the cowichan fold and thrust system vancouver island southwestern british columbia the cowichan fold and thrust system is a southwest verging linked thrust system formed by northeast southwest large scale crustal contraction involving the upper cretaceous nanaimo group and its wrangellian basement the geometry of the thrust system from plan and profile perspectives illustrates its thick skinned structural style the thrust system is interpreted to be a leading imbricate fan thrusting was most likely in sequence creating an estimated minimum 20 30 shortening at the basement cover interface both fault propagation and fault bend folding are evident the former being more common kinematic indicators and the geometry of the thrust system show no evidence of significant transpressional or transtensional displacement fields contraction postdates deposition of the nanaimo group and is indirectly dated as late eocene the sole fault of the thrust system is interpreted to rise from northwest to southeast on a series of deep lateral to oblique ramps and merge with the san juan fault system this explains the rapidly westward thickening wedge of thrusted wrangellian basement in the study area in the east the sole fault is interpreted to have locally permitted the nanaimo group to overthrust the san juan terrane investigation of the cowichan fold and thrust system reveals how shortening during westward progressing terrane accretion is accommodated in a forearc region dominated by a large slab of rigid semicontinental crust

upper crustal structure in puget lowland washington results from the 1998 seismic hazards investigation in puget sound a new three dimensional 3 d model shows seismic velocities beneath the puget lowland to a depth of 11 kin the model is based on a tomographic inversion of nearly one million first arrival travel times recorded during the 1998 seismic hazards investigation in puget sound ships allowing higher resolution mapping of subsurface structures than previously possible the model allows us to refine the subsurface geometry of previously proposed faults e g seattle hood canal southern whidbey island and devils mountain fault zones as well as to identify structures tacoma lofall and sequim fault zones that warrant additional study the largest and most important of these newly identified structures lies along the northern boundary of the tacoma basin we informally refer to this structure here as the tacoma fault zone although tomography cannot provide information on the recency of motion on any structure holocene earthquake activity on the tacoma fault zone is suggested by seismicity along it and paleoseismic evidence for abrupt uplift of tidal marsh deposits to its north the tomography reveals four large west to northwest trending low velocity basins tacoma seattle everett and port townsend separated by regions of higher velocity ridges that are coincident with fault bounded uplifts of eocene crescent formation basalt and pre tertiary basement the shapes of the basins and uplifts are similar to those observed in gravity data gravity anomalies calculated from the 3 d tomography model are in close agreement with the observed anomalies in velocity cross sections the tacoma and seattle basins are asymmetric the basin floor dips gently toward a steep boundary with the adjacent high velocity uplift locally with a velocity overhang that suggests a basin vetgent thrust fault boundary crustal fault zones grow from minor folds into much larger structures along strike inferred structural relief across the tacoma fault zone increases by several kilometers westward along the fault zone to lynch cove where we interpret it as a zone of south vergent faulting overthrusting tacoma basin in contrast structural relief along the seattle fault zone decreases west of seattle which we interpret as evidence that the n s directed compression is being accommodated by slip transfer between the seattle and tacoma fault zones together the tacoma and seattle fault zones raise the seattle uplift one of a series of east west trending pop up structures underlying puget lowland from the black hills to the san juan islands

an insight into crack density saturation rate and porosity model of the 2001 bhuj earthquake in the stable continental region of western india abstract the 2001 bhuj earthquake mw 7 6 source zone is examined in the light of crack density e saturation rate and porosity parameter using new data set derived from a large aftershock sequence recorded by the gujarat seismic network gsnet during november 2006 december 2009 processes of rupture initiations of the mainshock and its aftershock sequence are better understood by synthesizing the dynamic snapshots of the source zone using the new dataset pattern of crustal heterogeneities associated with high e high and high anomalies at depths varying from 20 km to 25 km is similar to those of earlier study by mishra and zhao 2003 the anomalous zone is found extended distinctly by 50 60 km in the lateral direction indicating the reinforcement of cracks and fractured volume of rock matrix due to long aftershock sequence since 2001 bhuj earthquake in the source area it is inferred that the presence of a fluid filled fractured rock matrix with super saturation may have affected the structural and seismogenic strengths of the source zone and is still contributing significantly to the geneses of earthquakes in and around the source zone anomalous pattern of high e with wider distribution of high indicates the existence of micro cracks in the lower crust while high suggests the cementation of cracks through permeation of residual magma metamorphic fluids into the hypocenter zone the results suggest that the existence of residual fluids in the fractured rock matrix in the mid to lower crust might have played a key role in triggering the 2001 mainshock and is still responsible for its continued long aftershock sequences

upper crustal structure beneath the strait of georgia southwest british columbia summary we present a new three dimensional 3 d p wave velocity model for the upper crustal structure beneath the strait of georgia southwestern british columbia based on non linear tomographic inversion of wide angle seismic refraction data our study part of the georgia basin geohazards initiative gbgi is primarily aimed at mapping the depth of the cenozoic sedimentary basin and delineating the near surface crustal faults associated with recent seismic activities e g m 4 6 in 1997 and m 5 0 in 1975 in the region joint inversion of firstarrival traveltimes from the 1998 seismic hazards investigation in puget sound ships and the 2002 georgia basin experiment provides a high resolution velocity model of the subsurface to a depth of 7 km in the southcentral georgia basin sedimentary rocks of the cretaceous nanaimo group and early tertiary rocks have seismic velocities between 3 0 and 5 5 km s 1 the basin thickness increases from north to south with a maximum thickness of 7 1 km depth to velocities of 5 5 km s 1 at the southeast end of the strait the underlying basement rocks probably representing the wrangellia terrane have velocities of 5 5 6 5 km s 1 with considerable lateral variation our tomographic model reveals that the strait of georgia is underlain by a fault bounded block within the central georgia basin it also shows a correlation between microearthquakes and areas of rapid change in basin thickness the 1997 1975 earthquakes are located near a northeast trending hinge line where the thicknesses of sedimentary rocks increase rapidly to the southeast given its association with instrumentally recorded moderate sized earthquakes we infer that the hinge region is cored by an active fault that we informally name the gabriola island fault a northwest trending southwest dipping velocity discontinuity along the eastern side of vancouver island correlates spatially with the surface expression of the outer island fault the outer island fault as mapped in our seismic tomography model is a thrust fault that projects directly into the lummi island fault suggesting that they are related structures forming a fault system that is continuous for nearly 90 km together these inferred thrust faults may account for at least a portion of the basement uplift at the san juan islands

