site investigations to evaluate flow liquefaction slides at sand heads fraser river delta l ecoulement par liquefaction spontanee est un processus recurrent dans la progradation du delta du fleuve fraser une portion du delta a ete etudiee au moyen d essais de penetration au cone sismique et de resistivite et au moyen de l analyse spectrale de la methode des ondes de surface et de l echantiollonage de sol et de vapeur dans un effort pour determiner la stratigraphie du site l etat in situ et le degre de saturation afin explorer l enigme de l ecoulement par liquefaction spontanee un ensemble de capteur de pression interstitielle et d inclinaison a ete installe comme partie de l etude pour mesurer les effets de la vidange resultant du reflux de la maree les resultats ont indique que le lit de la mer a faible profondeur etait partiellement sature contenant un volume significatif de gaz methane qui affectait la reponse dynamique de la pression interstitielle durant le jusant une reduction de la contrainte effective atteignant 16 5 kpa a ete mesuree de meme qu un delai de 50 min entre le reflux de la maree et la reaction de la pression interstitielle ceci suggere que les effets de tampon sont un facteur significatif dans le declenchement de la liquefaction spontanee des sables l ches non viellis la ou ils reposent sur des pentes sous marines les resultats des essais in situ montrent que les sables nouvellement deposes du fleuve fraser ont une forte susceptibilite a l ecoulement par liquefaction

aqueous and mineralogical analysis of arsenic in the reduced circumneutral groundwater and sediments of the lower fraser river delta british columbia canada abstract naturally occurring as in the groundwater and holocene sediments of the lower fraser river delta was studied at each of two sites in the delta vertical profiles of groundwater geochemistry were characterized and sediment cores were collected for petrographic analysis quantification of elemental abundances and sequential extraction analysis to characterize the pools of solid phase as total solid phase as concentrations were similar to those found in other deltaic environments ranging from 2 2 to 2 6 g g at a site of groundwater recharge with approximately 4 m of peat at the surface to 2 7 4 1 g g at a site adjacent to the fraser river where saline water is encountered within the aquifer at depth arsenic that was weakly bound or could be mobilized by reduction of fe oxides constituted between 20 and 50 of the total as in the sediments groundwater as concentrations showed distinct vertical trends and peaked at 9 g l at the peat influenced site and between 29 and 32 g l at the site with saline water at depth at both sites reduction of dissolved organic matter appears to be responsible for reducing conditions high dissolved fe up to 230 mg l high hco 3 and the presence of nh 4 and po 4 3 the peat impacted site implies that a direct source of dissolved organic c from peat is not a sufficient condition for significant as release into groundwater in the fraser river delta

late quaternary sea levels and crustal movements coastal british columbia late quaternary sea level fluctuations on the british columbia coast have been established from studies of terrestrial and marine sediments and landforms these studies indicate that the sea level history of mainland british columbia and eastern vancouver island is very different from that of the queen charlotte islands and western vancouver island specifically in the former areas there was a rapid rise of submerged coastal lowlands between about 13 000 and 10 000 years ago emergence culminated about 6000 9000 years ago depending on the locality when the sea relative to the land was 12 m or more lower than at present in some areas during middle and late holocene time relative sea level rose on the mainland coast and at least locally on eastern vancouver island resulting in inundation of coastal archaeological sites and low lying terrestrial vegetation tidal records and precise levelling suggest ongoing submergence of at least part of this region in contrast shorelines on the queen charlotte i

age and significance of earthquake induced liquefaction near vancouver british columbia canada in late 1994 sand dykes large sand blows and deformed strata were exposed in the walls of an excavation at annacis island on the fraser river delta near vancouver british columbia the features record liquefaction during a large earthquake about 1700 years ago this was perhaps the largest earthquake to affect the vancouver area in the last 3500 years similar less well dated features have been reported from several other sites on the fraser delta and may be products of the same earthquake three radiocarbon ages that closely delimit the time of liquefaction on annacis island are similar to the most precise radiocarbon ages on coseismically subsided marsh soils at estuaries in southern washington and oregon both the liquefaction and the subsidence may have been produced by a single great plate boundary earthquake at the cascadia subduction zone alternatively liquefaction at annacis island may have been caused by a large crustal or subcrustal earthquake of about the same age as a plate boundary earthquake farther west the data from annacis island and other sites on the fraser delta suggest that earthquakes capable of producing extensive liquefaction in this area are rare events further liquefaction analysis using historical seismicity suggests that current assessment procedures may overestimate liquefaction risk

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

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

estimating locations and magnitudes of earthquakes in southern california from modified mercalli intensities modified mercalli intensity mmi assignments instrumental moment magnitudes m and epicenter locations of thirteen 5 6 m 7 1 training set events in southern california were used to obtain the attenuation relation mmi 1 64 1 41 m 0 00526 h 2 63 log h where h is the hypocentral distance in kilometers and m is moment magnitude intensity magnitudes m i and locations for five 5 9 m 7 3 independent test events were consistent with the instrumental source parameters fourteen historical earthquakes between 1890 and 1927 were then analyzed of particular interest are the m i 7 2 9 february 1890 and m i 6 6 28 may 1892 earthquakes which were previously assumed to have occurred near the southern san jacinto fault a more likely location is in the eastern california shear zone ecsz these events and the 1992 m 7 3 landers and 1999 m 7 1 hector mine events suggest that the ecsz has been seismically active since at least the end of the nineteenth century the earthquake catalog completeness level in the ecsz is m 6 5 at least until the early twentieth century online material figures and table of empirical mmi site corrections

joint inversion of regional and teleseismic earthquake waveforms a least squares joint inversion technique for regional and teleseismic waveforms is presented the mean square error between seismograms and synthetics is minimized using true amplitudes matching true amplitudes in modeling requires meaningful estimates of modeling uncertainties and of seismogram signal to noise ratios this also permits calculating linearized uncertainties on the solution based on accuracy and resolution we use a priori estimates of earthquake parameters to stabilize unresolved parameters and for comparison with a posteriori uncertainties we verify the technique on synthetic data and on the 1983 borah peak idaho m 7 3 earthquake we demonstrate the inversion on the august 1954 rainbow mountain nevada m 6 8 earthquake and find parameters consistent with previous studies

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

holocene sea level change and delta growth fraser river delta british columbia the fraser river has been building its delta into the strait of georgia in southwestern british columbia for about the last 9000 years this period encompassed a relative sea level rise of some 13 m this study concerns the effects of the rise in sea level on the depositional evolution of the delta the lithostratigraphy of the delta was established by a series of drill cores four major lithostratigraphic units were defined peat organic rich silt interbedded silts and sands and massive sands these units were interpreted in terms of the delta s contemporary depositional environments including peat bog floodplain and intertidal zone analysis of the delta s depositional architecture and chronology indicates that the delta continued to prograde during the rise in sea level a marine transgression accompanied by the landward migration of marine and intertidal facies did not occur growth of the delta during the rise in sea level was accomplished by both vertical accretion and lateral progradation ver

the seismic facies of a tidally influenced holocene delta boundary bay fraser river delta b c abstract the tidally influenced fraser river delta in southwestern british columbia is actively prograding into the strait of georgia to investigate the geometry stratigraphy and structure of the post glacial fraser river delta sediments a high resolution shallow seismic reflection survey was carried out during the summer of 1987 in the boundary bay region of the delta twenty kilometers of optimum offset seismic reflection data were collected these high resolution seismic profiles were shot with 3 m spacing between geophones and a two way travel time of 200 ms four seismic facies were identified representing foreset topset slope failure and distributary channel deposits the foreset seismic facies has reflections that dip at angles of 1 to 7 towards boundary bay along strike the reflections have horizontal or mounded patterns horizontal continuous reflections typify the topset seismic facies the slope failure facies is located within the foreset seismic facies and consists of upper convex reflections and broken internal reflections the distributary channel seismic facies is characterized by concave downward reflections and is incised into the top of the foreset facies

