southern california permanent gps geodetic array continuous measurements of regional crustal deformation between the 1992 landers and 1994 northridge earthquakes the southern california permanent gps geodetic array pgga was established in 1990 across the pacific north america plate boundary to continuously monitor crustal deformation we describe the development of the array and the time series of daily positions estimated for its first 10 sites in the 19 month period between the june 28 1992 mw 7 3 landers and january 17 1994 mw 6 7 northridge earthquakes we compare displacement rates at four site locations with those reported by feigl et al 1993 which were derived from an independent set of global positioning system gps and very long baseline interferometry vlbi measurements collected over nearly a decade prior to the landers earthquake the velocity differences for three sites 65 100 km from the earthquake s epicenter are of order of 3 5 mm yr and are systematically coupled with the corresponding directions of coseismic displacement the fourth site 300 km from the epicenter shows no significant velocity difference these observations suggest large scale postseismic deformation with a relaxation time of at least 800 days the statistical significance of our observations is complicated by our incomplete knowledge of the noise properties of the two data sets two possible noise models fit the pgga data equally well as described in the companion paper by zhang et al this issue the pre landers data are too sparse and heterogeneous to derive a reliable noise model under a fractal white noise model for the pgga data we find that the velocity differences for all three sites are statistically different at the 99 significance level a white noise plus flicker noise model results in significance levels of only 94 43 and 88 additional investigations of the pre landers data and analysis of longer spans of pgga data could have an important effect on the significance of these results and will be addressed in future work

the winona basin structure and tectonics a compilation of published and new geophysical data from the winona basin off northern vancouver island has allowed a detailed interpretation of the sedimentary and tectonic history of the region to be made the basin is forming as a result of the asymmetric subsidence of a recently isolated lithospheric block that is slowly converging with the continental margin the crust beneath the basin is young 1 5 ma increasing in age from southeast to northwest and of normal oceanic thickness it is virtually non magnetic however probably because of its having been rapidly buried by turbidite sedimentation subsidence of the basin and uplift of the paul revere ridge began in the early pleistocene ca 1 8 ma and since that time up to 8 km of turbidite sediments has accumulated in the basin the nature of the fanning of the deposits suggests that the basin has been kept full throughout its history the minimum average supply rate necessary to accomplish this is about 70 106 mg year 1 this pleistocene ave

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

displacement of surface monuments horizontal motion measurements of tectonic deformation depend on both accurate instrumentation and adequate coupling of the apparatus to the earth s crust existing techniques capable of resolving the signals of interest strain rates of 10 n yr are mainly observatory based the limitation in the baselength of these instruments 1 km results in a requirement that the reference monuments be exceptionally stable 10 tzm yr however records from pition flat observatory california show that the actual horizontal displacements for massive near surface monuments eraplaced in competent weathered granite are of the order of 50 tzm yr the low noise level of the strain measurements at this site indicates that this magnitude of monument displacement is abnormally small until high accuracy geodetic techniques are developed sophisticated monuments or monument monitoring devices will be necessary to record faithfully continuous crustal deformation

current tectonics of northern cascadia from a decade of gps measurements 1 global positioning system gps measurements to study regional deformation were initiated in northern cascadia in the late 1980s and early 1990s on the basis of a decade of gps data we derive a crustal velocity field for nw washington sw british columbia the permanent and campaign gps velocities are defined with respect to north america in the itrf2000 reference frame velocity uncertainties are estimated using a model of time series noise spectra this new velocity field is the basis for interpretation of the tectonics of the northern cascadia subduction system gps velocities are interpreted in terms of interseismic loading of the megathrust using different coupling models our data confirm that the upper part of the megathrust is nearly fully locked an exponential model for the downdip transition zone gives slightly better agreement with the data compared to the common linear transition the landward decrease of forearc strain loading is smaller than predicted by any of the current subduction interseismic models this could be a consequence of a small 0 3 mm yr long term motion of the southern vancouver island forearc with respect to north america or of a concentration of interseismic strain across the elastically weaker cascadia volcanic arc in northern vancouver island our velocity field supports the existence of an independent explorer microplate currently underthrusting underneath north america at least up to brooks peninsula further north gps velocities indicate transient and or permanent deformation of northernmost vancouver island related to the interaction with the explorer microplate and possibly with the queen charlotte transform margin

