wider seismogenic zone at cascadia due to fluid circulation in subducting oceanic crust in the cascadia subduction zone the extent of the seismogenic portion of the plate interface is poorly resolved by seismicity due to the lack of a large megathrust event during the instrumental record therefore fault zone temperatures 150 to 350 c are used to estimate the limits of the seismogenic zone previous thermal models for the cascadia margin estimated that 350 c on the plate boundary occurs 40 70 km offshore in contrast models of interseismic deformation have been interpreted to indicate a seismogenic zone extending landward of the coastline to the updip edge of a region of episodic tremor and slip ets we examined cascadia subduction zone temperatures with thermal models that include the effects of fluid circulation in an ocean crust aquifer fluid circulation cools the subduction zone and widens the thermally defined seismogenic zone by shifting the 350 c isotherm at the plate boundary 30 55 km landward relative to results from simulations without fluid flow our thermal models indicate a 60 80 km wide transition zone between 350 c on the fault and the updip edge of ets under british columbia canada washington and oregon united states ets occurs at 410 550 c the location of the basalt to eclogite transition in the subducting crust provides an important constraint on the thermal models because hydrothermal circulation in the ocean crust aquifer produces only small surface heat flux anomalies on this margin with thick sediment in the trench and on the incoming plate

great earthquakes on canada s west coast a review this article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent geological survey of canada contribution 20090196 the first lithoprobe transect in 1984 across vancouver island had primary objectives to define the structure associated with subduction and constraints on the potential for great thrust earthquakes

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

short term and long term tremor migration patterns of the cascadia 2004 tremor and slow slip episode using small aperture seismic arrays 1 tectonic tremor has been recorded at many subduction zones including the nankai cascadia mexican and alaskan subduction zones this study the first to use small aperture seismic arrays to track tremor deployed three small aperture seismic arrays along the cascadia subduction zone during a tremor and slow slip episode in july 2004 the tremor was active during virtually all up to 99 minutes of the analyzed tremor episode using 5 min sample windows individual wave phases were tracked across the arrays and used to derive slowness vectors these were compared with slowness vectors computed from a standard layered earth model to derive tremor locations locations were stable within a volume roughly 250 km2 in epicenter and 20 km in depth for hours to days before moving to a new volume the migration between volumes was not smooth and the movement of the sources within the volume followed no specific pattern overall migration speeds along the strike of the subduction zone were between 5 and 15 km d smaller scale migration speeds between volumes reached speeds up to 2 km min uncertainties in the best locations were 5 km in epicenter and 10 km in depth for this data set and processing methodology tremor does not locate predominately on the primary subduction interface our favored model for the generation of tectonic tremor signals is that the tremor is triggered by stress and fluid pressure changes caused by slow slip and is composed at least in part of low frequency earthquakes broadly distributed in location

on corner frequencies attenuation and low frequency earthquakes we have recently suggested that the nearly constant duration of low frequency earthquakes lfes and equivalently the band limitation of tectonic tremor manifests a moment duration scaling that is fundamentally different from regular earthquakes and is most easily explained as rupture on asperities of roughly constant dimension in that work we employed qualitative arguments against potential bias by attenuation here we examine the role of attenuation more quantitatively through an analysis that avoids specification of particular source e g brune models and relies on the particle velocity spectral maximum as the definition of apparent corner frequency the analysis leads to the formal definition of a saturation frequency as the limiting value of apparent corner frequency as the true corner frequency tends infinity the saturation frequency a formal equivalent to fmax can be used to set bounds on path averaged quality factor q we apply these relations to deep crustal and intraslab earthquakes beneath vancouver island to estimate bulk crustal attenuation parameters that are subsequently used to correct apparent corner frequency measurements of lfes reported in our earlier work the attenuation bias due to bulk crustal structure is shown to be small with negligible effect on the principal conclusions of that study however a review of laboratory and seismic refraction measurements of attenuation in oceanic basalts and evidence for high p to s lfe corner frequency ratios raises the possibility that strong highly localized near source attenuation accompanying high pore fluid pressures could cause the bandlimited nature of lfes through the depletion of high frequencies

seismic interferometry using non volcanic tremor in cascadia 1 seismic interferometry to date has focussed on surface waves due to paucity of deep high frequency noise sources here we investigate use of non volcanic tremor on the cascadia subduction zone as a noise source to recover scattered body wave contributions to the green s function tremor data for 2004 2005 recorded at polaris bc stations twkb mgcb and lzb were filtered and cross correlated twkb and mgcb correlations generate a highly reproducible arrival at 4 5 s for combinations of vertical with radial or transverse components twkb and lzb correlations are less reproducible but yield a strong arrival at 3 5 s for 2005 upon consideration of source receiver geometry polarity and timing we interpret these arrivals to represent reflection conversions arrival times imply that these signals originate at depths between 9 and 12 km coincident with an interval of strong reflectivity imaged in the 1987 lithoprobe southern cordillera transect

crustal structure of the cascadia subduction zone southwestern british columbia from potential field and seismic studies the northern cascadia subduction zone is a region of convergence between the oceanic explorer and northern juan de fuca plates and the continental north american plate potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2 5 and 3 dimensional interpretations the lateral extent and thickness of the accreted wedge vary significantly along the zone the narrow metasedimentary pacific rim terrane lies immediately west of wrangellia and extends the length of vancouver island consistent with its emplacement by strike slip faulting following the accretion of wrangellia the ophiolitic crescent terrane is a narrow slice lying seaward of the pacific rim terrane but not extending northward beyond the juan de fuca plate in this region the crescent terrane was emplaced in a strike slip or obliquely convergent style

intraslab earthquakes dehydration of the cascadia slab we simultaneously invert travel times of refracted and wide angle reflected waves for three dimensional compressional wave velocity structure earthquake locations and reflector geometry in northwest washington state the reflector interpreted to be the crust mantle boundary moho of the subducting juan de fuca plate separates intraslab earthquakes into two groups permitting a new understanding of the origins of intraslab earthquakes in cascadia earthquakes up dip of the moho9s 45 kilometer depth contour occur below the reflector in the subducted oceanic mantle consistent with serpentinite dehydration earthquakes located down dip occur primarily within the subducted crust consistent with the basalt to eclogite transformation