provenance stratigraphic architecture and hydrogeologic influence of turbidites on the mid ocean ridge flank of northwestern cascadia basin pacific ocean abstract the northwestern edge of cascadia basin north pacific ocean is unusual because late pliocene to holocene turbidites lap onto juvenile oceanic crust of the juan de fuca ridge subsidence of the ridge flank combines with irregular westward progradation of the turbidite facies to create a stratigraphic section that coarsens and thickens upward the sand provenance is mixed individual turbidity currents have funneled into the area through several shelf slope and abyssal floor conduits including vancouver valley juan de fuca channel barkley canyon and nitinat canyon local bathymetric blockage deflection and reflection of flow paths by basement relief remobilization by intrabasinal submarine slides and debris flows episodic channel switching and sporadic overbank flooding have combined to produce erratic recurrence intervals for the turbidity currents only the tallest basement highs have remained isolated from turbidite deposition during the last 500 000 years spatial and temporal differences in sediment accumulation are important because they modulate the three dimensional responses of compaction and consolidation those changes in physical properties govern where and when hydraulic communication with the underlying basement shuts down the basal hemipelagic layer of cascadia basin transforms to an effective hydrologic seal seepage rates 1 mm yr once the sediment basalt interface is buried by 100 150 m of strata rapid accumulation of turbidites therefore accelerates the hydrogeologic conversion of igneous basement from open to sealed

sedimentary environments and postglacial history of the fraser delta and lower fraser valley british columbia the fraser river delta which is about 1000 km2 in area above low tide level has been built into the strait of georgia in southwestern british columbia during the holocene present day sedimentary

an assessment of source parameters of earthquakes in the cordillera of the western united states abstract source parameters for 50 earthquakes occurring between 1915 and 1988 have been used to examine the faulting characteristics of magnitude 5 5 to 7 8 earthquakes within the western u s cordillera an area extending from the eastern sierra nevada to the great plains and from northwestern montana to trans pecos texas source parameters used in this study include seismic information from first motion analyses body and surface waveform modeling geodetic studies and geological studies of surface faulting the principal results of this analysis include 1 all earthquakes occurred on faults dipping 38 or more and no evidence for listric of low angle planar faulting was found 2 all m 7 0 earthquakes occurred at depths 12 km and were composed of multiple subevents 3 most earthquakes 70 per cent had unilateral ruptures and 4 no individual subevent had a rupture length 21 km

geomagnetic paleosecular variation at hawaii around 3 ma from a sequence of 107 lava flows at kaena point oahu abstract we have sampled a sequence of 107 lava flows in the waianae series on oahu hawaii in two separate stretches the first 51 flows at kaena point extends from sea level to an altitude of 190 m and the second 56 flows follows the satellite tracking station road from 218 m to 360 m above sea level thermal and af demagnetization yield very similar results and reveal only normal polarities k ar dating at five different horizons combined with the paleomagnetic results indicate that the sequence was emplaced entirely in the normal polarity interval between the upper mammoth and the lower kaena polarity transitions 3 22 3 11 ma some of the flows are serially correlated filtering does not however drastically affect the final results inclinations are shallower than those expected from a centered dipole field although large 13 the inclination anomaly is consistent with results from other sites at the same latitude the scatter of the vgps about the geographic pole is consistent with available data from other regions at similar latitudes the scatter of the directions is also consistent with the predictions of recent statistical models of paleosecular variation therefore these results do not support the hypothesis of a pacific dipole window

a delta plain sheet sand in the fraser river delta british columbia canada abstract borehole data in the fraser river delta demonstrate that a nearly continuous sand unit generally 8 to 20 m thick underlies the surficial silts and sands of most of the delta plain the sand unit consists of one or more sharp based fining upward sequences and is interpreted to be a complex of distributary channel deposits distributary channel migration and the resulting accumulation of channel sands has occurred primarily in a tidal flat environment due to the interaction of tidal and fluvial processes and the high proportion of sand in the sediment load this sand unit thus provides a model for sand rich river and tide dominated delta plains jetties now maintain the position of the main channel of the fraser river where it crosses the tidal flats and prevent channel migration and further storage of sand in the lower delta plain as a result more of the available sand is being transported directly to the river mouth where failures transport the sand downslope consequently these structures contribute to instability at the river mouth

the superstition hills california earthquakes of 24 november 1987 the 24 november 1987 superstition hills earthquakes occurred on the conjugate northwest striking right lateral superstition hills fault and a previously unknown northeast striking left lateral structure defined by a lineation of hypocenters extending from the superstition hills fault to the brawley seismic zone master event locations of the earthquakes using caltech usgs seismic network data reveal the following the first main shock 33 4 9 n 115 47 7 h 10 6 km m s 6 2 0154 gmt on the northeast striking structure and its foreshocks co locate events on the northeast structure cluster in space and time and some aftershocks occur in the brawley seismic zone the second main shock 33 0 9 n 115 50 9 w h 1 9 km m s 6 6 1315 gmt on the superstition hills fault was 12 hr after the first main shock and initiated at shallow depth where the two trends join aftershocks of the second main shock lie in a northwest trend between the superstition hills and superstition mountain faults and have a sharply defined western edge the extent of the northwest trend aftershocks is not coincident with surface rupture on the superstition hills fault in general the earthquakes in the northeast trend are deep and the earthquakes in the northwest trend are shallow we compare the earthquake distribution to the distribution of crystalline basement rocks defined from a refraction study of fuis et al 1982 the earthquake locations and extent of aftershock activity appear to be controlled by the presence of crystalline basement rocks

dynamic characteristics of the yellow river mouth the yellow river delta is dynamic because the river carries a heavy sediment load and serious siltation occurs in the area massive sediment transport brings fertile soil to the river mouth leading to its expansion by 20 25 km2 per year the dramatic reduction in runoff and sediment in the delta due to increasing water demand has extended the period of flow cut off the river changes its delta channel about once every ten years the channel shifts affect the upstream flood stages since the river shifted to the present delta channel in 1976 the shape of the delta has changed remarkably sediment laden flow has been building channels by depositing sediment in low velocity areas and has extended the river mouth into the sea for more than 40 km the velocity of sea currents is enhanced by the growth of the river mouth spit at a rate of 1 m s 1 per 10 km extension which results in high capacity of sea currents carrying sediment away from the mouth the new river mouth area was silted up by 14 m and the old river mouth area was eroded by 5 m siltation takes place in the low flow season and scouring occurs in the flood season scouring occurs during the rising limb of a flood and siltation occurs during the receding limb of a flood the roughness of the river changes largely following the increase in discharge the minimum manning roughness occurs at non siltation and non erosion discharge tidal influence in the river is restricted to a section extending over only a few kilometres unlike in many other coastal areas the river water with its high sediment concentration develops into density current and flows underneath the seawater during flood this mechanism contributes to sediment transportation in the delta copyright 2000 john wiley sons ltd