the 2001 adana landslide and its destructive effects turkey the current study deals with a catastrophic landslide that occurred due to heavy rainfall in adana southern turkey in 2001 the research area which is one of the largest populated districts in adana and one of the most tropical places between the middle east and southern europe has about 25 000 people on the basis of geological and geotechnical surveys the landslide phenomena and the effects of landslide are examined to understand the slide mechanism of the landslide ground reconnaissance laboratory and in situ tests are conducted it is observed that the clayey and silty layers of the site are heavily saturated due to extreme rainfall in winter and consequently sheared the landslide consists of a block sliding in the upper portions and a debris flow soil flow component around the margins of the sliding blocks in the middle parts and at the toe although there was no loss of human life it has caused significant economic losses besides it is determined that if precautions are not taken around the landslide area larger landslides could definitely occur in the future

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

crustal velocity structure of the southern nechako basin british columbia from wide angle seismic traveltime inversion1this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia in the batholithsonland seismic project a refraction wide angle reflection survey was shot in 2009 across the coast mountains and interior plateau of central british columbia part of the seismic profile crossed the nechako basin a jurassic cretaceous basin with potential for hydrocarbons within sedimentary strata that underlies widespread volcanic rocks along this 205 km long line segment eight large explosive shots were fired into 980 seismometers forward and inverse modelling of the traveltime data were conducted with two independent methods ray tracing based modelling of first and secondary arrivals and a higher resolution wavefront based first arrival seismic tomography material with velocities less than 5 0 km s is interpreted as sedimentary rocks of the nechako basin while velocities from 5 0 6 0 km s may correspond to interlayered sedimentary and volcanic rocks the greatest thickness of sedimentary rocks in the basin is found in the central 110 km of the profile two sub basins were id

crustal structure of the colorado plateau arizona application of new long offset seismic data analysis techniques the colorado plateau is a large crustal block in the southwestern united states that has been raised intact nearly 2 km above sea level since cretaceous marine sediments were deposited on its surface controversy exists concerning the thickness of the plateau crust and the source of its buoyancy interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick a thick crust could support the observed topography of the colorado plateau isostatically while a thinner crust would indicate the presence of an underlying low density mantle this paper reports results on long offset seismic data collected during the 1989 segment of the u s geological survey pacific to arizona crustal experiment that extended from the transition zone into the colorado plateau in northwest arizona we apply two new methods to analyze long offset data that employ finite difference travel time calculations 1 a first arrival time inverter to find upper crustal velocity structure and 2 a forward modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals we find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern basin and range province to about 42 km beneath the northern transition zone and southern colorado plateau margin we observe some crustal thinning to 37 km thick and slightly higher lower crustal velocities farther inboard beneath the kaibab uplift on the north rim of the grand canyon the crust thickens to a maximum of 48 km we observe a nonuniform crustal thickness beneath the colorado plateau that varies by 15 and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations crustal compositions as inferred from seismic velocities appear to be the same beneath the colorado plateau as those in the basin and range province to the southwest implying that the plateau crust represents an unextended version of the basin and range some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the proterozoic era

basin shear wave velocities beneath seattle washington from noise correlation rayleigh waves tomography with short period rayleigh waves extracted using noise interferometry can refine s wave velocity v s models in urban areas with dense arrays of short period and broadband instruments we apply this technique to the seattle area to develop a new shallow v s model for use in seismic hazard assessment continuous data from the seismic hazards in puget sound ships array and local broadband stations have interstation distances of 90 km or less this spacing allows us to extract rayleigh waves with periods between 2 and 10 s that are sensitive to shallow basin structure this new v s model for the seattle basin is constructed using direct observations rather than using p wave velocity v p observations and a v p v s ratio as all previous 3d models at this scale have been constructed our results reveal greater detail in the upper 3 5 km than previous models earthquake simulations calculated using our new model better predict peak ground velocities pgv at periods between 1 and 2 s for two local earthquakes than the previous model used to calculate seattle s seismic hazard map frankel et al 2007 we collected data from two local earthquakes and ran finite difference simulations using our new velocity model as well as the previous velocity model used in development of the seattle seismic hazard maps to assess how well our model predicts ground motions relative to the previous model with a recent deployment of netquakes strong motion stations by the pacific northwest seismic network pnsn and the u s geological survey we are now able to make more comprehensive assessments of the predictions for recent events