extent and duration of the 2003 cascadia slow earthquake 1 inversion of continuous gps measurements from the pacific northwest show the 2003 cascadia slow earthquake to be among the largest of ten transients recognized here twelve stations bracketing slow slip indicate transient slip propagated bi directionally from initiation in the southern puget basin reaching 300 km along strike over a period of seven weeks this event produced for the first time resolvable vertical subsidence and horizontal displacement reaching six mm in southern washington state inverted for non negative thrust slip a maximum of 3 8 cm of slip is inferred centered at 28 km depth near the sharp arch in the subducting juan de fuca plate nearly all slip lies shallower than 38 km inverted slip shows a total moment release equal to mw 6 6 and a high degree of spatial localization rather than near uniform slip this suggests rupture concentrated along asperities holds for slow earthquakes as well as conventional events

semipermanent gps spgps as a volcano monitoring tool rationale method and applications abstract semipermanent gps spgps is an alternative to conventional campaign or survey mode gps sgps and to continuous gps cgps that offers several advantages for monitoring ground deformation unlike cgps installations spgps stations can be deployed quickly in response to changing volcanic conditions or earthquake activity such as a swarm or aftershock sequence spgps networks can be more focused or more extensive than cgps installations because spgps equipment can be moved from station to station quickly to increase the total number of stations observed in a given time period spgps networks are less intrusive on the landscape than cgps installations which makes it easier to satisfy land use restrictions in ecologically sensitive areas spgps observations are preferred over sgps measurements because they provide better precision with only a modest increase in the amount of time equipment and personnel required in the field we describe three applications of the spgps method that demonstrate its utility and flexibility at the yellowstone caldera wyoming a 9 station spgps network serves to densify larger preexisting networks of cgps and sgps stations at the three sisters volcanic center oregon a 14 station spgps network complements an sgps network and extends the geographic coverage provided by 3 cgps stations permitted under wilderness land use restrictions in the basin and range province in northwest nevada a 6 station spgps network has been established in response to a prolonged earthquake swarm in an area with only sparse preexisting geodetic coverage at three sisters the estimated precision of station velocities based on annual 3 month summertime spgps occupations from 2009 to 2015 is approximately half that for nearby cgps stations conversely spgps derived station velocities are about twice as precise as those based on annual 1 week sgps measurements after 5 years of spgps observations at three sisters the precision of velocity determinations is estimated to be 0 5 mm yr in longitude 0 6 mm yr in latitude and 0 8 mm yr in height we conclude that an optimal approach to monitoring volcano deformation includes complementary cgps and spgps networks periodic insar observations and measurements from in situ borehole sensors such as tiltmeters or strainmeters this comprehensive approach provides the spatial and temporal detail necessary to adequately characterize a complex and evolving deformation pattern such information is essential to multi parameter models of magmatic or tectonic processes that can help to guide research efforts and also to inform hazards assessments and land use planning decisions

continuous gps monitoring of elastic strain in the northern cascadia subduction zone previous monitoring and modeling of crustal deformation across the northern cascadia margin at vancouver is has provided strong evidence that the subduction thrust fault is locked and may generate future great earthquakes the recent establishment of the western canada deformation array wcda a network of continuous gps trackers in southwestern british columbia provides a new tool for monitoring crustal strain and thereby helps to assess the earthquake hazard in this region 17 months of continuous data from the three longest running wcda sites penticton victoria holberg indicate 1 a 7 mm yr easterly motion of victoria relative to penticton victoria is located in the forearc at the southeastern end of vancouver is 230 km from trench axis whereas penticton is located behind the arc 520 km from the trench axis and assumed fixed on the stable north american plate 2 a 3 mm yr northwesterly motion and 10 mm yr uplift with a large uncertainty of holberg relative to penticton holberg is located on the northern most part of vancouver is 50 km from the trench and near the plate triple junction and 3 the presence of probably seasonal variations in the apparent relative positions of the two coastal sites with respect to penticton the landward motion of victoria agrees with the deformation predicted by elastic dislocation models of the interseismic period for a great subduction thrust earthquake as constrained by previous deformation data although not as well resolved the motion of holberg roughly parallel to the margin is significantly different and is inconsistent with simple subduction models the non linear variations in measured position are dominated by an annual period probably resulting from seasonal biases in the precise orbit estimates or in the modeling of tropospheric delays or tidal effects