pre and post missoula flood geomorphology of the pre holocene ancestral columbia river valley in the portland forearc basin oregon and washington usa abstract geomorphic landscape development in the pre holocene ancestral columbia river valley 1 5 km width in the portland forearc basin 50 km length is established from depositional sequences which pre date and post date the glacial lake missoula floods the sequences are observed from selected borehole logs 150 in number and intact terrace soil profiles 56 in number in backhoe trenches four sequences are widespread including 1 a vertically aggraded pleistocene alluvial plain 2 a steep sided valley that is incised 125 150 m into the pleistocene gravel plain 3 missoula flood terraces 19 13 ka abandoned on the sides of the ancestral valley and 4 holocene flooding surfaces 11 8 ka buried at 70 30 m depth in the axial columbia river valley weathering rims and cementation are used for relative dating of incised pleistocene gravel units soil development on the abandoned missoula flood terraces is directly related to terrace deposit lithology including thin bw horizons in gravel irregular podzols in sand and multiple bw horizons in thicker loess capping layers radiocarbon dating of sand and mud alluvium in the submerged axial valley ties holocene flooding surfaces to a local sea level curve and establishes holocene sedimentation rates of 1 5 cm year 1 during 11 9 ka and 0 3 cm year 1 during 9 0 ka the sequences of pleistocene gravel aggradation river valley incision cataclysmic missoula flooding and holocene submergence yield complex geomorphic landscapes in the ancestral lower columbia river valley

rapidly accelerating subsidence in the greater vancouver region from two decades of ers envisat radarsat 2 dinsar measurements abstract rapidly accelerating ground subsidence in the south western part of british columbia the third largest metropolitan area in canada with over 2 3 million of inhabitants is estimated using the multidimensional small baseline subset msbas advanced differential interferometric synthetic aperture radar dinsar an effective processing strategy for multi mission multi temporal sar data the synthetic aperture radar sar data used in this study consists of seven independent data sets one ascending and one descending ers 1 2 and envisat frames together spanning july 1995 september 2010 and three radarsat 2 frames spanning february 2009 october 2012 during the july 1995 october 2012 period we observe fast ground subsidence with a maximum rate of about 2 cm year in the greater vancouver region that includes the fraser river delta and the cities of burnaby richmond surrey new westminster and vancouver the rapidly accelerating subsidence is observed beneath the vancouver international airport skytrain terminal as well as several agricultural and industrial locales these time series suggest that the subsidence rate at the studied regions does not decrease with time as suggested in previous studies but remains steady or increases these results also demonstrate the importance of acquiring and appropriately estimating longer time series as previous studies on the same greater vancouver area may have misinterpreted the long term ground deformation rate and direction and underestimated the potential hazard the long term impact of this subsidence on urban infrastructure can be significant and needs to be investigated further

the foreslope hills large scale fine grained sediment waves in the strait of georgia british columbia abstract the foreslope hills are a set of 20 m high 5 km long ridges at 230 350 m water depth in the south central strait of georgia occupying a 60 km 2 area at the base of the fraser river delta foreslope they are composed of clay silt and fine sand previous authors have interpreted them as 1 a single mass slide deposit 2 mud diapirs 3 in situ rotational failures and 4 creep deformation features but collaborative evidence for these interpretations is lacking seafloor surface morphological renders derived from recent multibeam sonar bathymetry data show linear evenly spaced symmetrical ridges with ridge bifurcation resembling wave ripples but on a much larger scale detailed multichannel and single channel seismic reflection data show sinusoidal internal reflections suggesting upslope migration the ridges occur within the uppermost portion of the holocene section in a very high sedimentation rate environment consequently they are probably less than 3000 years old moored current meter records for the region show reversing currents largely oriented normal to the ridge crests averaging 10 cm s with a maximum of 50 cm s this evidence combined with increasing documentation of large scale sedimentary bedforms in slope and rise environments suggests that the foreslope hills should be re interpreted as sediment waves upslope migration is typical of many large scale fine grained sediment waves the preferred physical interpretation for the creation and maintenance of the foreslope hills as sediment waves is the lee wave model requiring a phase shift in density stratified flow as it passes over the wavy topography maximum measured bottom currents are easily strong enough to entrain silt and clay delivered to the site by turbidity currents or by sediment plume fallout from fraser river outflow either strong flood tides or severe biannual density intrusions due to their intense stratification may be susceptible to lee wave formation and the creation of sediment waves leading to the observed pattern of upslope migration

origin and variability of sedimentary facies of the fraser river delta foreslope british columbia abstract five sedimentary facies have been recognized in piston cores from the fraser river delta foreslope 1 river plume suspension deposited massive silty clay with minor organics 2 suspension deposited silty clay to clayey silt with rare parallel silt and very fine sand laminations deposited under minor traction 3 clayey silt with silt or very fine sand interbeds from mixed suspension and low density turbidity current traction deposition 4 silty very fine to medium grained sand with less than 10 thin clay or silt interbeds predominantly deposited from high density turbidity currents and 5 organic rich disorganized chaotic beds facies distribution patterns suggest that the fraser delta foreslope is dissected by a number of both active and inactive channels these channels have served as a mechanism for coarse grained sediment to bypass the delta plain and upper foreslope and be introduced directly to the lower foreslope and basin floor it is hypothesized that these channels debouch as a series of depositional lobes and slump related failure deposits in the prodelta basinal environment the direct introduction of clastic sediment into water depths exceeding 250 m suggests a process analogue for the interpretation of shelf edge deltas in the rock record thus the fraser river delta may offer a unique opportunity to model active sedimentation associated with a lowstand of sea level

geomorphology and potential slope instability on the fraser river delta foreslope vancouver british columbia le delta du fleuve fraser est localise dans une des regions ayant les plus hauts risques sismiques au canada et comprend pour une bonne part des sables fluviaux meubles tres connus comme possedant un potentiel eleve de liquefaction sismique des etudes recentes de la rive ouest du delta du fraser indiquent que la liquefaction cyclique durant un fort tremblement de terre est possible engendrant potentiellement des deformations incluant des extensions laterales un ramollissement cyclique et possiblement une coulee l on presente une evaluation preliminaire de la stabilite statique des pentes apres secousse dans laquelle les analyses ont ete limitees aux valeurs estimees des resistances au cisaillement ultimes ou a l etat d equilibre deduites de correlations avec les donnnees du terrain et aux contraintes de cisaillement calculees en partant de la bathymetrie du lit de mer existant l on a trouve que le coefficient postliquefaction de securite contre le glissement etait inferieur a l unite pour la pente de la terrasse superieure roberts en supposant une limite inferieure de resistance au cisaillement ultime equivalente a 0 06 p 0 les donnees pour evaluer la validite de cette hypothese sont insuffisantes cependant il y a abondance de depots de rupture le long de la base de la pente du delta

palaeosols in westphalian coal bearing and red bed sequences central and northern england westphalian a c coal bearing strata and red beds in the pennine basin of central and northern england contain a wide variety of palaeosols in coal bearing facies associations alluvial gley semi gley organic and rare well drained palaeosols reflect the predominance of poor drainage conditions during deposition in red beds more evolved ferruginous and ferallitic palaeosols testify to free drainage conditions during deposition catenary relationships can be inferred between most of the palaeosols in both facies associations providing further information on the palaeogeomorphology of the depositional systems and assisting in the understanding of the environments and conditions of coal seam formation

record of large late pleistocene outburst floods preserved in saanich inlet sediments vancouver island canada abstract two anomalous gray silty clay beds are present in odp cores collected from saanich inlet vancouver island british columbia canada the beds which date to about 10 500 14c yr bp 11 000 calendar years bp contain tertiary pollen derived from sedimentary rocks found only in the fraser lowland on the mainland of british columbia and washington just east of the strait of georgia abundant illite muscovite in the sediments supports a fraser lowland provenance the clay beds are probably distal deposits of huge floods that swept through the fraser lowland at the end of the pleistocene muddy overflow plumes from these floods crossed the strait of georgia and entered saanich inlet where the sediment settled from suspension and blanketed diatom rich mud on the fiord floor the likely source of the floods is late pleistocene ice dammed lakes in the fraser and thompson valleys which are known to have drained at about the time the floods occurred