spatio temporal analysis of a complex landslide in the northern apennines italy by means of dendrochronology dendrochronology is used to determine the spatial and temporal development of movements in a complex landslide located in the northern apennines the analysis of trees growing on the landslide body enabled us to date their colonisation pattern which starts from the 1920s and increases mainly after 1950 since then sampled trees recorded with growth anomalies the movements involving the landslide body in 1956 1957 1982 1983 1990 1995 and 1998 these movements are reactivations of the main landslide that probably occurred in the last decades of the 19th century in 1980 trees on the upper landslide body were involved in a rock fall due to the expansion of the main scarp microsections cut out from selected tree ring sequences allowed seasonal dating of the highlighted movements which mostly occurred in early spring meteorological and seismic time series were analyzed in order to relate them with reactivations and to search for correlations and triggering events the seismic activity although characterized by low magnitudes could be able either to trigger the movements 1990 or to destabilize the ground where rainfalls could have been playing the first role in triggering the reactivation probably in 1982 1983 and 1995 the other reactivations seem to be linked either to long lasting rainfalls or to monthly rainfall peaks mostly following dry periods

transient modeling of regional rainfall triggered shallow landslides this study investigates the transient modeling of regional rainfall triggered shallow landslides in unsaturated soil using the richards equation to model shallow landslides within a distributed regional scale framework infinite slope stability analysis coupled with the hydrological model with consideration of the fluctuation of time dependent pore water pressure and the soil water characteristic curve proposed by van genuchten was developed the validity of the proposed model is established through several test problems by comparing the numerical results with the analytical solutions a new procedure to set up wide range shallow landslide analysis and to integrate regional distribution variations for input data such as geology groundwater level hydrogeological characteristics and rainfall intensity and duration was presented the results obtained demonstrate that the computed distribution of the safety factor is consistent with the distribution of actual landslides in addition the fluctuation of pore water pressure in unsaturated soil dominates the stability of landslides during typhoons accompanied by heavy rainfall the findings observed in this study are a fundamental contribution to environmental effects for landslides in areas with higher occurrence and vulnerability to extreme precipitation

regional p wave velocity structure of the northern cascadia subduction zone 1 this paper presents the first regional three dimensional p wave velocity model for the northern cascadia subduction zone sw british columbia and nw washington state constructed through tomographic inversion of first arrival traveltime data from active source experiments together with earthquake traveltime data recorded at permanent stations the velocity model images the structure of the subducting juan de fuca plate megathrust and the fore arc crust and upper mantle beneath southern vancouver island the megathrust above the juan de fuca plate is characterized by a broad zone 25 35 km depth having relatively low velocities of 6 4 6 6 km s this relative low velocity zone coincides with the location of most of the episodic tremors recently mapped beneath vancouver island and its low velocity may also partially reflect the presence of trapped fluids and sheared lower crustal rocks the rocks of the olympic subduction complex are inferred to deform aseismically as evidenced by the lack of earthquakes within the low velocity rocks the fore arc upper mantle beneath the strait of georgia and puget sound is characterized by velocities of 7 2 7 6 km s such low velocities represent regional serpentinization of the upper fore arc mantle and provide evidence for slab dewatering and densification tertiary sedimentary basins in the strait of georgia and puget lowland imaged by the velocity model lie above the inferred region of slab dewatering and densification and may therefore partly result from a higher rate of slab sinking in contrast sedimentary basins in the strait of juan de fuca lie in a synclinal depression in the crescent terrane the correlation of in slab earthquake hypocenters m 4 with p wave velocities greater than 7 8 km s at the hypocenters suggests that they originate near the oceanic moho of the subducting juan de fuca plate

pn velocities beneath continental collision zones the turkish iranian plateau summary tomographic imaging of the uppermost mantle velocity in the middle east shows normal pn velocities 8 0 8 2 kms beneath the black sea and the southern caspian sea in contrast low pn velocities 7 9 km s i are found beneath the aegean sea and much of the turkish iranian plateau a region of exceptionally low pn velocities c7 6 km s beneath the lesser caucasus mountains and along the turkish iranian border coincides with a region of high sn attenuation and extensive neogene volcanism all these features suggest that near solidus conditions exist within the uppermost mantle beneath the turkish iranian plateau such conditions may result either from the decompression melting associated with upwelling convection cells or by the infiltration of water released from subducted lithosphere into the mantle above the presence of a partially melted uppermost mantle weakens the lithosphere beneath the turkish iranian plateau thus allowing it to become the locus of deformation in the arabian eurasian collision zone

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

three dimensional crustal structure of the mariana island arc from seismic tomography 1 a three dimensional 3 d seismic refraction survey was acquired over the mariana volcanic arc at 14 5 18 5n and 145 147e first arrival traveltimes from this survey and from a separate 2 d survey acquired approximately perpendicular to the arc have been simultaneously inverted for a 3 d p wave velocity model using seismic tomography subject to smoothness constraints the active arc which initiated only 3 4 ma ago has an average crustal thickness of 18 km approximately 40 km to the east the inactive remnant of the rifted eocene arc has an average crustal thickness of 21 km due primarily to a thicker lower crustal layer with velocities of 6 5 7 0 km s 1 crustal production clearly varies both temporally and spatially with some crustal layers including the igneous forearc crust varying in thickness by a factor of up to 2 along strike average p wave velocities within the upper crust of the modern arc are 240 360 m s 1 lower than in the eocene arc but are 280 m s 1 higher within the lower crust middle crust with velocities of 6 0 6 5 km s 1 is best developed beneath the eocene arc these results suggest an evolution of arc structure with increasing age we infer closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the middle to lower crust as a result of intracrustal differentiation although tonalitic rocks may predominate in the transition from upper to middle crust the bulk of the crust is essentially basaltic