improved ocean tide loading corrections for gravity and displacement canada and northern united states within 100 km of the north american coastline ocean tide loading effects are 1 order of magnitude larger than the present day precision of absolute gravity 1 gal and continuous differential global positioning system gps measurements 3 8 mm in relation to the precision of these measurements ocean loading effects on gravity tend to be larger than the effect of loading on gps measurements even at sites hundreds of kilometers from the coast ocean tide loading can affect superconducting gravimeters at a level of 1 order of magnitude higher than the precision with which tidal constituents amplitudes can be resolved 0 1 gal in order to reduce contamination of high precision geodetic measurements by ocean tide loading a numerical representation of the nearshore ocean tide load constituents m2 s2 n2 o1 k1 has been developed using available ocean tide numerical modeling results for canadian waters the nearshore ocean tides are represented by a grid of cells ranging in size from 0 125 to 0 5 in which the amplitude and phase lag of tidal constituents are constant and which are bounded in general by lines of constant latitude and longitude the detailed nearshore tidal representation was originally designed to supplement the global 1 1 model of schwiderski but computations have also been carried out using the fes95 2 global model ocean tide loading computations that include the more detailed coastal ocean tide representations yield 10 20 changes in tidal correction values a significant reduction in the variance of absolute gravity residuals is observed at vancouver island absolute gravity stations when the more detailed coastal model is used but a considerable unexplained residual remains at holberg at the northwestern tip of vancouver island computations based on the fes95 2 model yield slightly smaller gravity residuals at west coast sites but only at the measurement precision level ocean loading effects on daily gps positions in the western canada deformation array are adequately predicted by the schwiderski global ocean tide loading model data from the canadian superconducting gravimeter installation gatineau quebec for m2 and o1 were compared with ocean tide load modelling results for the schwiderski and fes95 2 global ocean tide models enhanced by the nearshore model with the enhancement the two models agree with each other to within the expected computational uncertainty of 0 1 gal for both m2 and o1 both models predict an ocean loading effect at o1 within 0 1 gal of the observed effect however the predicted loading effect for m2 is still 0 2 gal smaller than the observed effect which suggests that there is still room for improvement in semidiurnal ocean tide models for the east coast of north america the enhanced ocean tide loading models will become more important in the future when gravity and position are measured at subcentimeter precision over periods shorter than 24 hours

seismotectonics of the explorer region 1 the explorer region offshore western canada is a tectonically complex area surrounded by the pacific north america and juan de fuca plates existing tectonic models for the region differ fundamentally proposed plate configurations range from multiple independent plate fragments to an explorer plate now fused to north america along the continental margin and cut by pacific north america transform faults in the west we present new seismological data constraining the region s current tectonics we use three component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4 combined with 34 harvard centroid moment tensor solutions they represent the region s largest earthquake source parameter data set obtained by robust waveform modeling techniques in addition we perform joint epicenter determination to relocate larger earthquakes recorded since 1918 the source parameters and improved locations provide a consistent tectonic picture earthquake slip vector azimuths along the pacific plate boundary change smoothly and are significantly less northerly oriented than the pacific north america plate motion direction requiring an independent explorer plate the present day pacific explorer boundary is formed by transform faults subparallel to the revere dellwood wilson fault plate motion vectors indicate that the winona block is part of the explorer plate current explorer motion is more northerly than indicated by magnetic anomalies prior to 2 ma implying a recent change possibly coinciding with a northwestward ridge jump near explorer plate s northern end transferring the winona block from the pacific to the explorer plate in response to these plate motion changes the region north of the western sovanco fracture zone was assimilated into the pacific plate the region around the eastern sovanco fracture zone characterized by broadly distributed seismicity is composed of well defined sets of conjugate faults bounding rotating crustal blocks earthquake fault strikes agree with the dominant northwest southeast fault sets however the conjugate sets must be also active to fully accommodate present day explorer plate motion the sw portion of the strike slip nootka fault zone the explorer juan de fuca plate boundary is well defined by focused seismicity however its full extent under nootka island remains unresolved the explorer north america boundary shows sporadic low magnitude seismicity our explorer north america rotation pole predicts convergence varying from negligible at the boundary s northwest end to 2 cm yr at the se end this convergence can be accommodated either by subduction or by crustal thickening extending to the north american continent we favor subduction based on low deformation rates observed by onshore gps sites the present explorer plate system configuration is a result of stepwise reorientation of the explorer ridge system each step successively reducing the subduction rate relative to north america