geomagnetic paleointensities at hawaii between 3 9 and 2 1 ma preliminary results abstract thellier thellier experiments have been conducted on 454 samples from different sequences of the waianae and koolau volcanoes in oahu hawaii the sampled interval as determined by k ar dating spans from 3 9 to 2 1 ma results from 89 samples from 41 flows meet restrictive reliability criteria and provide the first paleointensity estimates from hawaii for the sampled period with good temporal resolution on this time interval the vadm shows ample fluctuations from a minimum of 2 3 1022 am2 to a maximum of 11 5 1022 am2 about an average value of 7 0 2 3 1022 am2 suggesting that fluctuations of the geomagnetic field strength comparable in amplitude to the time averaged value may be a characteristic of the field during both the reverse mammoth chron and the normal chron between the mammoth and the kaena the field strength appears to increase with time

depth of seismicity in the imperial valley region 1977 1983 and its relationship to heat flow crustal structure and the october 15 1979 earthquake focal depths from over 1000 earthquakes occuring between 1977 and 1983 in the imperial valley southern peninsular ranges are used to study relationships between the depth of seismicity heat flow and crustal structure this study used relocated a and b quality events from the california institute of technology catalog that were carefully selected to insure focal depth precision of 2 km regional variations in focal depth appear to be related to regional heat flow variation whereas local variations in focal depth especially in the central imperial valley may be related to crustal structure these variations are studied by rheologic modeling a comparison of focal depths of earthquakes occurring before and after the october 15 1979 m 6 6 earthquake indicates that aftershocks during the first 2 months of the sequence were 2 3 km deeper than earthquakes occurring in other time periods the deepest earthquakes in imperial valley are spatially associated with a subbasement dome near the northern end of the imperial fault this dome coincides with the region where the imperial fault undergoes a transition from stick slip behavior to aseismic fault creep models of slip during the 1979 mainshock are also compared with premainshock and postmainshock seismicity a relocation of the 1940 m 7 1 mainshock suggests that this sequence began by rupturing the same portion of the fault that experienced maximum slip during the 1979 mainshock

a re examination of the 1947 manix california earthquake sequence and comparison to other sequences within the mojave block the 10 april 1947 manix california earthquake m l 6 2 is the largest earthquake to have occurred in the mojave block in historic time left lateral movement was observed along the manix fault strike of n70 e following the earthquake however the large aftershocks of the sequence as located by richter and nordquist 1951 align themselves along a trend of n30 w this led richter 1958 to conclude that rupture during the main shock was along the trend of the aftershocks and that the surface faulting was a secondary feature in this study relocation of the larger aftershocks of the sequence verifies the previously observed trend of n30 w suggesting the aftershocks occurred along a fault conjugate to the manix fault body waveform modeling of the main shock indicates that it was composed of two subevents the location of the second subevent with respect to the first subevent suggests unilateral rupture to the southwest along the manix fault although the focal mechanisms of both events are consistent with either faulting along the manix fault or a conjugate fault both subevents occurred at relatively shallow depths 5 to 7 km a study of other recent earthquakes of m l 3 0 within the mojave block indicates that the majority of earthquakes in the region occur at shallow depths 7 km although the heat flow in the area is 10 to 20 mw m2 lower than in surrounding regions where deeper seismicity is present

basin architecture and density structure beneath the strait of georgia british columbia georgia basin is located within one of the most seismically active and populated areas on canada s west coast over the last decade geological investigations have resolved important details concerning the basin s shallow structure and composition yet until recently relatively little was known about deeper portions of the basin in this study new seismic velocity information is employed to develop a 3 dimensional density model of the basin comparison of the calculated gravity response of this model with the observed gravity field validates the velocity model at large scales at smaller scales several differences between model and observed gravity fields are recognized analysis of these differences and correlation with independent geoscience data provide new insights into the structure and composition of the basin fill and underlying basement specifically four regions with thick accumulations of unconsolidated pleistocene and younger sediments which were not resolved in the velocity model are id

local magnitudes and apparent variations in seismicity rates in southern california redetermination of local magnitudes for moderate earthquakes recorded by the southern california seismographic network scsn from 1932 to 1990 has shown that the magnitudes have not been consistently determined over that time period the amplitudes of ground velocities recorded on wood anderson instruments were systematically overestimated prior to 1944 compared to present reading procedures leading to a significant overestimation of local magnitudes in addition the change from human to computerized estimation of event magnitude from a suite of amplitudes in 1975 led to slightly lower event magnitudes for the time after 1975 compared to the time before these changes contribute to an apparently higher rate of seismicity in the 1930s and 1940s than later in the catalog which had been interpreted as a decrease in seismicity rate after the 1952 kern county m w 7 5 earthquake wood anderson amplitudes have been reread and consistent magnitudes recalculated using uniform procedures for all earthquakes with a catalog magnitude of 4 5 and greater within the scsn from 1932 to 1943 and those with a catalog magnitude of 4 8 and greater from 1944 to 1990 so as to create a complete list of all earthquakes with a modern local magnitude of 5 0 or greater using these new magnitudes we find that the rate of m l 5 0 and greater earthquakes in southern california over this 59 year period to be poissonian with no changes in rate significant above the 90 level from this rate in any 30 year period the poissonian probability of a m 6 earthquake is 99 7 the probability of an m 7 earthquake is 65 and the probability of an m 8 event is 18

optical dating studies of quaternary organic rich sediments from southwestern british columbia and northwestern washington state the suitability of optical dating using 1 4 ev infrared excitation for determining the time of deposition or compaction of organic rich sediments and peat is assessed with measurements on seven samples from six different lithostratigraphic units one is of zero age two have associated 14c ages three are known to have been deposited during an interglaciation and one is 1 ma old the samples yield satisfactory optical ages ranging from 0 to over 100 ka we conclude that the muir point formation southern vancouver island and the whidbey formation northwestern washington state were both deposited during 18o stage 5 as previously deduced from other evidence the age obtained from the 1 ma sample was significantly too low the optical dating method is simpler and more precise than thermoluminescence dating and is recommended for future work

applications of land based high resolution seismic reflection analysis to quaternary and geomorphic research abstract the availability in the last few years of low cost microcomputers and multichannel seismographs has made possible the application of high resolution digital seismic reflection profiling to quaternary problems seismic analysis of the subsurface can provide continuous two dimensional sections containing detailed structural and stratigraphic information a review of the general principles of shallow seismic reflection methods includes the nature of seismic sources the use of shotgun shells the application of the optimum offset method data processing and the issue of site dependency a brief overview is provided of seismic and sequence stratigraphy four case studies illustrate the application of reflection seismic profiling 1 champlain sea sediments late wisconsinan on the flank of the gatineau hills near quyon quebec 2 the definition of bedrock valleys beneath pleistocene deposits near shawville quebec 3 the fill of a linear depositional basin okanagan valley british columbia 4 the seismic signatures of topset and foreset beds slope failures and a distributary channel of the fraser river delta british columbia