lithoprobe southern vancouver island cenozoic subduction complex imaged by deep seismic reflections reply the lithoprobe seismic reflection project on vancouver island was designed to study the large scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting juan de fuca plate in this paper we interpret two lithoprobe profiles from southernmost vancouver island that were shot across three important terrane bounding faults leech river san juan and survey mountain to determine their subsurface geometry and relationship to deeper structures associated with modem subduction the structure beneath the island can be divided into an upper crustal region consisting of several accreted terranes and a deeper region that represents a landward extension of the modern offshore subduction complex in the upper region the survey mountain and leech river faults are imaged as northeast dipping thrusts that separate wrangellia a large mesozoic paleozoic terrane from two smaller accreted terranes the leech river schist mesozoic rock

shear wave constraints on a deep crustal reflective zone beneath vancouver island analysis of teleseismic receiver functions recorded in the northern cascadia subduction zone show that large p to s converted phases are generated at the boundaries of a dipping layer of deep crustal reflectors the e zone beneath central vancouver island these data provide the first shear wave constraints on this layer which is clearly observed in seismic reflection sections and is coincident with a region of low electrical resistivity interpreted from a magnetotelluric survey modeling of the amplitudes polarities and arrival times of the p to s converted phases demonstrates that the e zone is a prominent low velocity layer with an s velocity 1 0 0 2 km s lower than the regions above and below the e zone dips 7 5 in the direction n10 e 20 and extends from depths of 36 5 to 41 km at alb b on central vancouver island the estimated s velocity contrast when considered with refraction p velocities yields a poisson s ratio of 0 34 for this zone given the uncertainty in vp and vs this value may range from 0 27 to 0 38 the low p and s velocities and high poisson s ratio combined with electrical resistivity measurements suggest a porosity of 0 1 1 9 and crack aspect ratios of 0 001 0 01 values consistent with a region containing thin fluid saturated cracks and thus supportive of the interpretation of this feature as a major shear zone

crustal velocity structure in the eastern insular and southernmost coast belts canadian cordillera seismic refraction data recorded along a 330 km cross strike profile through the eastern insular and southernmost coast belts of the canadian cordillera are interpreted using an iterative combination of traveltime inversion and amplitude forward modelling the resultant model is characterized by large lateral variations in velocity the most significant of these variations is a decrease in upper and middle crustal velocities to the east of the surface trace of the harrison fault which likely represents the transition from crust of the insular superterrane to that of the intermontane superterrane this interpretation is consistent with some present geological models that place the possible probable location of the suture between the two superterranes less than 20 km east of the harrison fault velocities at the base of the upper crust average 6 4 and 6 2 km s west and east of the fault respectively mid crustal velocities average 6 6 6 9 km s to the west and 6 35 6 45 km s to the east of the fault low

imaging a steeply dipping subducting slab in southern central america article i nfo subduction of oceanic lithosphere leads to large changes in seismic velocities that are observable through converted and scattered teleseismic waves in this study we use teleseismic data from a dense broadband seismic experiment in central america to image a slab that dips n60 although strongly bent up going p to s mode conversions show a strong signal from 60 200 km depth reverberations do not reach the surface because the slab dips steeply here the subducted oceanic plate is successfully imaged to 200 km depth in nicaragua where station geometry is adequate but only at periods near 6 s at depths n100 km compared with 1 2 s periods for shallower structure the long period nature of the deeper conversion indicates a gradational boundary rather than a discontinuity probably marking the thermal boundary layer at the slab wedge interface a sharper discontinuity is imaged near the top of the slab at depths b80 km probably denoting the cocos plate moho seismicity lies above the boundary at depths b80 km and below the inferred thermal boundary at greater depth the change of character of the discontinuity with depth may re ect partial eclogitization of the subducting crust and associated reduction of the velocity contrast at the moho with seismicity lying within subducting crust but the dominant velocity change migrating out of it with increasing depth in the upper plate images map the moho in both costa rica and nicaragua with moho conversions weakest directly beneath the arc complex dipping structure at 25 50 km depth beneath the nicaragua forearc perhaps reveals the deep roots of an accreted arc terrane

neighbourhood inversion of teleseismic ps conversions for anisotropy and layer dip summary the inversion of teleseismic receiver functions for lithospheric structure is difficult due to the non linearity of the problem which is greatly increased in the presence of dipping interfaces and layer anisotropy given an efficient ray theoretical tool for forward modelling teleseismic seismograms we perform a directed monte carlo search technique using the neighbourhood algorithm of sambridge enabling us to search 20 30 parameters in a reasonable amount of computer time tests on synthetic data reveal inherent velocity depth trade offs in typical data sets due to the limited moveout present in teleseismic ps the azimuth of the anisotropic symmetry axis and the strike of a dipping interface prove to be well resolved given adequate backazimuthal coverage we apply this technique to two single station data sets the first from permanent station pgc vancouver island british columbia displays dipping low velocity sediment layers in the mid crust the second from a station at the northern end of the tibetan plateau operating in 1991 and 1992 requires a sequence of thick crustal anisotropic layers to explain the observed pattern of receiver function arrivals

nature of the low velocity zone in cascadia from receiver function waveform inversion abstract a dipping low velocity zone lvz is a ubiquitous structural element of subduction zones worldwide in this study we map seismic attributes characterizing the lvz beneath the cascadia subduction zone from northern vancouver island to northern california using receiver function waveform inversion throughout this region the lvz is characterized by high v p v s ratios mean 2 77 strong s velocity contrasts 50 and thicknesses averaging 3 37 km the lvz is immediately underlain by a second weaker layer exhibiting moderate v p v s ratios mean 1 85 with mean thickness of 4 62 km we interpret the combined structure in terms of subducting oceanic crust based on classical structural petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores the lvz is identified with pervasively hydrated high porosity pillow basalts and sheeted dikes of layer 2 with possible contributions from sediments layer 1 fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above and a low porosity gabbroic mafic cumulate dominated layer 3 below

high resolution image of the oceanic moho in the subducting nazca plate from p s converted waves short period p s converted waves outline the oceanic moho of the descending nazca plate in northern chile at 24 s comparison with hypocentral locations obtained with temporary local seismic networks show that earthquakes inside the subducting oceanic lithosphere of the nazca plate are located predominantly in the oceanic crust at depths between 70 and 120 km and probably in the upper mantle at depths below 120 km the results suggest that dehydration embrittlement as mechanism for intermediate depth earthquakes is active both in the former oceanic crust and upper mantle of subducting lithosphere