plate motion at the ridge transform boundary of the south cleft segment of the juan de fuca ridge from gps acoustic data 1 we measure the present day plate velocity of the juan de fuca ridge 25 km off axis to be 63 6 3 6 mm a at s67 2 e 7 9 degrees 1 relative to the pacific plate pa this velocity was derived from gps acoustic gpsa measurements in 2000 2001 2002 and 2003 that observed the position of a seafloor array 44 43 n 130 03 w 2900 m depth with a repeatability of 4 6 mm three transient events at the juan de fuca ridge and blanco transform account for 10 of this motion in viscoelastic modeling suggesting that the observed gpsa pa velocity is due primarily to steady state plate dynamics subtracting the modeled transient motion gives a velocity of 57 3 3 9 mm a at s72 9 e 12 1 degrees 1 which is consistent at the 95 confidence level with the velocity calculated from the wilson 1993 0 0 78 ma euler pole therefore this site is interpreted to be in a region of continuous full rate plate motion a robust result of this study which holds with and without correcting for transient motions these results provide direct geodetic evidence that spreading occurs predominantly within 25 km of the axis at this intermediate spreading rate ridge previously reported geodetic monitoring across the 1 km wide axial valley from 1994 1999 and 2000 2003 shows no significant extension chadwell et al 1999 hildebrand et al 1999 chadwick and stapp 2002 w w chadwick personal communication 2006 and seismic monitoring shows no activity this suggests the crust between 0 5 and 25 km off axis accommodates 26 mm of aseismic deformation each year through some combination of near axis fault motion and elastic strain accumulation

strain accumulation in western washington the juan de fuca plate is subducted beneath the north american plate off the coast of washington at a rate of about 40 mm yr n68 e the average principal strain rates extension reckoned positive measured in northwestern washington are as follows olympic peninsula 25 km south of port angeles from 1982 through 1990 e 1 0 011 0 027 strain yr n31 e 6 6 and e 2 0 092 0 028 strain yr n59 w 6 6 and near seattle from 1972 through 1985 e 1 0 027 0 019 strain yr n22 e 6 4 and e 2 0 036 0 013 strain yr n68 w 6 4 both strain measurements are consistent with uniaxial contraction in the direction of plate convergence uplift rates inferred from tide gage recordings are about 4 mm yr on the pacific coast and near 0 mm yr farther inland near seattle these deformation rates are consistent with a model of the cascadia subduction zone in which the plate interface beneath the continental slope and outer continental shelf is locked but free to slip farther landward the limited downdip extent of the locked segment of the plate interface is consistent with a shallow depth 20 km of the isotherm 450 c that defines the brittle ductile transition small thrust events diagnostic of seismic subduction should then occur only offshore and at shallow depths the principal strain rates measured from 1972 through 1983 in the back arc region near richland washington are e 1 0 016 0 013 strain yr n03 w 34 and e 2 0 024 0 013 strain yr n87 e 34

precise relative locations for earthquakes in the northeast pacific region we report that double difference methods applied to cross correlation measured rayleigh wave time shifts are an effective tool to improve epicentroid locations and relative origin time shifts in remote regions we apply these methods to seismicity offshore of southwestern canada and the u s pacific northwest occurring along the boundaries of the pacific and juan de fuca including the explorer plate and gorda block plates the blanco mendocino revere dellwood nootka and sovanco fracture zones host the majority of this seismicity largely consisting of strike slip earthquakes the explorer juan de fuca and gorda spreading ridges join these fracture zones and host normal faulting earthquakes our results show that at least the moderate magnitude activity clusters along fault strike supporting suggestions of large variations in seismic coupling along oceanic transform faults our improved relative locations corroborate earlier interpretations of the internal deformation in the explorer and gorda plates north of the explorer plate improved locations support models that propose northern extension of the revere dellwood fault relocations also support interpretations that favor multiple parallel active faults along the blanco transform fault zone seismicity of the western half of the blanco appears more scattered and less collinear than the eastern half possibly related to fault maturity wemore use azimuthal variations in the rayleigh wave cross correlation amplitude to detect and model rupture directivity for a moderate size earthquake along the eastern blanco fault lastly the observations constrain the seismogenic zone geometry and suggest a relatively narrow seismogenic zone width of 2 to 4 km less

three dimensional dislocation model for great earthquakes of the cascadia subduction zone there have been no historical cascadia great subduction thrust earthquakes but there is good recent evidence that very large earthquakes have occurred in the past and that strain is building up toward a future great event geodetic measurements in the coastal region from northern california to southern british columbia show vertical and horizontal deformation as expected for the strain accumulation of a locked thrust fault the segment of the subduction thrust that is locked and may rupture in future great events has previously been estimated through two dimensional 2 d elastic dislocation modeling of interseismic deformation geodetic data in this study a general 3 d dislocation model for thrust faults has been developed that accommodates curved fault geometry and nonuniform interseismic locking or coseismic rupture the model is based on die surface deformation due to shear faulting in an elastic half space the 3 d model of the cascadia subduction zone calculates the surface deformation for a locked zone or a rupture zone of variable width along the margin the bend in the margin trend and subducting slab end effects are included there is a downdip transition zone between interseismic completely locked and free slip portions of the fault or between coseismic full rupture and no displacement an initial 3 d model based upon 2 d dislocation models and upon thermal constraints was adjusted to optimize the fit of the predicted interseismic surface deformation to current deformation geodetic data the best fit model has the thrust locked along the whole margin with an average locked zone width of 60 km and a transition zone width of 60 km the two zones lie mainly offshore beneath the continental shelf and slope the locked and transition zone widths vary smoothly along the margin being greater off northern washington where the thrust dip is shallow and narrower off central oregon assuming that the locked plus transition zones approximate the maximum coseismic rupture area these widths permit amw 9 earthquake