interactions between the landers and hector mine california earthquakes from space geodesy boundary element modeling and time dependent friction the m w 7 1 hector mine earthquake happened 7 years after the 1992 m w 7 3 landers earthquake within a system of right lateral strike slip faults comprising the eastern california shear zone because estimates of recurrence intervals of m 7 earthquakes on these faults range anywhere from 1500 to 50 000 years the close temporal spacing of these two earthquakes suggests that there is some interaction between the two current models of simple coulomb static stress interactions between the earthquake ruptures do not predict an obvious cause and effect relationship the interaction between the computed normal and right lateral shear stress reductions induced by the landers earthquake at the hector mine hypocenter is unclear we use a combination of space geodesy boundary element modeling and computer modeling of time dependent fault friction to investigate the interaction between the two earthquakes we compare stress changes on the hector mine rupture plane induced by the landers earthquake with the detailed slip distribution inferred using a combination of gps and insar data the slip distributions of both earthquakes are also used to infer the magnitude of the shear stress drop on each earthquake rupture and the orientation of the remote background stress consistent with these two recent events for each earthquake the azimuth of the remote maximum compressive stress was approximately 17 6 and the magnitude of the shear stress drop was 8 1 mpa on the landers rupture and 10 2 mpa on the hector mine rupture using a simple spring and slider model as a proxy for the rate and state frictional response of the hector mine faults to landers earthquake imposed stress steps we find that while a decrease in normal stress at the hector mine hypocenter would by itself have caused the hector mine earthquake to nucleate immediately the simultaneous decrease in shear stress may have caused a delay in the peak shear loading consideration of an acceptable range of rate and state friction parameters secular strain rates and crustal stiffnesses leads us to conclude that this delay could have been 0 to 40 years

the stability of fine grained sediments from the fraser river delta the stability and erodibility of benthic fine grained sediments on the foreshore and upper foreslope of the fraser river delta were measured using thein situbenthic flume sea carousel the erosion threshold c 0 which varied between 0 11 and 0 50 pa was inversely related to erosion rate it was also proportional to sediment wet bulk density b in the form c 0 7 10 4 b 0 47 this trend agreed with those obtained from four other sites across canada that encompassed a wide range in sediment densities 1000 b 2000 kg m 3 erosion threshold shields parameter c was positively related to grain reynolds number over the silt clay size range 0 01 re 5 c 2 86re0 58 this was similar to earlier findings for water saturated sediment this trend is also in continuity with published threshold values measured in biostabilized sand 5 re 50 copyright 1997 academic press limited three erosion types were observed type ia erosion was a surface phenomenon caused by the presence of a thin organic fluff layer it occurred at current speeds less than 0 3 m s 1 type 1b erosion asymptotically decaying with time characterized the mid portions of each erosion time series and occurred at current speeds of 0 3 0 9 m s 1 type i ii erosion transitional was largely found on the foreslope and occurred at current speeds in excess of 0 8 m s 1 the erosion process began in all cases under turbulent transitional flows the change from type i to type ii erosion corresponded to the onset of turbulent rough flows 60 re 70 this is supported by video observations which showed type i erosion to be largely the result of entrainment of small aggregates and flocs whereas type ii erosion was the result of enlargement of surface irregularities and subsequent undercutting and release of large aggregates mean erosion rate em was independent of either azimuthal current speed or bed shear stress it was largely constant throughout each deployment and varied between 1 4 and 7 4 10 4 kg m 2 s 1

thermal mechanical controls on seismicity depth distributions in the san andreas fault zone earthquake depth distributions in continental crust can be related to factors which determine the depth of the transition from brittle to ductile failure these factors include temperature strain rate and lithology the san andreas fault zone of northern and central california is an ideal place to test the relative importance of these factors since there is abundant earthquake data and our understanding of the tectonic interaction of the pacific and north american plates provides constraints for several parameters in the san andreas fault zone we have found an approximately 3 km increase from northern to central california of the median depth of events and the depth above which 90 of events occur in addition depth distributions of earthquakes vary systematically from northern to central california both the maximum depth of earthquakes and the depth of the peak of seismic activity are shallower in northern california utilizing critical strength envelopes we have found that unlike the oceanic case this variation is not thermally controlled rather a combination of low strain rate and low stress level cause the seismicity in northern california to be shallow relative to central california

biosedimentological zonation of boundary bay tidal flats fraser river delta british columbia boundary bay tidal flats on the inactive southern flank of the fraser delta have surface sediments consisting almost entirely of very fine to fine well to very well sorted sands which show a gradual fining shorewards trend five floral sedimentological zones form distinct biofacies these are from the shoreline seaward the saltmarsh algal mat upper sand wave eelgrass and lower sand wave zones the lower limit of the saltmarsh lies at a constant level above which the maximum duration of continuous exposure rises abruptly from 12 to 40 days similarly at the lower limit of the algal mat zone the maximum duration of continuous exposure jumps from 1 to 2 days and at the upper limit of the eelgrass zone from 0 5 to 0 8 days these correlations between exposure and zonation are suggested to be causal in the algal mat and eelgrass zones microtopography of biogenic origin only a few centimetres high creates lateral heterogeneity within the zonal biofacies in the upper sand wave zone very low amplitude a 0 1 m symmetrical sand waves 30 m of probable storm wave origin have a similar effect in the lower sand wave zone sand waves a 0 5 m 60 m are formed by tidal currents or wave action and physical sedimentary structures dominate over biogenic ones the densities of the following macrofaunal organisms which produce distinctive biogenic sedimentary structures were determined on two surveyed transects callianassa californiensis and upogebia pugettensis both thalassinidean shrimps three polychaete worms abarenicola sp spio sp and clymenella sp the bivalve mya arenaria and the gastropods batillaria attramentaria and nassarius mendicus callianassa excavate unlined temporary feeding burrows whereas upogebia build mud lined permanent dwelling burrows upogebia are restricted to below mean sea level where continuous exposure is 0 5 days whereas callianassa extend up to a level just below mean higher high water where maximum continuous exposure rises abruptly from 4 to 9 days this difference in range is probably due to the latter s greater anoxia tolerance a necessary adaptation for life in an unlined feeding burrow

pleistocene glacimarine sedimentation on the continental slope off vancouver island british columbia abstract cordilleran ice sheet activity has had a significant influence on glacimarine sediment transport and deposition on the western canadian continental slope resulting in distinct sediment facies here we present lithogenic sediment grain size distributions grains 250 m g 1 63 m 10 to 60 m and 2 to 4 m and mass accumulation rates mar from a marine core md02 2496 collected 50 km offshore of vancouver island british columbia the lithogenic fraction of sediment deposited on the slope increased as grain size decreased and sediment sorting increased at 30 ka when the cordilleran ice sheet grew into southern british columbia during the fraser glaciation as the ice sheet advanced onto the continental shelf near core md02 2496 between 19 and 17 ka the proximal ice margin dramatically increased lithogenic sediment mars glacimarine sedimentation at this time was associated with cyclic deposition of generally coarse poorly sorted sediment an 80 year cyclicity suggests the influence of 1 outburst floods 2 lunar or 3 solar cycles on tidewater outlet glacier discharge rapid ice sheet retreat at 17 ka is indicated by ice rafted debris ird deposition followed by the deposition of fine well sorted silt suggestive of iceberg discharge followed by rock flour in a meltwater plume lithogenic sediment mars decreased dramatically at the bolling with a brief return to coarse poorly sorted sediments during the younger dryas glacimarine sedimentation decreased by 10 ka as the cordilleran ice sheet extent diminished to the areal extent of modern ice fields