dispersion of regional body waves at 100 150 km depth beneath alaska in situ constraints on metamorphism of subducted crust phase delays at high frequencies are observed in body waves that travel in the alaska slab along its strike at 100 150 km depth the delays between 2 6 hz energy and the direct 0 5 1 hz arrival are 0 5 1 5 s for p waves and 1 5 4 s for s waves such dispersion suggests a waveguide structure that parallels the slab perhaps near its top a channel that is 2 6 km thick and 2 5 5 slower than surrounding mantle can explain the observations the thickness of the layer is comparable to that of subducted oceanic crust or somewhat thinner the layer may be crust that is slow at these depths the required velocity anomaly is too small to be due to a continuous layer of metastable gabbro yet too large to represent an eclogite layer it may indicate a mixture of the two or persistence of hydrated mineral assemblages to depth

seismic reflection imaging of the subducting juan de fuca plate mckenzie and parker s early suggestion1 that the juan de fuca plate is underthrusting north america has since been confirmed by numerous studies evidence for plate convergence along this junction includes a sediment filled trench2 7 intense compressional deformation of the continental slope and shelf sediments2 7 seafloor magnetic anomalies that extend beneath the continental slope2 4 inland andesitic volcanoes8 paired low high heat flow and gravity anomalies9 10 active onshore deformation11 12 onshore crustal earthquakes with compressional axes parallel to the direction of plate convergence13 sinistral strike slip earthquakes along the nootka fault fig 1 14 and a well defined benioff wadati zone6 15 16 here we report the results of a seismic reflection survey that has delineated two slabs of oceanic lithosphere underlying vancouver island one that is currently being subducted and one that is underplated these findings lead us to speculate that successive underplating of oceanic lithosphere may be an important process in the evolution and growth of continents

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a comparison of the receiver structure beneath stations of the canadian national seismograph network three component broadband data from the recently deployed canadian national seismograph network provide a new opportunity to examine the structure of the crust and upper mantle beneath the canadian landmass receiver function analysis is an ideal method to use with this data set as it can provide constraints on the s velocity structure beneath each station of this seismograph network this analysis method is particularly useful in that it provides site specific information i e within 5 15 km of the station low velocity layers can be identified and it is possible to examine structure to upper mantle depths in this study receiver functions were computed for each of the 19 stations that made up the seismograph network in june 1994 five stations sampling a variety of tectonic environments including the appalachian orogen the canadian shield the western canadian sedimentary basin and the cascadia subduction zone were chosen for detailed modelling the results presented here are the first estimates of the s velocity structure beneath these five stations for those stations where comparisons can be made with seismic reflection and refraction results there is excellent agreement in eastern canada simple receiver functions and clear moho ps conversions at most stations indicate a relatively transparent crust and a moho depth of 40 45 km in northwestern canada moho ps phases indicate a crustal thickness of 33 38 km beneath inuvik northwest territories the moho is interpreted as two velocity steps separated in depth by 5 km and an upper mantle low velocity zone is near 47 km depth in western canada the data indicate a mid crustal low velocity zone beneath edmonton the moho of the subducting juan de fuca plate is interpreted at 52 krn depth beneath southern vancouver island several stations exhibiting complex receiver functions warrant further study they include stations at scheffewille quebec in the canadian shield deer lake newfoundland on the boundary of the grenville province and the appalachian orogen and yellowknife northwest territories at the intersection of the churchill and slave provinces and the western plains

seismic reflection constraints on imbrication and underplating of the northern cascadia convergent margin an interpretation of the deep structure of the continental shelf offshore southern vancouver island subject to constraints from other geophysical data is derived by combining seismic reflection profiles shot in 1989 with those from an earlier 1985 survey accretionary wedge sediments which extend landward beneath the volcanic crescent terrane comprise two primary units both of which have shortened through duplex formation the maximum thickness of the crescent terrane 6 8 km occurs just seaward of its contact with the inboard largely metasedimentary pacific rim terrane the e region of reflectivity first detected dipping landward beneath vancouver island is regionally extensive being observed on all the seismic profiles the e reflectivity thins seaward and splits into two or more strands that probably link into major faults within the accreted sedimentary wedge reflections from the interplate decollement beneath the outer continental shelf separate from the downgoing plate continue into the

sedimentary underplating at the cascadia mantle wedge corner revealed by seismic imaging earth s largest earthquakes occur at the boundary between subducting oceanic crust and the overriding plate yet the position of the plate boundaries remains uncertain analysis of zones of low seismic wave velocities beneath the northern cascadia subduction zone implies that the plate boundary here may be deeper than previously thought

new constraints on subduction zone structure in northern cascadia summary a detailed passive seismic experiment was carried out across southwestern british columbia and northwestern washington to investigate the structure of the subducting juan de fuca plate and mantle wedge in cascadia and its relation to intraslab seismicity as part of the polaris project 31 three component broad band stations were deployed in an approximately linear array spanning southern vancouver island the gulf and san juan islands watcom county and the british columbia lower mainland p wave coda from 41 teleseismic events have been employed in formal inversions for fine scale shear velocity structure our results indicate a structure very similar to that identified across a comparable profile in central oregon the continental moho is evident at the eastern end of the profile near 35 km depth but disappears towards georgia strait puget sound a prominent low s velocity zone is clearly evident below southern vancouver island dipping eastwards through georgia strait puget sound and coincides with the e reflection zone originally identified in lithoprobe studies structure below the e layer is much less prominent and varies intermittently along the array based on the observations and interpretations of similar structures beneath oregon alaska and south america and its projection to mantle depths we suggest that the low velocity e layer represents the dehydrating oceanic crust of the subducting juan de fuca plate this interpretation is consistent with recent seismicity studies that place shallow wadati benioff events within the oceanic mantle and implies that the oceanic crust is 6 8 km shallower beneath vancouver island than previously assumed as in oregon we interpret the diminished signature of oceanic crust below a depth of 45 km to signal the presence of eclogitization which in turn supplies water to serpentinize the overlying forearc mantle

the reflection seismic survey of project tipteq the inventory of the chilean subduction zone at 38 2 s summary we describe results of an active source seismology experiment across the chilean subduction zone at 38 2 s the seismic sections clearly show the subducted nazca plate with varying reflectivity below the coast the plate interface occurs at 25 km depth as the sharp lower boundary of a 2 5 km thick highly reflective region which we interpret as the subduction channel that is a zone of subducted material with a velocity gradient with respect to the upper and lower plate further downdip along the seismogenic coupling zone the reflectivity decreases in the area of the presumed 1960 valdivia hypocentre the plate interface itself can be traced further down to depths of 50 60 km below the central valley we observe strong reflectivity at the plate interface as well as in the continental mantle wedge the sections also show a segmented forearc crust in the overriding south american plate major features in the accretionary wedge such as the lanalhue fault zone can be identified at the eastern end of the profile a bright west dipping reflector lies perpendicular to the plate interface and may be linked to the volcanic arc