upper crustal structure of southwestern british columbia from the 1998 seismic hazards investigation in puget sound 1 this paper applies nonlinear three dimensional travel time tomography to refraction data recorded during the 1998 seismic hazards investigation in puget sound ships to derive the first large scale high resolution upper crustal velocity model for southwestern british columbia a minimum structure p wave velocity model is constructed using 175 000 first arrival travel times picked from data recorded by 58 temporary onshore stations the model details forearc crustal structures related to terrane accretion and subsequent basin formation to a depth of about 10 km the metchosin igneous complex correlative with the eocene crescent siletz terrane in washington is imaged as a laterally extensive wnw trending high velocity anomaly underlying southernmost vancouver island and much of the strait of juan du fuca northeast of the strait of georgia the southwesterly dip of the contact between the wrangellia terrane rocks of vancouver island and the coast plutonic complex suggests wrangellia rocks are down faulted against the plutonic complex at the southwestern end of the strait of juan de fuca the 50 km long wnw trending clallam basin has a maximum thickness of 5 6 km near the eastern end of the strait of juan de fuca port townsend basin has an inferred thickness of approximately 4 5 km the southern end of the 9 km thick georgia basin is bounded by a high velocity basement ridge beneath the strait of georgia clusters of well located earthquakes have a prominent nw trend and coincide spatially with rapid lateral velocity changes clusters of microearthquakes there are associated with the intersection of several east trending structural highs within this nw trend

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

p wave velocity structure of the earth s crust beneath vancouver island refraction data were recorded from three shot points out to a maximum distance of 330 km as part of the 1980 vancouver island seismic project visp80 these vertical component data are partially reversed and so can be interpreted in terms of two dimensional structures by iterative modeling of p wave travel times and amplitudes the structure of the upper crust is the best constrained part of the model it consists generally of a gradually increasing velocity from 5 3 km s at the surface to 6 4 km s at 2 km depth to 6 75 km s at 15 5 km depth where the velocity increases sharply to 7 km s below 20 km depth the model becomes speculative because the data provide only indirect constraints on velocities at these depths an interpretation that fits the observed times and amplitudes has a low velocity zone in the lower crust and a moho at 37 km depth the only significant departure from this general structure is beneath the central part of vancouver island where the 15 5 km boundary in the model atta

tomographic image of the southern california mantle p wave teleseismic travel time delays recorded by the southern california array are inverted by the method of backprojection tomography to obtain images of variations in the p wave velocity structure to a depth of 750 km two major upper mantle features arc resolved one beneath the transverse ranges region and another beneath the salton trough region the transverse ranges feature appears as a curtainlike east trending high velocity anomaly this feature is 60 km thick extends most deeply on its eastern end to 250 km and attains a maximum velocity 3 greater than average southern california mantle of the same depth the salton trough feature which is not as well resolved as the transverse ranges feature is composed of low velocities in the upper 70 100 km these p wave velocities arc depressed 3 4 compared to average southern california mantle tests of the inversion indicate the major aspects of the imaged structure are authentic

tectonic deformation in western washington from continuous gps measurements the analysis of 3 years of continuous data from 7 permanent gps stations along the western washington section of the cascadia subduction zone indicates that the direction of the observed horizontal velocities with respect to station drao nominally representing the stable north american continent is roughly parallel to the relative plate convergence direction of the juan de fuca and north america plates and that their magnitude decreases away from the trench most of this deformation can be attributed to inter seismic strain accumulation due to the locking of the thrust interface when the dislocation model predictions are subtracted from the observed velocities there is evidence for an additional n s oriented contraction at a rate of 4 mm yr over a distance of 250 km this signal presumably represents a more long term deformation pattern than the periodic accumulation and release of elastic strain connected with subduction earthquakes and is most likely related to the occurrence of shallow earthquakes in western washington that are characterized by predominantly n s oriented maximum principal stress

forearc structure beneath southwestern british columbia a three dimensional tomographic velocity model 1 this paper presents a three dimensional compressional wave velocity model of the forearc crust and upper mantle and the subducting juan de fuca plate beneath southwestern british columbia and the adjoining straits of georgia and juan de fuca the velocity model was constructed through joint tomographic inversion of 50 000 first arrival times from earthquakes and active seismic sources wrangellia rocks of the accreted paleozoic and mesozoic island arc assemblage underlying southern vancouver island in the cascadia forearc are imaged at some locations with higher than average lower crustal velocities of 6 5 7 2 km s similar to observations at other island arc terranes the mafic eocene crescent terrane thrust landward beneath southern vancouver island exhibits crustal velocities in the range of 6 0 6 7 km s and is inferred to extend to a depth of more than 20 km the cenozoic olympic subduction complex an accretionary prism thrust beneath the crescent terrane in the olympic peninsula is imaged as a low velocity wedge to depths of at least 20 km three zones with velocities of 7 0 7 5 km s inferred to be mafic and or ultramafic units lie above the subducting juan de fuca plate at depths of 25 35 km the forearc upper mantle wedge beneath southeastern vancouver island and the strait of georgia exhibits low velocities of 7 2 7 5 km s inferred to correspond to 20 serpentinization of mantle peridotites and consistent with similar observations in other warm subduction zones estimated dip of the juan de fuca plate beneath southern vancouver island is 11 16 and 27 at depths of 30 40 and 50 km respectively