cold climate peat formation in canada and its relevance to lower permian coal measures of australia abstract canada is a vast cold country large expanses of wetlands occur whose characters change predictably over continental distances along north south temperature gradients and coastal inland precipitation trends the changing climatic conditions during the holocene have influenced the initiation and rates of peat formation for instance most of the peats of south central canada are not older than 5000 years b p those in the arctic regions are fossil deposits of the hypsithermal times of 7 8000 years b p maintained at the surface because of the low rates of peat oxidation an analysis of the environments of formation and of the stratigraphy of recent peats which formed under cold to cold temperate climate in canada forms the basis for a comparative study and a better understanding of the permo carboniferous coals of gondwanaland brazil south africa and australia much of the stratigraphic complexities that occur in the peat sequences are blurred out during organic decomposition and metamorphism leading to coal formation however some of the major features of the sequences that develop in string fens from basal sedge peats up to sedge woody peats and in boreal and cold temperate bogs and swamps from basal lacustrine deposits with some reworked and algal rich sediments grading upward into sedge peats to sphagnum sedge peat to sphagnum woody or just woody peats may be recognizable in banded coal seams it is suggested that future detailed comparative microfacies analyses of the cold peat sequences of canada and the lower permian coal seams of australia will allow better reconstructions and understanding of ancient wetlands without exclusively using the less appropriate albeit better known recent models of subtropical swamps

liquefaction features on the fraser delta evidence for prehistoric earthquakes sand dykes and sills and vented sand boils on the fraser river delta and adjacent serpentine river floodplain in southwestern british columbia record one or more major liquefaction events the source of the dykes is a shallow subsurface saturated sand unit that was deposited in foreslope and distributary channel environments during holocene progradation of the fraser delta the dykes cut steeply through a crust of delta topset muds and locally flatten out as sills in overlying peaty sediments at two sites evidence was found for venting of sand onto a subaerial or intertidal surface liquefaction and upward movement of sand and water locally deformed the intruded sediments causing some subsidence and uplift of the delta surface all observed liquefaction features are younger than ca 3500 bp and at least some are younger than 2400 bp it is not clear however whether they formed during one or several separate events a review of possible causes suggests that the liquefaction features probably are seis

intershield geochemical differences among hawaiian volcanoes implications for source compositions melting process and magma ascent paths as hawaiian volcanoes develop their lavas systematically change in composition and isotopic ratios of sr nd and pb these trends provide important constraints for understanding plume related volcanism as a volcano migrates away from the hotspot there are also geochemical differences between hawaiian shields in particular lavas from adjacent shields such as kilauea and mauna loa on hawaii and koolau and waianae on oahu have significant differences in abundances of some major and incompatible elements and isotopic ratios of sr nd and pb some incompatible element abundance ratios such as zr nb and sr nb are correlated with intershield differences in sr and nd isotope ratios but these isotopic ratios are not correlated with intershield differences in major element composition or even parent daughter abundance ratios such as rb sr and sm nd moreover at kilauea and mauna loa the intershield differences have apparently persisted for a relatively long time perhaps 100 ka these intershield geochemical differences provide important constraints on plume volcanism specifically i each volcano must have distinct magma ascent paths from the region of melt segregation ii the 25 50 km distance between adjacent but geochemically distinct shields requires that the sources vary on a similar scale and that the melt production region is similarly restricted the absence of correlations between lava compositions and radiogenic isotope ratios provides evidence for significant differences in melting process such as each shield forming by a different mean extent of melting with melt segregation at different mean pressures two types of models are consistent with the intershield geochemical differences i a relatively large radius ca 40 km plume conduit with a systematic spatial distribution of geochemical heterogeneities or ii a small radius less than 20 km plume conduit composed of geochemically distinct diapirs because relatively small radius diapirs of limited vertical extent are too small to create the large hawaiian shields a possible alternative is a continuous conduit containing solitary waves which transport geochemically distinct packets of material

foreshocks 1966 1980 in the san andreas system california the spatial and temporal distributions of seismicity preceding moderate ml 5 0 main shocks in the san andreas fault system in california have been analyzed to recognize and characterize the patterns of foreshock occurrence of 20 main shocks in the san andreas system 7 or 35 per cent have been preceded by immediate foreshock sequences that included events within 1 day and 5 km of the main shocks a possible correlation of the rate of foreshock occurrence with type of faulting was found such that none of the four main shocks with reverse faulting had foreshocks while 44 per cent of the strike slip earthquakes had foreshocks some enhanced seismic activity was also observed at relatively large distances from the main shock 13 to 30 km 1 to 5 days before 40 per cent of the main shocks but this activity cannot be clearly distinguished from the background seismicity of the seven immediate foreshock sequences only two had the swarm like appearance of the class ii foreshocks defined by mogi the other foreshock sequences appear to be single events sometimes with their own aftershocks preceding the respective main shocks four of these sequences are spatially correlated with distinct physical discontinuities in their faults between the hypocenters of the foreshock and main shock and similar discontinuities may also be associated with the other sequences the durations of the foreshock sequences were found to decrease as the depths of the main shocks increase from 3 to 11 km which has been interpreted as a dependence on stress to account for this stress dependence of the duration and the presence of discontinuities a model for foreshocks occurrence is presented this model proposes that foreshocks may represent a process of delayed multiple rupture and that the delay between occurrence of foreshock and main shock might represent the time needed for static fatigue to break the stronger rock at the discontinuity in the fault

new k ar ages of shield lavas from waianae volcano oahu hawaiian archipelago abstract a geochronological study utilized the unspiked potassium argon k ar technique to obtain ages from the two main volcanic members of the shield stage of the waianae volcano hi these new dates are further constrained using a combination of stratigraphic relationships magnetostratigraphy and major element geochemistry exposed shield lavas encompass 0 85 ma with reliably dated tholeiitic lavas from the main shield ranging from 3 93 0 08 to 3 54 0 04 ma and a later shield stage ranging in age from 3 57 0 04 to 3 08 0 04 ma these data suggest that the total extent of waianae shield activity was significantly more than 1 ma the age of faulting in two flank zones is constrained to be about 3 4 ma preliminary estimates of lava accumulation rates vary from about 0 3 to 2 0 mm a calculated rates show no systematic variation with location in the volcano or with time

transitional field behavior during the gilbert gauss and lower mammoth reversals recorded in lavas from the wai anae volcano o ahu hawaii using a portable fluxgate magnetometer we identified two polarity transitions one a reversed to normal r n in a 600 m section called pu u heleakala and second a normal to reversed n r in a 300 m section known as pu u kamai leunu of superposed basalt flows on the western flank of wai anae volcano one of the two volcanoes that compose the hawaiian island of o ahu volcanic stratigraphic studies suggest that the r n reversal corresponds to the gilbert gauss polarity boundary and the n r reversal correlates with the lower mammoth polarity transition we drilled an average of six to eight samples from each of the lava flows 36 flows from the heleakala and 29 from the kamai leunu spanning the two sections lightning induced secondary magnetization is common in some flows requiring detailed alternating field demagnetization to remove it and to isolate the primary directions thermal demagnetization proved to be ineffective and if employed could lead to incorrect identification of lightning dominated remanence as primary our laboratory experiments indicate that we have successfully identified one transition zone on each section composed of at least seven to eight transitional lava flows the transitional vgps of the gilbert gauss reversal are located in northwestern africa sri lanka and borneo whereas for the lower mammoth reversal the transitional vgps are located in northern africa and in the southern atlantic these observations are not in agreement with the hypothesized antipodal longitudinal bands over the americas and the western pacific rim the two transitions are characterized by periods of high negative inclinations corresponding to vgps in africa about 180 away from the site longitude the obtained vgp paths could be explained by a transitional field affected by anomalous geomagnetic fields that exist beneath the pacific hemisphere these recurrent nonaxisymmetric components in the transitional fields suggest that lateral variations in lower mantle properties influence the geodynamo