detection and location of low frequency earthquakes using cross station correlation source geometry and simple propagation of s waves generated by low frequency earthquakes lfes in northern cascadia results in strong waveform coherence on horizontal channels for stations at smaller epicentral distances as recognized by previous workers this cross station similarity can be exploited for detection of lfes in tremor we develop a cross station correlation approach based on waveform coherence and travel time consistency that exploits a full complement of network stations and we demonstrate its application to separate arrays on vancouver island and washington state our approach yields thousands of impulsive lfe detections per slow slip episode and hundreds of events between episodes lfe epicentral distributions reveal the presence of well defined regions of high asperity density with those regions farther downdip that are active during interepisodic tremor and slow slip periods the lfe epicenters also display pronounced spatiotemporal clustering that compares favorably with independent tremor and lfe template catalogs and that defines rapid tremor reversals with complex propagation patterns the lfe source depths can be estimated for some detections in which p waves are identified on the vertical channel through correlation with the horizontal component s waveforms lfe hypocenters cluster between two recently developed plate interface models below southern vancouver island our approach enables the identification of high signal to noise ratio impulsive lfe detections from targeted regions on the plate boundary for generation of lfe templates to be used in studies of structure and seismogenesis in cascadia online material additional maps of station density for southern vancouver island epicenter locations for northern washington and additional time distance plots for southern vancouver island

deep electrical structure of the northern cascadia british columbia canada subduction zone implications for the distribution of fluids long period magnetotelluric data have been used to image the deep electrical structure of the cascadia subduction zone in british columbia canada zones of elevated electrical conductivity were found in both the forearc and backarc regions and are interpreted as a consequence of the fluid release from subducting slab a shallow zone of high conductivity beneath vancouver island is likely due to fluids that are trapped above the subducting plate east of this structure is a conductive 0 03 s m forearc mantle wedge that also exhibits low seismic velocities and may be serpentinized a free fluid phase is required to account for this enhanced conductivity elevated conductivities are observed in the upper mantle throughout the backarc 0 01 s m and strongly support the hypothesis of a shallow convecting asthenosphere this enhanced upper mantle conductivity can be explained by either hydrogen ion diffusion in olivine minerals or by a few percent partial melting 4

imaging slow slip fronts in cascadia with high precision cross station tremor locations 1 we apply a new method to obtain accurate locations of tremor sources beneath southern vancouver island unlike more standard cross time methods which compare waveforms from different time windows at the same station this cross station method compares waveforms from the same time window at widely separated stations it performs well relative to cross time methods when the response to an impulsive tremor source is dominated by the main arrival rather than coda and when multiple colocated sources are active within the time window examined we focus on a region roughly 10 km across that was monitored by the polaris deployment from 2003 to 2006 relative location errors appear to be 1 km allowing us to image in great detail rapid and small scale tremor migrations that arise behind the main slow slip front the secondary fronts tend to a start at or within about 1 km of the main tremor front and propagate back along strike at rates of 10 20 km h b less commonly do the reverse ending at the main front or c propagate up or downdip at or within 1 2 km of the main front estimated stress drops in the secondary events are comparable to that in the main event implying that their 25 50 times greater propagation speed results from a similarly greater slip speed

correlation of tremor activity with tidal stress in the northern cascadia subduction zone 1 we analyze hourly data from five tremor episodes in the northern cascadia subduction zone over the period 2003 2005 provided by the tremor activity monitoring system tams all five tremor episodes correspond to slow slip events observed by gps fourier decomposition is used to separate the hourly tremor counts for each episode into long period 0 2 2 cpd components the tidal component of the observations is compared with theoretical stress variations at depths of 20 30 and 40 km with 40 km being the depth of the interpreted subduction thrust interface the stress variations are predicted by a 2 d ocean tide loading model combined with estimates of stress variations from earth tides we find that the shear stress in the thrust direction and the compressive normal stress on shallow dipping surfaces correlates with the data significantly better than the confining stress over the range of depths investigated the relative amplitudes of tidal shear stress and compressive normal stress result in positive coulomb stress favoring slip peak tremor activity occurs at times of maximum tidal shear stress in the thrust direction which would assist slow slip and would suggest that tidal tremor and slip are colocated the response of the tremor to tidal shear stress is roughly proportional to the mean activity level controlled by tectonic conditions of stress and pore pressure a significant nontidal daily variation in tremor activity of unknown origin is identified

new geophysical insight into the origin of the denali volcanic gap summary volcanic gaps are segments of subduction zones that lack the volcanic activity usually found at these convergent margins they are regions where the necessary conditions to produce melt may appear favourable but where volcanoes are surprisingly absent from the surface in this study we present a new model that can explain the occurrence of such volcanic gaps it is based on seismic imaging and geodynamic modelling of the denali volcanic gap a 400 km wide region at the eastern end of the alaska aleutian subduction zone here the thick crust of the pacific plate and yakutat terrane subduct at shallow angle beneath north america a high resolution seismic profile clearly images the subducting crust undergoing progressive dehydration between 50 and 120 km depth and a negative subhorizontal velocity contrast at 60 km depth in the overlying mantle wedge we interpret this 60 km discontinuity as marking the top of a layer of partial melt that pools at the base of the overriding plate in steadystate subduction models melt accumulates at the apex of a vaulted mantle wedge the pinch zone from where it may break through the overlying lithosphere to the surface beneath the denali volcanic gap the pinch zone is absent or greatly reduced because shallow subduction of the yakutat terrane progressively cools the system and causes the slab to advance and replace the hot core of the mantle wedge this regime can be seen as the opposite of subduction rollback it prevents the formation of a pinch zone reduces the length of the melting column and causes melt to pool at the base of the overriding plate thus inhibiting magma generation and extraction

deep high amplitude reflections from a major shear zone above the subducting juan de fuca plate seismic reflection data show that a regionally extensive band of landward dipping reflections exists above the subducting juan de fuca plate at the western canadian continental margin the reflections truncate at depth a major terrane boundary mapped near the surface and must have a structural origin many reflections are very strong having reflection coefficients locally as high as 10 20 these amplitudes are consistent with theoretical calculations of reflections from sharp poirosity contrasts if the pores are very thin however seismic data from the accreted sedimentary melange farther seaward from which much of the deep reflective zone is derived suggest that significant porosity contrasts do not occur alone because higher reflection amplitudes are found in regions of inferred high strain both geometric and amplitude constraints imply that the deep reflective layering represents intensely sheared rocks possibly originating close to the subduction decollement