identifying active structures using double difference earthquake relocations in southwest british columbia and the san juan islands washington this paper applies double difference earthquake relocation techniques to investigate sources of seismicity in southwest british columbia canada and the san juan islands washington the study area is a complex region of deformation and has the potential for large earthquakes in the north americancrust double difference earthquake relocation techniques are applied to identify otherwise hidden active struc tures that may pose a hazard to nearby population and infrastructure we present evi dence for previously unrecognized active structures using precise relative earthquake relocations obtained using both catalog and waveform cross correlation data results have significantly reduced errors over routine catalog locations and show lineations in areas of clustered seismicity in southwest british columbia these lineations or streaks appear to be hidden structures that do not disrupt near surface sediments however in thesanjuanislandstheidentifiedlineationcouldberelatedtorecentlymappedsurface expressions offaults identifiedfromseismic reflection andmultibeam bathymetric sur veys we use avariety of velocity models for therelocations and find that inappropriate models lead to artifacts at layer boundaries and increased vertical errors

three dimensional crustal velocity structure beneath the strait of georgia british columbia summary the strait of georgia is a topographic depression straddling the boundary between the insular and coast belts in southwestern british columbia two shallow earthquakes located within the strait m 4 6 in 1997 and m 5 0 in 1975 and felt throughout the vancouver area illustrate the seismic potential of this region as part of the 1998 seismic hazards investigation of puget sound ships experiment seismic instruments were placed in and around the strait of georgia to record shots from a marine source within the strait we apply a tomographic inversion procedure to first arrival traveltime data to derive a minimum structure 3 d p wave velocity model for the upper crust to about 13 km depth we also present a 2 d velocity model for a profile orientated across the strait of georgia derived using a minimum parameter traveltime inversion approach this paper represents the first detailed look at crustal velocity variations within the major cretaceous to cenozoic georgia basin which underlies the strait of georgia the 3 d velocity model clearly delineates the structure of the georgia basin taking the 6 km s 1 isovelocity contour to represent the top of the underlying basement the basin thickens from between 2 and 4 km in the northwestern half of the strait to between 8 and 9 km at the southeastern end of the study region basin velocities in the northeastern half are 4 5 6 km s 1 and primarily represent the upper cretaceous nanaimo group velocities to the south are lower 3 6 km s 1 because of the additional presence of the overlying tertiary huntingdon formation and more recent sediments including glacial and modern fraser river deposits in contrast to the relatively smoothly varying velocity structure of the basin velocities of the basement rocks which comprise primarily palaeozoic to jurassic rocks of the wrangellia terrane and possibly jurassic to mid cretaceous granitic rocks of the coast belt show significantly more structure probably an indication of the varying basement rock lithologies the 2 d velocity model more clearly reveals the velocity layering associated with the recent sediments huntingdon formation and nanaimo group of the southern georgia basin as well as the underlying basement we interpret lateral variation in sub basin velocities of the 2 d model as a transition from wrangellian to coast belt basement rocks the effect of the narrow onshore offshore recording geometry of the seismic experiment on model resolution was tested to allow a critical assessment of the validity of the 3 d velocity model lateral resolution throughout the model to a depth of 3 5 km below the top of the basement is generally 10 20 km

case for very low coupling stress on the cascadia ssubduction fault a fundamental problem in plate tectonics is the shear strength of major plate boundary faults this translates to the question whether the generally observed small earthquake stress drops of 3 10 mpa on major faults release most of the accumulated stress or only a small fraction of it there is strong evidence that the san andreas fault a major transform plate boundary is weak 20 mpa shear resistance it is not yet clear whether subduction thrust faults are also weak we present two types of evidence from the northern cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault comparable to that estimated for the san andreas fault first the well defined surface heat flow and heat generation allow negligible frictional heating on the plate interface the average shear stress on the fault must thus be very low over a time scale of a few million years second focal mechanism solutions for small crustal earthquakes in the southern vancouver island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress this inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low one explanation for the weakness of the fault is the presence of near lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence the conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked accumulating strain energy toward a future great earthquake the surface deformation responds to the small 20 mpa temporal changes of the stress field associated with the subduction earthquake cycle this transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin the weakness of the cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards for example a subduction earthquake may induce large strike slip earthquakes in the forearc that affect a large area

earthquake ground motion and 3d georgia basin amplification in southwest british columbia shallow blind thrust scenario earthquakes abstract finite difference modeling of 3d long period 2 s ground motions for large m w 6 8 scenario earthquakes is conducted to investigate the effects of the georgia basin structure on ground shaking in greater vancouver british columbia canada scenario earthquakes include shallow blind thrust north america na plate earthquakes simulated in locations congruent with linear clusters of shallow seismicity that is potential active faults a slip distribution model of the m w 6 7 northridge california blind thrust earthquake with the hypocenter modified to 5 km depth is used to characterize the source rupture process two sets of simulations are performed for a given scenario earthquake using models with and without georgia basin sediments the ratio of predicted peak ground velocity pgv for the two simulations is applied here as a quantitative measure of amplification due to 3d basin structure a total of eight shallow blind thrust na plate scenario earthquakes are simulated within 100 km of greater vancouver overall predicted ground motions are higher in the down dip direction of each epicenter due to the source radiation pattern hence scenario earthquakes located south of vancouver produce the highest motions in the city the average maximum pgv at stiff soil sites across greater vancouver considering all eight scenario earthquakes is 17 8 cm s modified mercalli intensity vii the average increase in peak motion due to the presence of georgia basin sediments is a factor of 4 1 the effective duration of moderate level 3 4 cm s shaking within greater vancouver is an average of 22 s longer when georgia basin sediments are included in the 3d structure model online material snapshots and videos of wave propagation and peak ground velocity maps