seismic investigation of the coast plutonic complex insular belt boundary beneath the strait of georgia the strait of georgia a topographic depression between vancouver island and the mainland of british columbia is considered to be the boundary between two tectonic provinces the coast plutonic complex on the east and the insular belt to the west the allochthonous nature of the insular belt has been established mainly on the basis of paleomagnetic measurements various tectonic models to explain the geological differences between the two provinces have been proposed one of these suggests that the boundary is an old transform fault zone and is represented currently by a thrust fault along the eastern side of the strait of georgia other models propose that the coast plutonic complex is a feature superimposed by tectonic and metamorphic events after the accretion of the insular belt such models do not require a major crustal discontinuity along the strait of georgia in may 1982 a seismic refraction survey using a 32 l air gun and a radio telemetering sonobuoy system was carried out in the strait of ge

the paraglacial geomorphology of the fraser lowland southwest british columbia and northwest washington abstract the fraser lowland is interpreted as a complex paraglacial landsystem in the sense that all its landform units landform and sediment associations and thus the whole landsystem have been modified by nonglacial processes conditioned by glaciation the late pleistocene and holocene processes of modification include slope erosion glacimarine marine glacifluvial fluvial glacilacustrine lacustrine organic mass movement and aeolian this complex paraglacial landsystem has transitioned from proglacial through marine to its present dominantly fluvial character landforms representative of each sediment redistribution process are described and a conceptual model is developed with relative sea level and sediment source changes as the primary drivers the sediment wave and sediment exhaustion models are compared and illustrated in the context of this paraglacial land system anthropogenic modification during the past century the anthropocene has complicated the paraglacial story and is shown to impact the incidence of natural hazards especially in the context of hydroclimate change

aquifer heterogeneity from sh wave seismic impedance inversion we collected sh wave seismic reflection data over a shallow aquifer in southwestern british columbia to investigate the use of such data in hydrogeologic applications we used this data set in developing a methodology that uses cone penetrometer data as an integral part of the inversion and interpretation of the seismic data a bayesian inversion technique converts the seismic amplitude variations to velocity variations honoring the probabilities of the priors and adhering to a geologically reasonable sparseness criterion velocity measurements acquired with the cone penetrometer provide velocity profiles and vertical seismic profiling vsp data all of which are valuable in properly constraining the bayesian inversion the differentiation of lithologies in this data set sand and clay is accomplished by first using a normalization procedure to remove the impact of effective stress which dominates the velocity variation in the upper 10 to 20 m the final transformation of seismic velocities to void ratio for the sand dominated regions is made using laboratory derived measurements it provides an image of the heterogeneity of the near surface aquifer

palynology of holocene top set aggradational sediments of the fraser river delta british columbia abstract palynological study of radiocarbon aged top set holocene sediments of the fraser delta british columbia reveals a record of tidal marsh river marsh and river swamp environments which indicate former sea level positions and rates of sea level change interbedded silts and fine sands containing pinus and cyperaceae pollen and monolete fern spores occur between 15 1 and 14 2 m depth and are interpreted as freshwater dominated delta front marsh deposits organic rich silts containing grass and skunk cabbage pollen and horsetail spores between 14 2 7 8 m depth represent an emergent delta top marsh environment regularly flooded by the fraser river arboreal pollen types pine spruce and alder dominate organic rich silts between 7 8 and 5 8 m but rosaceae pollen rises to dominance in peatier sediments between 4 7 5 8 m these two zones record natural vegetation succession from marsh to swamp due to increased relative elevation of the site an increase in arboreal pollen in organic rich silts between 4 7 and 2 3 m signals a drop in relative elevation and return to regular flooding declines in percentages of arboreal pollen and increases in rosaceae and skunk cabbage pollen indicate preliminary stages of bog development and disappearance of fluvial influence above 2 3 m organic rich silts formed mainly in a fluvially dominated marsh setting as top set aggradation kept pace with sea level rise between approximately 7960 and 4410 yr b p an apparent slowing or cessation in sea level rise allowed peaty sediments to form around 6000 yr b p the rate of sea level rise again declined at 4410 yr b p causing organic rich silts to be replaced by peat deposition the study provides insights into the history of sea level change and aggradation in the fraser river delta

dikes and the petrology of waianae volcano oahu we summarize the distribution orientation and composition of the dikes of waianae volcano and attempt to relate this summary to the evolution of the volcano dikes are located in the caldera in a well defined northwest rift zone and in less well defined south and northeast rift zones the active axis of the south rift zone may have shifted with time the dikes generally are parallel to their rift zones and are oriented in many directions in the caldera area they commonly dip about 70 to 80 and are generally less than a meter thick most are virtually identical in texture mineralogy and composition to nearby extrusive rocks a few have unusual low magnesium silicic compositions not yet observed in flows these may be classified as icelandites trends defined by the compositions of tholeiitic dikes are similar to but extend further than crystal fractionation trends of tholeiitic flows of waianae and other hawaiian volcanoes the basalts can be divided into a more silica saturated strongly tholeiitic subgroup and a less saturated mildly tholeiitic subgroup the icelandites lie on trend with the second group most of the analyzed rift zone dikes are strongly tholeiitic most of the analyzed caldera dikes are mildly tholeiitic all of the differentiated high silica low magnesium dikes found are inside the caldera near the eruptive center

stability of the fraser river delta front the distributary front deposits of the delta consist mainly of interlayered sand and silt and lie in one of the most seismically active zones in canada subsurface deposits at the north tend to be siltier than those at the south sediments on the surface of the delta slope range from clayey silts at the north to pure sands at the south slope angles vary from 23 at the head of the slope to 1 2 within 2 km beyond the tidal flats hydraulic bedforms observed on the sandy slopes southeast of the main channel gradually increase in size from small ripples to large asymmetric sand waves 2 3 m high and approximately 30 m long they are generated primarily by flood tidal currents which scour the seabed to a water depth in excess of 100 m formation of gullies which crease the delta slope probably is initiated principally by failure of oversteepened deposits at a channel mouth previous studies have suggested that the large hummocks or ridges at the base of the slope off the main channel have been formed by the co

asymmetry in holocene river deltas patterns controls and stratigraphic effects abstract understanding controls on sediment distribution in river deltas is paramount to predicting facies relationships and stratal architecture most classifications of deltas have focused on subaerial plan morphology as a simplistic function of river tide and wave energy new work however is revealing the importance of subaqueous deposits and in particular shore parallel sediment redistribution in the shaping of delta planform delta asymmetry has emerged as an important characteristic reflective of patterns of alongshore sediment dispersal asymmetry has been described from onshore and offshore environments from several different types of modern deltas but aspects of asymmetry have not been fully documented and the degree to which these patterns are recorded in deltaic strata is not yet known this study is a comprehensive literature review of sediment distributions in modern deltas focusing on studies with high resolution geomorphic geophysical and geochronological datasets we reviewed 27 deltas using over 100 papers primarily from the past 15 20 years morphological facies and stratigraphic aspects were analyzed across the entire spectrum of deposits from the delta apex to the most distal muds of the prodelta we define quantifiable indices of asymmetry describing upcurrent vs downcurrent distribution of sediment volume sediment area sediment caliber and distributary channels most deltas are asymmetrical to some degree with respect to one or more of these parameters many deltas are increasingly skewed toward the downcurrent side from proximal to distal sub environments i e delta plain delta front prodelta some deltas are skewed toward the upcurrent side in one sub environment and toward the downcurrent side in another sub environment sand is preferentially deposited on the downcurrent side of most deltas but distributary channels tend to develop toward the upcurrent side the highest sand mud ratios are often on the upcurrent side in the lower delta plain but in the delta front these ratios are highest on the downcurrent side these complex patterns of asymmetry reflect different combinations of controls including discharge partitioning lobe abandonment and localized transgression plume deflection by coastal currents dominance of longshore current direction variable subsidence and other factors these processes may result in upcurrent downcurrent variations in clinoform geometry rates of progradation and stratal lapping relationships asymmetry has multiple aspects manifestations and controls even within a single delta but the long term preservation potential of these patterns likely varies depending on depositional setting and shoreline trajectory studies of ancient deltas will be better informed by recognizing the wide variety of controls on sediment distribution and avoiding the tendency toward a single model of delta asymmetry