evidence for both crustal and mantle earthquakes in the subducting juan de fuca plate 1 we investigate the relationship between high precision hypocentres in the subducting juan de fuca plate beneath southwest british columbia and the velocity structure from receiver function analysis and reflection seismic data 108 earthquakes at depths of 40 to 65 km were relocated using a double difference algorithm to obtain precise earthquake locations correlation of the relocated seismicity with structural information shows a concentration of earthquakes near the top of the subducting oceanic crust and a deeper layer of seismicity in the uppermost mantle of the subducting juan de fuca plate the strong correlation of the seismicity with the structure of the subducting plate suggests that seismic failure is caused by changes in mechanical strength within the plate supporting the hypotheses that phase transformations and thermal petrological conditions play an important role in the seismogenesis of double seismic zones index terms 7209 seismology earthquake dynamics and mechanics 7218 seismology lithosphere and upper mantle 7230 seismology seismicity and seismotectonics 7220 seismology oceanic crust citation cassidy j f and f waldhauser evidence for both crustal and mantle earthquakes in the subducting juan de fuca plate geophys res lett 30 2 1095 doi 10 1029 2002gl015511 2003

the northern cascadia subduction zone at vancouver island seismic structure and tectonic history the structure and tertiary tectonic history of the northern cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the

possible emplacement of crustal rocks into the forearc mantle of the cascadia subduction zone 1 seismic reflection profiles shot across the cascadia forearc show that a 5 15 km thick band of reflections previously interpreted as a lower crustal shear zone above the subducting juan de fuca plate extends into the upper mantle of the north american plate reaching depths of at least 50 km in the extreme western corner of the mantle wedge these reflectors occur in rocks with p wave velocities of 6750 7000 ms 1 elsewhere the forearc mantle which is probably partially serpentinized exhibits velocities of approximately 7500 ms 1 the rocks with velocities of 6750 7000 ms 1 are anomalous with respect to the surrounding mantle and may represent either 1 locally high mantle serpentinization 2 oceanic crust trapped by backstepping of the subduction zone or 3 rocks from the lower continental crust that have been transported into the uppermost mantle by subduction erosion the association of subparallel seismic reflectors with these anomalously low velocities favours the tectonic emplacement of crustal rocks

crustal anisotropy in the forearc of the northern cascadia subduction zone british columbia summary this paper aims to identify sources and variations of crustal anisotropy from shear wave splitting measurements in the forearc of the northern cascadia subduction zone of southwest british columbia over 20 permanent stations and 15 temporary stations were available for shear wave splitting analysis on 4500 event station pairs for local crustal earthquakes results from 1100 useable shear wave splitting measurements show spatial variations in fast directions with margin parallel fast directions at most stations and margin perpendicular fast directions at stations in the northeast of the region crustal anisotropy is often attributed to stress and has been interpreted as the fast direction being related to the orientation of the maximum horizontal compressive stress however studies have also shown anisotropy can be complicated by crustal structure southwest british columbia is a complex region of crustal deformation and some of the stations are located near large ancient faults to use seismic anisotropy as a stress indicator requires identifying which stations are influenced by stress and which by structure we determine the source of anisotropy at each station by comparing fast directions from shear wave splitting results to the maximum horizontal compressive stress orientation determined from earthquake focal mechanism inversion most stations show agreement between the fast direction and the maximum horizontal compressive stress this suggests that anisotropy is related to stress aligned fluid filled microcracks based on extensive dilatancy anisotropy these stations are further analysed for temporal variations to lay groundwork for monitoring temporal changes in the stress over extended time periods determining the sources of variability in anisotropy can lead to a better understanding of the crustal structure and stress and in the future may be used as a monitoring and mapping tool

hypocenter location by pattern recognition 1 a novel approach to hypocenter location is proposed on the basis the concept of pattern recognition a new data misfit criterion for location is introduced which measures discrepancies between the observed arrival times of an event and those of nearby previous events in the arrival pattern misfit measure travel times predicted by an earth model are effectively replaced by information from an ensemble of previous observations thin plate spline interpolation and generalized cross validation are applied to interpolate and smooth the resulting misfit function which may then be used in standard location algorithms synthetic experiments show that in certain circumstances it is possible to achieve locations with errors smaller than those in the underlying database it is suggested that the arrival pattern approach exploits information on lateral heterogeneous earth structure contained in the database to constrain locations the arrival pattern approach is illustrated by relocating 395 ground truth events from the nevada test site 482 earthquakes from the marianas subduction zone and 457 earthquakes from the atlantic mid ocean ridge it is shown that picking errors and unmodeled small scale lateral heterogeneity are the most significant sources of event mislocation and that errors in the original locations of the database events make a much smaller contribution

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

seismic reflection imaging of two megathrust shear zones in the northern cascadia subduction zone at convergent continental margins the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single thin interface known as a megathrust1 great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together2 earthquakes of lower magnitude occur within the underlying oceanic plate and have been linked to geochemical dehydration reactions caused by the plate s descent3 4 5 6 7 here i present deep seismic reflection data from the northern cascadia subduction zone that show that the inter plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a 5 km thick 110 km wide region of imbricated crustal rocks earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter plate boundary zone this implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate slip on the shallower megathrust shear zone which may occur by aseismic slow slip will transport crustal rocks into the upper mantle above the subducting oceanic plate and may in part provide an explanation for the unusually low seismic wave speeds that are observed there8 9

evidence for underthrusting beneath the queen charlotte margin british columbia from teleseismic receiver function analysis summary the queen charlotte fault zone is the transpressive boundary between the north america and pacific plates along the northwestern margin of british columbia two models have been suggested for the accommodation of the 20 mm yr 1 of convergence along the fault boundary 1 underthrusting 2 internal crustal deformation strong evidence supporting an underthrusting model is provided by a detailed teleseismic receiver function analysis that defines the underthrusting slab forward and inverse modelling techniques were applied to receiver function data calculated at two permanent and four temporary seismic stations within the queen charlotte islands the modelling reveals a 10 km thick low velocity zone dipping eastward at 28 interpreted to be underthrusting oceanic crust the oceanic crust is located beneath a thin 28 km eastward thickening 10 continental crust