regional scale high plasticity clay bearing formation as controlling factor on landslides in southeast spain complex landslides in clay bearing sediments are investigated in two moderate relief regions of southeast spain both regions more than 100 km apart show landslides affecting the same flysch formation which outcrops widely in the central and western betic cordillera along the contact between the external south iberian domain and internal alboran domain zones intense rainfall episodes can be considered as the main triggering factor for slope failures in these two areas we have chosen two landslides diezma and riogordo landslides one from each area of study to investigate their morphological and geotechnical features in order to establish the relative importance of the different controlling factors from a kinematic point of view the two features studied in detail can be referred as to rotational failures evolving downhill to slow earthflows the movement was concentrated on several surfaces developed on a clay rich layer mostly constituted by smectite this clay mineral is of critical relevance to the mechanical behaviour of soils and flysch like formations being very consistent at dry conditions but rapidly losing its strength at wet conditions thus softened smectite rich clay layers with high water contents can approach the properties of a lubricant which in turn can be critical for slope stability in addition to their high plasticity these clays have a high swelling potential which can induce significant vertical overpressure thus reducing even more the strength properties of the flysch formation in southeast spain a region with a mediterranean rainfall regime slope stability can be seriously influenced by the presence of these smectite rich clay layers in a formation of regional extent as is the case of the flysch formation therefore this lithologically controlled factor should be taken into account when evaluating landslide hazard in the betic cordillera

mantle wedge structure in the miyagi prefecture forearc region central northeastern japan arc and its relation to corner flow pattern and interplate coupling 1 we applied three dimensional 3 d seismic tomography to both active source data and local earthquake data recorded at onshore and offshore seismic stations in the miyagi prefecture forearc region of central northeastern japan to obtain a detailed seismic velocity model of the mantle wedge the velocity model showed a clear boundary that divides the mantle wedge into an oceanward high velocity toe and a landward low velocity region that extends trenchward from beneath currently active arc volcanoes the top of the seaward dipping boundary is about 20 30 km landward from the coastline and its bottom is approximately beneath the coastline we consider that this velocity boundary represents the trenchward limit of corner flow and the high velocity toe represents a stagnant mantle wedge the high spatial resolution of the tomographic image we produced allowed us to identify a spatial correlation between velocity variations in the mantle wedge immediately above the plate boundary and the spatial extent of the rupture areas of the large interplate earthquakes that occur repeatedly in the region our results suggest that variations of the volume fraction of serpentinization in the mantle wedge cause the irregularity of rupture patterns of megathrust earthquake sequences we also identified a swarm of active microseismicity in the mantle wedge well above the seismogenic plate boundary this activity may be related to dehydration of a detached seamount

the role of rainfall in the landslide hazard the case of the avigliano urban area southern apennines italy abstract mass movements varying in type and size some of which are periodically reactivated affect the urban area of avigliano the disturbed and remoulded masses consist of sandy silty or silty clayey plastic material interbedded with stone fragments and conglomerate blocks five landslides that were markedly liable to rainfall associated instability phenomena were selected the relationships between landslides and rainfall were investigated using a hydrological and statistical model based on long term series of daily rainfall data the model was used to determine the return period of cumulative daily rainfall over 1 180 days the resulting hydrological and statistical findings are discussed with the aim of identifying the rainfall duration most critical to landslides the concept of a precipitation threshold was generalized by defining some probability classes of cumulative rainfall these classes indicate the thresholds beyond which reactivation is likely to occur the probability classes are defined according to the return period of the cumulative rainfall concomitant with landslide reactivation

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

crustal structure and relocated earthquakes in the puget lowland washington from high resolution seismic tomography 1 the availability of regional earthquake data from the pacific northwest seismograph network pnsn together with active source data from the seismic hazards investigation in puget sound ships seismic experiments has allowed us to construct a new high resolution 3 d p wave velocity model of the crust to a depth of about 30 km in the central puget lowland in our method earthquake hypocenters and velocity model are jointly coupled in a fully nonlinear tomographic inversion active source data constrain the upper 10 15 km of the model and earthquakes constrain the deepest portion of the model a number of sedimentary basins are imaged including the previously unrecognized muckleshoot basin and the previously incompletely defined possession and sequim basins various features of the shallow crust are imaged in detail and their structural transitions to the mid and lower crust are revealed these include the tacoma basin and fault zone the seattle basin and fault zone the seattle and port ludlow velocity highs the port townsend basin the kingston arch and the crescent basement which is arched beneath the lowland from its surface exposure in the eastern olympics strong lateral velocity gradients consistent with the existence of previously inferred faults are observed bounding the southern port townsend basin the western edge of the seattle basin beneath dabob bay and portions of the port ludlow velocity high and the tacoma basin significant velocity gradients are not observed across the southern whidbey island fault the lofall fault or along most of the inferred location of the hood canal fault using improved earthquake locations resulting from our inversion we determined focal mechanisms for a number of the best recorded earthquakes in the data set revealing a complex pattern of deformation dominated by general arc parallel regional tectonic compression most earthquakes occur in the basement rocks inferred to be the lower tertiary crescent formation the sedimentary basins and the eastern part of the olympic subduction complex are largely devoid of earthquakes clear association of hypocenters and focal mechanisms with previously mapped or proposed faults is difficult however seismicity structure and focal mechanisms associated with the seattle fault zone suggest a possible high angle mode of deformation with the north side up we suggest that this deformation may be driven by isostatic readjustment of the seattle basin