current deformation and the width of the seismogenic zone of the northern cascadia subduction thrust evidence has been obtained for the accumulation of elastic strain across the northern cascadia subduction zone that may be released in a future very large subduction thrust earthquake vertical and horizontal strain rates across the southern vancouver island region have been determined through 1 long term trends in tide gauge data 2 changes in repeated accurate leveling surveys 3 changes in repeated high accuracy gravity profiles and 4 horizontal shortening observed in repeated precise positioning surveys the outer coast is uplifting at a rate of a few millimeters per year decreasing landward and shortening is occurring across the 100 km wide coastal region at a rate of about 0 1 microstrain per year mm km 1yr 1 the results are compared with the distribution of strain accumulation predicted from elastic dislocation and viscoelastic models for a subduction thrust fault the location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern vancouver island and northern washington provided the locked portion is restricted to a 60 km wide band offshore beneath the continental shelf and slope this conclusion also results from modeling the coseismic subsidence on the outer coast of vancouver island about 300 years ago deduced from paleoseismicity data the unusually narrow downdip extent of the subduction thrust seismogenic zone that extends little if at all beneath the coast is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments the high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km the distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of vancouver and victoria from this source the narrow width may also limit the earthquake size however events of magnitude well over 8 are possible

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

relative sea level change inferred from intertidal sediments beneath marshes on vancouver island british columbia abstract relative sea level change at the time of and since the most recent great earthquake at the cascadia subduction zone is estimated from intertidal sediments at three marshes on western vancouver island british columbia we compare the elevation of the pre earthquake surface which is marked by a tsunami sand sheet with the modern depositional elevation range of the sediment type upon which the sand was deposited at a site south of the nootka fault zone which is the northern boundary of the subducting juan de fuca plate tidal mud overlies the pre earthquake marsh surface the stratigraphy at this site indicates 0 2 1 6 m of coseismic submergence and 1 1 m of subsequent emergence in contrast two sites to the north lack obvious stratigraphic evidence for coseismic land level change and record between 0 1 and 1 7 m of post earthquake submergence these results indicate a difference in tectonic environment across the nootka fault zone and suggest that plate boundary rupture during the last great cascadia earthquake probably did not extend north of central vancouver island

the cenozoic history of the coastal landscape of the southern cape province south africa a review abstract we discuss the latest viewpoints and the problems involved in reconstructing the cenozoic history of the southern cape coastal landscape based on planation levels evidence of sea level fluctuations plateau clays and fluvial deposits silcretes and coastal sequences limestones lignites and cover sands we propose a tentative chronology of the cenozoic history of the southern cape controversy still surrounds the development of the coastal platform there being two schools of thought the upper erosion surface is an early to mid tertiary peneplain preserved by a ferricrete and silcrete cuirasse surficial boulder beds gravels sands and clays attest to extensive fluvial action on the proto coastal forelands a combination of residual and transported material seems the best explanation of the origins of the weathered clays on the coastal platform the recent pleistocene to holocene sand cordons onshore and offshore in the wilderness embayment exhibit a regular spatial and temporal pattern together with the older sands covering the coastal plateau six phases of development have been identified the interpretation of the knysna formation of sands clays and lignites is still uncertain the lignite beds are probably the key to understanding the stratigraphy of the whole region

thermobaric structure of the franciscan complex in the pacheco pass region diablo range california examination of the thermobaric structure of jadeite glaucophane bearing franciscan rocks in the pacheco pass region of the diablo range show that they are subhorizontal and the structurally intermediate horizon has the highest metamorphic grade mineral parageneses of more than 700 metagraywackes plus 38 metagraywacke samples from up to 1200 vertical feet of drill core were examined modal compositions of about 100 metagraywackes were determined and more than 2600 microprobe analyses were completed to quantify compositions of jadeitic pyroxene from about 80 metagraywackes the jadeite jd zone is defined by the assemblage jadeitic clinopyroxene quartz pm albite lawsonite pm sodic amphibole pm aragonite this zone occurs at an intermediate structural position sandwiched between subhorizontal jadeitic clinopyroxene free albite ab zones transition zones occur both above and below the jd zone the jd zone covers an area 20 km wide and 400 m thick the nearly flat lying boundaries betwee

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