lithoprobe southern vancouver island seismic reflection sees through wrangellia to the juan de fuca plate multichannel seismic reflection profiles obtained on vancouver island show that above a zone of decoupling between the north american and juan de fuca plates wrangellia is underplated by two accreted terranes of probable oceanic origin both the zone of decoupling and wrangellia are disrupted by easterly dipping faults some of which are thrusts the crustal structure of wrangellia comprises paleozoic and mesozoic volcanic and sedimentary rocks disposed as roof pendants upon and large irregular masses within a ubiquitous jurassic plutonic and metamorphic complex

slab low velocity layer in the eastern aleutian subduction zone summary local earthquakes in the vicinity of the alaskan peninsula s shumagin islands often produce arrivals between the main p and s arrivals not predicted by standard traveltime tables based on traveltime and polarization these anomalous arrivals appear to be from p to s conversions at the surface of the subducted pacific plate beneath the recording stations the p to s conversion occurs at the top of a low velocity layer which extends to at least 150 km depth and is 8 f 2 per cent slower than the overlying mantle the slab is 7 per cent faster than the mantle the low velocity layer contains the foci of the earthquakes in the upper plane of the double seismic zone and confines ps ray paths to lie within it these observations indicate that layered structures persist to positions well past the surface location of the volcanic front reactions forming high pressure minerals do not yield slab like velocities until beyond the point that subduction zone magma genesis occurs if the subducted oceanic crust forms the layer it is subducted essentially intact

reflection signature of seismic and aseismic slip on the northern cascadia subduction interface at the northern cascadia margin the juan de fuca plate is underthrusting north america at about 45 mm yr 1 ref 1 resulting in the potential for destructive great earthquakes2 3 the downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake thought to have been in 1700 4 occurred before instrumental recording thermal and deformation studies5 indicate that off southern vancouver island the interplate interface is presently fully locked for a distance of 60 km downdip from the deformation front great thrust earthquakes on this section of the interface with magnitudes of up to 9 4 5 have been estimated to occur at an average interval of about 590 yr ref 3 further downdip there is a transition from fully locked behaviour to aseismic sliding where high temperatures allow ductile deformation with the deep aseismic zone exhibiting slow slip thrust events6 here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked to a broad reflection band at greater depth where aseismic slip is thought to be occurring this change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones

high resolution imaging of rapid tremor migrations beneath southern vancouver island using cross station cross correlations we develop a cross station method to detect and locate tremor and low frequency earthquakes lfes based on the original work of armbruster et al 2014 that compares waveforms from the same time window at stations separated by roughly 10 km to improve the signal to noise ratio we first rotate the horizontal components into the empirical shear wave particle motion direction the large scale rapid tremor reversals beneath southern vancouver island are best recorded by stations that exhibit pronounced shear wave splitting which obscures this optimal direction we correct for splitting using the stacked templates of 11 low frequency earthquake families obtained in this region by bostock et al 2012 we find that the style of rapid tremor migrations rtms evolves as the main front passes over a region very close to the main front numerous small scale migrations occur with recurrence intervals far shorter than tidal periods these usually propagate along the main front even when that is not parallel to dip several larger rtms propagating along the main front have prominent elongation orthogonal to the propagation direction inconsistent with the interpretation that their large propagation speed is an apparent velocity caused by the slow main front intersecting a preexisting linear structure on the plate interface farther behind the main front rtms gradually progress to being tidally modulated and have generally slower propagation speeds many reversal like rtms are observed some of which evolve from fronts initially propagating along the main front these reversals are sometimes coherent across regions of low tremor density

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

spatial temporal patterns of seismic tremors in northern cascadia 1 we study in detail the two consecutive episodic tremor and slip ets events that occurred in the northern cascadia subduction zone during 2003 and 2004 for both sequences the newly developed source scanning algorithm ssa is applied to seismic waveform data from a dense regional seismograph array to determine the precise locations and origin times of seismic tremors in map view the majority of the tremors occurred in a limited band bounded approximately by the surface projections of the 30 km and 50 km depth contours of the plate interface the horizontal migration of tremor occurrence is from southeast to northwest with an average speed of 5 km d in cross section tremors in both sequences span a depth range of over 40 km across the interface with the majority occurring in the overriding continental crust in particular 50 55 of them are located within 2 5 km from the strong seismic reflector bands above the plate interface the lack of vertical migration implies that a slow diffusion process in the vertical direction cannot be responsible for tremor occurrences the source spectra of tremors clearly lack high frequency content 5 hz relative to local earthquakes we propose two possible models to explain the relationship between slip and tremors the first one regards ets tremors as the manifestation of hydroseismogenic processes in response to the temporal strain variation associated with the episodic slip along the lower portion of the plate interface downdip from the locked zone in the second model tremors and slip are associated with the same process along the same structure in a distributed deformation zone across the plate interface neither model can be dismissed conclusively at this stage

the removal of free surface interactions from three component seismograms summary the effect of the free surface can be removed from three component seismic recordings to recover the incident upgoing wavefield if the slowness and azimuth of the current wavefront are known as a function of time for a single threecomponent station it is usually possible to estimate an azimuth for an event from the first arriving p waves but slowness estimates are less reliable when more than one wavetype is presented in the seismic wavetrain however the free surface correction operators are generally slowly varying functions of slowness and so some error in slowness can be tolerated effective approximations for the removal of the free surface effects can be made for hard rock sites to cover slowness bands for the main regional phases pn pg sn and lg by applying these operators in turn over group velocity windows appropriate to the particular phases the relative amplitude of the p sv and sh contributions to the wavefield can be estimated because the free surface amplification effects have been removed the amplitudes can be compared directly and provide useful constraints on the radiation characteristics of the source this procedure is therefore helpful for developing discrimination measures for different classes of sources

a magnetotelluric sounding across vancouver island detects the subducting juan de fuca plate magnetotelluric1 mt and geomagnetic2 depth sounding measurements have been made at 18 sites 7 with remote reference3 across vancouver island fig 1 beneath which the juan de fuca plate is underthrusting4 5 vancouver island is part of the old accretionary wrangellia terrane6 and is located in the central portion of a 350 km forearc region that extends from the sediment filled trench7 to the garibaldi volcanic belt8 gravity9 and seismic refraction studies10 suggest that the descending plate has a shallow northeasterly dip of 8 16 with perhaps higher dips beneath northeastern vancouver island a high resolution seismic reflection survey11 along lines 1 and 3 fig 1 located a zone of strong acoustic reflections the e horizon11 near the top of the subducting juan de fuca plate at depths of 23 34 km our model of the broad band mt data defines a sloping highly conducting zone also near the top of the subducting plate at the same depths as the seismic e horizon the high electrical conductivity in this zone the e conductor is the result of saline fluid within the pore spaces of sedimentary and mafic materials of the upper oceanic crust that have been subducted to those depths beneath vancouver island these are the first data which clearly define a dipping conductive layer associated with the boundary region between converging plates they have significant implications for thrust earthquakes and metamorphic reactions that occur in subduction zones