the southern whidbey island fault an active structure in the puget lowland washington information from seismic reflection profiles outcrops boreholes and potential field surveys is used to interpret the structure and history of the southern whidbey island fault in the puget lowland of western washington this northwest trending fault comprises a broad as wide as 6 11 km steep northeast dipping zone that includes several splays with inferred strike slip reverse and thrust displacement transpressional deformation along the southern whidbey island fault is indicated by along strike variations in structural style and geometry positive flower structure local unconformities out of plane displacements and juxtaposition of correlative sedimentary units with different histories the southern whidbey island fault represents a segment of a boundary between two major crustal blocks the cascade block to the northeast is floored by diverse assemblages of pre tertiary rocks the coast range block to the southwest is floored by lower eocene marine basaltic rocks of the crescent formation the fault probably originated during the early eocene as a dextral strike slip fault along the eastern side of a continental margin rift bending of the fault and transpressional deformation began during the late middle eocene and continues to the present oblique convergence and clockwise rotation along the continental margin are the inferred driving forces for ongoing deformation evidence for quaternary movement on the southern whidbey island fault includes 1 offset and disrupted upper quaternary strata imaged on seismic reflection profiles 2 borehole data that suggests as much as 420 m of structural relief on the tertiary quaternary boundary in the fault zone 3 several meters of displacement along exposed faults in upper quaternary sediments 4 late quaternary folds with limb dips of as much as 9 5 large scale liquefaction features in upper quaternary sediments within the fault zone and 6 minor historical seismicity the southern whidbey island fault should be considered capable of generating large earthquakes m s 7 and represents a potential seismic hazard to residents of the puget lowland

crustal heterogeneity in bulk velocity beneath the 2001 bhuj earthquake source zone and its implications in order to resolve an enigmatic issue relating to the existence of fluid related or temperature related anomalies at the mainshock hypocenter of the 2001 bhuj india earthquake m w 7 6 an estimate of the 3d bulk sound velocity structure is made from the inverted high quality p and s arrival times from a total of 368 aftershocks recorded by 12 temporary seismic stations installed following the 2001 bhuj earthquake results reveal strong lateral and vertical heterogeneity in bulk velocity graphic 1 beneath the source zone the 2001 bhuj mainshock and its aftershock source zones are associated with anomalously high bulk sound velocity high graphic 2 indicating high bulk elastic strength of the source rocks at the mainshock hypocenter due to high pore pressures of the fully saturated cracked rocks associated with solute precipitation through the processes of acoustic fluidization and cementation the interpretation of bulk velocity tomograms suggests that the processes of mineral dehydration and permeation of sea surface water through several active quaternary faults down to the deep crust might have contributed to in situ fluid related material heterogeneity in bulk velocity within the fluid filled fractured rock matrix in the paleorift zone at the 2001 bhuj mainshock hypocenter which in turn increased pore pressure lowered the effective stress and brought the system into a brittle failure high graphic 3 in the intersecting fault geometry in the fractured rock matrix at the mainshock hypocenter can be taken as evidence for the strong role of fluids in association with the intraplate earthquake of the indian peninsula 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif

characterization of active faulting beneath the strait of georgia british columbia southwestern british columbia and northwestern washington state are subject to megathrust earthquakes deep intraslab events and earthquakes in the continental crust of the three types of earthquakes the most poorly understood are the crustal events despite a high level of seismicity there is no obvious correlation between the historical crustal earthquakes and the mapped surface faults of the region on 24 june 1997 a m l 4 6 earthquake occurred 3 4 km beneath the strait of georgia 30 km to the west of vancouver british columbia this well recorded earthquake was preceded by 11 days by a felt foreshock m l 3 4 and was followed by numerous small aftershocks this earthquake sequence occurred in one of the few regions of persistent shallow seismic activity in southwestern british columbia thus providing an ideal opportunity to attempt to characterize an active near surface fault we have computed focal mechanisms and utilized a waveform cross correlation and joint hypocentral determination routine to obtain accurate relative hypocenters of the mainshock foreshock and 53 small aftershocks in an attempt to image the active fault and the extent of rupture associated with this earthquake sequence both p nodal and cmt focal mechanisms show thrust faulting for the mainshock and the foreshock the relocated hypocenters delineate a north dipping plane at 2 4 km depth dipping at 53 in good agreement with the focal mechanism nodal plane dipping to the north at 47 the rupture area is estimated to be a 1 3 km diameter circular area comparable to that estimated using a brune rupture model with the estimated seismic moment of 3 17 10 15 n m and the stress drop of 45 bars the temporal sequence indicates a downdip migration of the seismicity along the fault plane the results of this study provide the first unambiguous evidence for the orientation and sense of motion for active faulting in the georgia strait area of british columbia

analysis of physical testing of rainfall induced soil slope failures rainfall is a significant factor that triggers slope failures around the world this paper reports a series of physical tests which were conducted to simulate rain induced slope failures the experiments dealt with two scenarios including 1 rainwater infiltration into the slope and 2 slope failures induced by artificial rainfall with different initial conditions slope deformation and slope failures were observed and possible mechanisms were interpreted based on the experimental results the results confirm the hypothesis that pore water pressure and water content in a loose soil slope change rapidly and that water infiltration into cracks in the slope has a great impact on landslide development the observed slope failures can be divided into three types overall sliding failure partial sliding failure and flow slide the effect of slope gradient rainfall intensity and distribution of initial suction on the slope deformation and failure process are also summarized for possible applications under the similar conditions