seismic moment rates in the utah region an estimate of the long term seismicity of utah is obtained through the calculation of seismic moment rates from available geologic data on quaternary faulting the recurrence rates calculated from these geologic seismic moment rates agree well with recurrence rates estimated from historical earthquake data a return period of 80 to 190 yr for a magnitude 7 0 to 7 5 earthquake in the utah region is predicted by historical seismic and geologic data the estimation of recurrence rates from geologic data requires the use of a moment versus magnitude relationship for earthquakes characteristic of the utah region spectral studies of 19 utah earthquakes define this relationship in addition moment tensors and other source parameters are estimated for these earthquakes a resultant strain tensor for utah indicates the amount of horizontal east west extensional strain is nearly equal to the amount of vertical compressional strain in the region

postglacial deltaic sediments southern fraser river delta british columbia the fraser river delta the largest delta on the west coast of canada has been built into the strait of georgia during the holocene drill hole and seismic reflection records reveal a succession of sedimentary units deposited during early holocene progradation of the delta these overlie an irregular surface developed on pleistocene drift mud and silt similar to sediments presently accumulating off the mouth of fraser river in the southern strait of georgia are conformably overlain by a thick unit of sandy foreset beds dipping gently to the south southwest into boundary bay and deposited in a foreslope environment the foreset unit is sharply overlain by a much thinner topset sequence comprising silt and sand deposited in intertidal fluvial channel and overbank environments and peat deposited in swamps and bogs fifteen accelerator mass spectrometry radiocarbon dates on shell and wood indicate that most of the deltaic sediments south of the main channel of fraser river were deposited between ca 7

ka ena volcano a precursor volcano of the island of o ahu hawai i ka ena and wai alu ridges form prominent submarine ridges nw of the island of o ahu hawai i we evaluate whether or not either one of these ridges represents a submarine extension of wai anae volcano on o ahu using new bottom observations geophysical surveys and geochemical data acquired on new samples from the region wai alu ridge has the morphology of a submarine rift zone but is too shallow for its distance from the o ahu shoreline ka ena ridge also is unusually shallow and is surmounted by two topographic shields ka ena and wai alu ridges have similar magmatic and volcanic evolutionary histories beginning ca 5 ma with a submarine shield phase of volcanism that produced high sio 2 low feo tholeiites with higher 208 pb 204 pb than in the adjacent wai anae volcano late shield volcanism included transitional and alkalic rock types with lower sio 2 and enrichment in incompatible elements especially p 2 o 5 nb zr ti and light rare earth elements the transition from shield to late shield stage occurred as the edifice was beginning to emerge from the sea geological observations and k ar ages indicate that ka ena emerged above sea level ca 3 5 ma reaching a maximum height of 4000 m above the abyssal ocean floor and 1000 m above sea level relatively weak gravity anomalies topographic lineaments and the orientation of dike complexes indicate a volcanic structure that is independent of wai anae volcano thus volcanic structure geochemistry and age all indicate a precursor volcano to the island of o ahu which we call ka ena volcano after emergence ka ena volcano tilted 2 to the south we estimate a total volume of 20 27 10 3 km 3 for ka ena volcano taking into account overlapping geometry of concurrently active volcanoes sample compositions from the ka ena landslide deposit are entirely consistent with derivation from ka ena whereas most samples from the wai anae slump are likely derived from wai anae volcano uniformly oriented dikes in the wai anae nw rift zone likely reflect buttressing by a preexisting ka ena volcano unusual isotopic compositions of some wai anae samples including unique hydrous silicic lavas probably reflect interaction with underlying ka ena crust a newly recognized lava flow field on the southern flank of ka ena ridge extends the previously known distribution of secondary volcanism in the kaua i channel putative submarine volcanic activity in the region in 1956 cannot have built a large edifice and is unlikely to have produced pumice that was found on o ahu shores this eruptive activity therefore remains unconfirmed

m 6 earthquakes in southern california during the twentieth century no evidence for a seismicity or moment deficit abstract a broadly based report on seismic hazards in southern california wgcep 1995 concluded that the predicted seismicity exceeds that observed since 1850 a subsequent independent analysis argued that infrequent huge m 8 earthquakes are needed to explain the low rate of large earthquakes jackson 1996 frequency magnitude relationships and earthquake reporting suggest that the 1903 to 1997 catalog we present here with a b value of 1 0 and a rate of m 6 shocks of 0 42 to 0 49 yr 1 is nearly complete in contrast the 1850 to 1994 catalog used by wgcep is incomplete before the turn of the century and thus its reported seismicity rate of 0 32 m 6 shocks yr 1 is too low principally because the wgcep 1995 model results in b values of up to 4 0 for regions of lesser and blind faults the rate of m 6 shocks off the san andreas system predicted by the wgcep 1995 model is three times greater than that observed in this century because they obtained b 0 4 for m b 2 2 for m 7 3 on major faults their expected rate of m 7 san andreas shocks is twice as high as observed thus part of the seismicity and moment discrepancy identified by wgcep was caused by use of an incomplete catalog and part was caused by inappropriate b values we obtain a southern california moment release rate of 8 to 12 10 18 n m yr 1 which cannot be distinguished from the moment release estimated by fault slip or the moment accumulation inferred from plate motions or geodetically measured shear strain we thus find no evidence for a moment deficit significant aseismic moment release or for rare m 8 earthquakes off the san andreas fault system finally the number of m 6 earthquakes per decade does not depart significantly from a poisson process during this century and thus we find no evidence that the rate of seismicity is increasing now or at any other time since 1900

surface ruptures on cross faults in the 24 november 1987 superstition hills california earthquake sequence left lateral slip occurred on individual surface breaks along northeast trending faults associated with the 24 november 1987 earthquake sequence in the superstition hills imperial valley california this sequence included the m s 6 2 event on a left lateral northeast trending cross fault between the superstition hills fault shf and brawley seismic zone which was spatially associated with the left lateral surface breaks six distinct subparallel cross faults broke at the surface with rupture lengths ranging from about formula to 10 km and maximum displacements ranging from 30 to 130 mm about half a day after the m s 6 2 event an m s 6 6 earthquake nucleated near the intersection of the cross faults with the shf and rupture propagated southeast along the shf whereas right lateral slip on the shf occurred dominantly on a single trace in a narrow zone the cross fault surface slip was distributed over several stands across a 10 km wide zone also whereas afterslip accounted for a large proportion of total slip on the shf there is no evidence for afterslip on the cross faults we present documentation of these surface ruptures a simple mechanical model of faulting illustrates how the foreshock sequence may have triggered the main rupture displacement on other cross faults could trigger an event on the southern san andreas fault by a similar mechanism in the future