how the crust meets the mantle lithoprobe perspectives on the mohorovicic discontinuity and crust mantle transition application of regional geophysical and geological methods throughout two decades of canada s lithoprobe project provides new opportunities to analyze the mohorovicic discontinuity moho and crust mantle transition the transect format employed during lithoprobe in which 10 specified regions of canada were targeted for approximately a decade each between 1984 and 2003 permitted teams of scientists to focus on geological geophysical and tectonic issues for each transect as a primary objective was to enhance knowledge of the structure of the crust and lithosphere an obvious target in each transect was the nature and origin of the moho and crust mantle transition accordingly the combined results provide new perspectives on the moho and the relationship of the moho to the crust mantle transition perhaps the most important result is that the continental geophysical moho is a deceptively simple feature it has a variety of signatures at different scales that preclude a single universally applicable interpretation in methods that provide large scale information such as regional seismic studies it is a relatively abrupt refraction velocity contrast that often displays a dramatic downward decrease in seismic reflectivity however its origin in a geological or tectonic sense is perhaps best determined by careful analyses of structural details near the geophysical moho which are complex and varied in some areas within canada it appears that the geophysical moho may be old and perhaps remains from the time the crust formed in other areas it appears to be a relatively young feature that was superimposed onto older crustal fabrics

crust and upper mantle of kamchatka from teleseismic receiver functions teleseismic receiver functions rfs from a yearlong broadband seismological experiment in kamchatka reveal regional variations in the moho anisotropy in the supra slab mantle wedge and along the eastern coast ps converted phases from the steeply dipping slab we analyze both radial and transverse component rfs in bin averaged epicentral and backazimuthal sweeps in order to detect ps moveout and polarity variations diagnostic of interface depth interface dip and anisotropic fabric within the shallow mantle and crust at some stations the radial rf is overprinted by near surface resonances but anisotropic structure can be inferred from the transverse rf using forward modeling to match the observed rfs we find moho depth to range between 30 and 40 km across the peninsula with a gradational crust mantle transition beneath some stations along the eastern coast anisotropy beneath the moho is required to fit the transverse rfs at most stations anisotropy in the lower crust is required at a minority of stations modeling the amplitude and backazimuthal variation of the ps waveform suggests that an inclined axis of symmetry and 5 10 anisotropy are typical for the crust and the shallow mantle the apparent symmetry axes of the anisotropic layers are typically trench normal but trench parallel symmetry axes are found for stations apa and ess both at the fringes of the central kamchatka depression transverse rfs from east coast stations kro tum zup and petare fit well by two anisotropic mantle layers with trench normal symmetry axes and opposing tilts strong anisotropy in the supraslab mantle wedge suggests that the mantle lithosphere beneath the kamchatka volcanic arc is actively deforming strained either by wedge corner flow at depth or by trenchward suction of crust as the pacific slab retreats d 2002 elsevier science b v all rights reserved

comparison of diagenetic and low grade metamorphic evolution of chlorite in associated metapelites and metabasites an integrated tem and xrd study chlorite is a common sheet silicate that occurs in various lithologies over a wide grade range involving diagenesis and low grade metamorphism thus the reaction progress of chlorite overs a unique oppor tunity for direct correlation of zonal classification of metasedimentary rocks based on illite crystallinity with metabasite mineral facies to provide such correlation chlorite crystallinity indices apparent mean crystallite sizes and lattice strains crystallite size distributions and compositions of chlorite from coexisting metapelites and metabasites were determined by x ray divraction xrd scanning electron microscopy sem transmission electron microscopy tem analytical electron microscopy aem and electron microprobe emp methods samples were from palaeozoic and mesozoic formations of the bukkium innermost western carpathians hungary that underwent alpine cretaceous orogenic metamorphism metapelites range in grade from late diagenesis to epizone whereas metabasites vary from prehnite pumpellyite through pumpellyite actinolite to greenschist facies despite significant diverences in composition mineral assemblages and textures reaction progress as measured in part by chlorite crystallinity in metapelites paralleled that in metabasites chlorite crystal linity and mean crystallite size increase and the proportion of mixed layers in chlorite decreases whereas the calculated lattice strain does not change significantly with increasing metamorphic grade similar trends but especially at higher grades significant diverences were found in mean crystallite size values using various methods for xrd line profile analyses the increase in crystallite size with increasing grade was demonstrated also by direct tem measurements on ion milled whole rock samples but with a larger scatter of data at higher grades in spite of the diverent kinds of mixed layering in chlorite mg rich smectitic mostly random local corrensite like units in metabasites and fe rich berthierine and diocta hedral smectite in metapelites xrd calculated and tem measured parameters were found to be reliable tools for measuring reaction progress and metamorphic grade of the same degree in both lithotypes

three dimensional empirical traveltimes construction and applications summary a new approach for predicting traveltimes is proposed which makes use of sophisticated smoothing and interpolation techniques applied to observations from large earthquake catalogues the technique produces 3 d empirical traveltimes directly from a database of observations and does not impose any arbitrary spatial scalelength on the data for example by smoothing observations over a pre determined distance interval this feature allows the spatial variability in data density as well as multiple length scales of heterogeneity to be handled in a natural way after removing the empirical traveltimes from observed traveltimes the remaining residuals are spatially uncorrelated and in this sense all of the heterogeneity signal in the data is accounted for traveltimes predicted with the new approach compare favourably with those from 1 d and 3 d earth models application to nuclear blast data shows that traveltime residuals are reduced by 62 per cent and for earthquake residuals a reduction of up to 42 per cent is achieved across a range of teleseismic phases our results show that the pattern of regional variability in p wave traveltimes can also be recovered accurately in the epicentre determination of 25 nuclear blasts the use of empirical traveltimes in place of 3 d global velocity models reduced the average mislocation by over 59 per cent this new approach can also be used to de noise large arrival time databases and to identify outliers the methodology is quite flexible and can equally well be applied to local and regional seismicity