2d electrical resistivity imaging of some complex landslides in lucanian apennine chain southern italy we use high resolution electrical resistivity imaging to delineate the geometry of complex landslides in the lucanian apennine chain of southern italy to identify the discontinuity between the landslide material and bedrock and to locate possible surfaces of reactivation the lucanian apennine chain is characterized by high hydrogeological hazard and shows a complete panorama of mass movements in this area all typologies of landslides markedly predisposed and tightly controlled by the geostructural characteristics are found rotational and translational slides rototranslational slides earth and mudflows as well as deep seated gravitational slope phenomena with a predominance of rototranslational slides evolving as earthflow slides three test sites characterized by complex geology and a high hydrogeologic hazard are studied the giarrossa and varco izzo earthflow slides are located to the west and east of the town of potenza whereas the latronico slide is located close to the town of latronico

dynamics of a complex mass movement triggered by heavy rainfall a case study from nw turkey abstract following a period of heavy precipitation a large and complex mass movement namely the dagkoy landslide occurred in the west black sea region of turkey on may 21 1998 this paper describes the conditioning factors of the landslide and interprets the mass transport processes in terms of a movement scenario geology geomorphology and vegetation cover were considered as the conditioning factors of the failure observations showed that the gently sloping about 10 area is mostly covered by dense forest trees at the crown where the motion initiated significant intersection of the collapsed slope with dip of the local marls seems to have contributed to the formation and geometry of the landslide the distance from the crown down to the toe of the landslide measured more than 600 m with about 0 6 km 3 total earth material displaced the landslide has both a block sliding characteristics in the upper portions and a debris flow soil flow component around the margins of the sliding blocks in the middle parts and at the toe the proposed scenario for the landslide reveals that the movement was initiated near crown as a result of the excess water content in the marls at the end of 3 days of heavy rainfall the early perturbations transverse cracks ridges etc lasted for 6 7 h after which the central part of the zone started to move as a soil flow in which very large intact blocks were transported even though the movement was very rapid 1 2 m min there was no loss of life however the movement destroyed 38 houses one mosque and a considerable amount of farmland

crustal seismic velocity and density structure of the intermontane and coast belts southwestern cordillera seismic refraction wide angle reflection data were recorded along a 450 km profile across the intermontane coast and insular belts of the canadian cordillera crust and upper mantle structure was interpreted from traveltime inversion and forward amplitude modelling and the resultant seismic velocities were used to constrain modelling of the bouguer gravity data along the profile a high velocity high density block in the upper 8 km of crust was interpreted as the subsurface extension of harrison terrane the harrison fault at its eastern boundary may extend to at least 8 km depth and perhaps 20 km throughout the crust both seismic velocities and densities are in general high beneath the insular belt low beneath the coast and western intermontane belts and intermediate beneath the eastern intermontane belt however densities are unusually low in the lower crust beneath the coast belt 2800 kg m3 relative to velocities 6 6 6 8 km s this indicates that coast belt plutonic material is present

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

3d seismic refraction traveltime tomography at a groundwater contamination site we have applied traveltime tomography to 3d seismic refraction data collected at hillair force base utah in an approximately 95 40 m area over a shallow 20 m groundwater contamination site the purpose of this study is to test the ability of 3d first arrival time data to characterize the shallow environment and aid remediation efforts the aquifer is bounded below by a clay aquiclude into which a paleochannel has been incised and acts as a trap for dense nonaqueous phase liquiddnaplcontaminants aregularizednonlineartomographicapproachwasappliedto187 877 first arrival traveltimes to obtain the smoothest minimumstructure 3d velocity model the resulting velocity model contains a velocity increase from less than 300 to 1500 m s in the upper 15 m the model also contains a north southtrendinglow velocityfeatureinterpretedtobethepaleochannel based on more than 100 wells in the area checkerboard testsshow7 5 10 mlateralresolutionthroughoutmostofthe model the preferred final model was chosen after a systematic test of the free parameters involved in the tomographic approach includingthestartingmodel thefinalvelocitymodel compares favorably with a 3d poststack depth migration and 2d waveform inversion of coincident reflection data while the long wavelength features of the model reveal the primary target of the survey the paleochannel the velocity model is likely a very smooth characterization of the true velocity structure particularly in the vertical direction given thesizeofthefirstfresnelzoneforthesedata

seismic structure across the active subduction zone of western canada the vancouver island seismic project was conducted in 1980 to study the structure of the subducting oceanic juan de fuca plate and the overriding continental america plate the principal seismic refraction line line i was a 350 km onshore offshore profile perpendicular to the continental margin an array of 32 receivers was located on the america plate on the mainland and across vancouver island and extended offshore with three ocean bottom seismometers obs s two shots were fired at the eastern end of the line and 17 shots were located along the westernmost 100 km of the profile control for the interpretation of the onshore offshore profile was provided by a reversed refraction profile along the length of vancouver island and by a marine refraction profile recorded on the obs s modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two dimensional media the major features of the refraction structural model are that 1 the oceanic lithosphere dips at 3 or less beneath the continental slope so the bend in the subducting slab occurs landward of the foot of the slope 2 the oceanic lithosphere dips at 14 16 beneath the continental shelf until it passes beneath the continental moho at 37 km depth below western vancouver island 3 an upper mantle reflector may correspond to the base of the subducting lithosphere and 4 a segment of high velocity material above the downgoing crust with velocity 7 7 km s and depth range 20 25 km may represent a remnant of subducted lithosphere perhaps detached when the subduction zone jumped westward to its present position

metchosin igneous complex southern vancouver island ophiolite stratigraphy developed in an emergent island setting the metchosin igneous complex of southern vancouver island has three components gabbro stocks formerly sooke gabbro a newly recognized sheeted dike complex and a basaltic volcanic sequence formerly the metchosin volcanics the sheeted dikes occupy a position transitional between the gabbro stocks and submarine volcanics which they tie together in an ophiolitic pseudostratigraphy the submarine volcanics shoal upward and pass into subaerial sheet flows the complex formed in an area of regional extension as an emergent island