the fate of the downgoing oceanic plate insight from the northern cascadia subduction zone abstract in this study we use teleseismic receiver function analysis to image the seismic structure of the juan de fuca oceanic plate during its subduction beneath the north american plate seismic data have been recorded at 58 seismic stations deployed along the northern cascadia subduction zone harmonic decomposition of the receiver function data set along a trench normal profile allows us to image both the isotropic and the anisotropic structure of the plate slab our images highlight the presence of a highly anisotropic region at 40 70 km depths across the cascadia subduction zone the detected seismic anisotropy is interpreted to be related to both metamorphic facies e g blueschists and fluid released during the dehydration of the subducting mantle the processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab i e basalts gabbro layer and upper mantle as the oceanic plate sinks in the upper mantle such variations make it almost impossible to recognize the plate boundary as a characteristic velocity jump at depth neither positive nor negative along the cascadia subduction zone based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved we propose a 4 stage model of the evolution of the juan de fuca oceanic plate during its subduction beneath the north american plate

split from slip and schist crustal anisotropy beneath northern cascadia from non volcanic tremor 1 polarization and splitting analyses of minute long windows of non volcanic tremor at 3 component stations in the vicinity of southern vancouver island provide new evidence for the presence of anisotropy within the crustal portion of the north american plate beneath cascadia as reported previously tremor particle motions are predominantly horizontal and inferred to manifest an upgoing wavefield composed primarily of s waves under this assumption estimates of incidence angle and back azimuth can be deduced from orientation of the smallest principle direction of motion when subjected to standard s wave splitting analysis most stations yield systematic and reproducible fast polarization directions estimates of original polarization direction are more scattered and splitting times often exhibit multimodal distributions numerical simulations wherein band limited synthetic seismograms comprising contributions from multiple sources are analyzed for splitting neatly reproduce these characteristics and thereby provide a framework for interpretation a number of stations in our study sit astride the san juan fault which based on reflection studies dips approximately 60 to 70 to the north underlying these stations is the leech river complex consisting of strongly foliated greenschist facies phyllites with steeply dipping foliations striking parallel to fast polarization directions of split s waves detailed laboratory velocity measurements at elevated pressures show that as little as 2 to 3 km of vertically dipping leech river phyllite are required to produce the observed splitting these phyllites have some of the highest s wave anisotropies measured to date and are part of a once continuous belt of anisotropic crust extending over 2000 km along the western north american coast from southern vancouver island to southwest alaska

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

dipping seismic reflectors electrically conductive zones and trapped water in the crust over a subducting plate the lithoprobe program across the northern cascadia subduction zone at vancouver island has obtained geophysical data that suggest a dipping zone of trapped free pore water in the overlying continental crust land and marine multichannel seismic reflection profiles show several bands of reflectors the primary one at a depth of about 30 km is about 5 km thick dips inland and appears to coincide with an electrically conductive layer defined by magnetotelluric data the downgoing oceanic crust is imaged as a thin reflector dipping at about 15 and 10 km deeper than the primary reflector a location that is consistent with seismicity and seismic refraction data both the reflectors and the conductor have been modeled by near horizontal lamellae with 1 4 saline water porosity detailed geothermal measurements defining heat flow that decreases inland indicate that the dipping bands are nearly isothermal at a temperature of 400 500 c it is suggested that free water generated by dehydration of the downgoing oceanic plate migrates upward until reduced temperature results in hydration reactions and mineral precipitation that forms an impermeable barrier low vertical permeability is required to keep the water from separating to the top of the layer this can be reconciled with the low electrical resistivity if pore interconnection is through thin grain boundary tubes as predicted by theoretical equilibrium pore geometry although very small effective grain size is necessary an abrupt transition from high permeability to very low permeability is predicted at a several percent porosity in agreement with the modeled porosity such pore geometry with low permeability may be restricted to temperatures above about 400 c i e in greenschist to amphibolite facies conditions where grains will deform to equilibrium geometry and cracks are annealed the region above the reflective and conductive band is estimated to be within blueschist conditions consistent with the inferred high seismic velocity from refraction data and high density from gravity data

gravity anomalies crustal structure and seismicity at subduction zones 2 interrelationships between fore arc structure and seismogenic behavior an ensemble averaging technique is used to remove the long wavelength topography and gravity field associated with subduction zones short wavelength residual anomalies are attributed to the tectonic structure of subducting and overthrusting plates a paired positive negative fore arc anomaly is observed consisting of a long 1000 km linear trench parallel ridge landward of the deep sea terrace basin ridges have amplitudes of 1500 3000 m and 160 240 mgal wavelengths of 150 200 km and high gravity anomaly to topography ratios 50 75 mgal km 1 the ridge crests correlate with the downdip limit of coseismic slip and strong interplate coupling and in cascadia the updip limit of tremor epicenters the ridge crest may be interpreted as defining the boundary between the velocity weakening and seismogenic region of the subduction interface and the downdip frictional transition zone in tonga kermadec the kuril islands and chile landward ridges are associated with extinct volcanic arcs paired anomalies are attributed to the preferential subduction erosion of the outer fore arc and a spatially varying combination of a lower crustal underplating beneath the inner fore arc b the transformation of interseismic strain into permanent geologic strain via faulting folding or buckling of the inner fore arc and c the relative trenchward migration of extinct volcanic arcs in regions operating with a net crustal deficit along strike transitions in fore arc morphology and seismogenic behavior are related to preexisting crustal structure of subducting and overthrusting plates fore arcs have the added potential of recording the time integrated response of the upper plate to subduction processes and fore arc structure should be considered in tandem with seismological observations

dating of recent lake sediments in the united kingdom and ireland using spheroidal carbonaceous particle scp concentration profiles spheroidal carbonaceous particles scps produced from fossil fuel combustion are found in lake sediments and provide a historical record of atmospheric pollutant deposition the sediment record of these particles is remarkably consistent and the main features are now seen throughout europe and beyond once the scp record from a region has been established using accurately dated cores scp profiles can be used to determine dates at other sites in the region this technique was developed in sweden brit until recently has had limited use in the uk and ireland as the most useful dating feature the subsurface peak was not present in all cores in recent cores this can now almost always be observed dating features for uk and irish cores are found to differ slightly between regions the potential exists for extra dates to be allocated to a profile by using characterisation of scp surfaces to determine changes in fuel type use dates could then be allocated to these changes using documentary sources to derive combustion histories in future years scp dating will become increasingly valuable especially for mid to late nineteenth century sediments as continual decay renders 210 pb progressively less useful for this time interval

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses