D.C. Mosher, Geological Survey of Canada - Atlantic; H.A. Christian, Geological Survey of Canada - Atlantic; J.A. Hunter, Geological Survey of Canada - Ottawa; J.L. Luternauer, Geological Survey of Canada - Pacific, Vancouver

multibeam image larger image [GIF, 34.9 kb, 290 X 250, notice]

The Greater Vancouver district of SW British Columbia, Canada, is a highly urbanized and industrialized area which lies in the most seismically active region in Canada, overlying a subduction zone. The Fraser River delta is the most seismically vulnerable area within this region. A significant effort has been made to identify potential geohazards and refine assessments of potential earthquake ground surface response on the delta, both on- and offshore. These studies involve state-of-the-art geological, geophysical, and geotechnical techniques, including coring, drilling, geotechnical and geophysical boring, electromagnetic, seismic reflection, refraction, and shear wave surveying. This research has led to the identification of numerous failure features, possibly related to past earthquakes, and to the prediction that the top 10 to 20 m of sediment over much of the delta, on- and offshore, is susceptible to seismic liquefaction. The subsurface geology is complex, which suggests ground surface response modeling should take into account 3-D and ground motion amplification effects. At the slope break, in the offshore, slope angles reach 18° and the soils are some of the weakest measured, indicating post-liquefaction flow sliding is likely.

The city of Vancouver and the densely populated lower mainland region of southwest British Columbia are situated over an active subduction zone, with the trench axis about 150 km to the west of Vancouver. This setting makes the region subject to frequent seismic activity and contributes to a higher risk of large damaging earthquakes than in any other part of Canada (Rogers, 1994). The area encompasses the largest delta in Canada, the Fraser River delta, which is undergoing rapid urban development. The Fraser River Delta is a thick accumulation of sands and silts deposited entirely within the Holocene. These soils are considered to be loose or soft in engineering terms, requiring increased factors of safety for earthquake loading under the current National Building code guidelines (National Research Council, 1995, p. 149, Table 4.1.9.1.C; Sy et al., 1991). Ground motion amplification of earthquake shaking and liquefaction of cohesionless water-saturated soil could occur in such materials. The lower mainland region, including Vancouver, has a population in excess of 2 million with an annual increase of several percent. Approximately 250,000 people live on the delta proper, and it is the region receiving most of the recent population increase. In addition to this urbanization, the delta is host to a $74M/year farming industry, expanding industrial capacity, an international airport, the busiest ferry terminal in the world, the largest shipping facility in Canada, a $260M/year fishery, and a submarine hydroelectric cable corridor which supplies power to Vancouver Island and the capital city of the province. In light of the seismic hazard and the intense urbanization and important economic contribution of the region, the Geological Survey of Canada (GSC), in cooperation with academic, private, and public agencies, has conducted studies in the Fraser River delta to identify potential geohazards and refine assessments of potential earthquake ground surface response. These data are to assist policy makers and engineers on development and construction decisions, ultimately to protect the citizens of the region.

Figure 1. Cartoon of the tectonic setting of southwest British Columbia - Plate motion and configuration

Earthquakes that may present a hazard to the area occur in three distinct source regions (Rogers, 1994) :

1. Continental crust earthquakes, which may occur locally at magnitudes estimated at up to 7.5. These earthquakes are frequent and normally small in the region, the most significant historic event being a M=7.3 in central Vancouver Island in 1946.

2. Deeper earthquakes within the subducted oceanic plate. These subcrustal earthquakes may be large but are generally in the depth range of 45 to 65 km below surface. They include damaging earthquakes in southern Puget Sound in 1949 (M=7.0) and 1965 (M=6.5).

3. very large earthquakes on the subduction boundary between lithospheric plates. The subduction boundary off the west coast of Vancouver Island has not produced any earthquakes in historic times, but paleoseismic evidence shows they occur at intervals of centuries (Atwater, 1987; Adams, 1990; Clague and Bobrowsky, 1994; Hyndman, 1995). Analysis of contemporary crustal deformation reveals that strain is accumulating for a future event (Dragert et al., 1994).

In terms of the local geologic setting of the Fraser River delta, it is located in a structural depression of Late Cretaceous and Tertiary clastic sediments near the west margin of the North American plate. This depression forms what is now the Strait of Georgia. Overlying these basement rocks and underlying delta sediments are Quaternary deposits resulting from various stages of Pleistocene glaciation.

The Fraser River began to build its delta into the Strait of Georgia about 10,000 years ago (Clague et al., 1983; Clague et al., 1991; Luternauer et al., 1993). As the land rebounded during glacial retreat, sea level fell relative to the land, reaching a level more than 12 m below its present position at 8000 years BP (Mathews et al., 1970). By about 5000 years BP, sea level was no more than a few metres below present. Sometime before this, the Fraser River, which had previously flowed southwest into Boundary Bay, began to discharge west into the Strait of Georgia (Clague et al., 1991). During the last 5,000 years, the Fraser River Delta has prograded westward into the strait. Considerable channel meandering and anastomosing is known to have occurred, even in historical times, although channel locations are now fixed by dikes and jetties.

Figure 2. Location diagram of Fraser River delta. Unconsolidated delta sediments to the 250 m isobath in the offshore are outlined with the dark line.

Progradation of the delta into the Strait of Georgia has resulted in a 40 km-long coastal zone (Fig. 2). Tidal flats extend about 9 km from the diked edge of the delta to the subtidal slope. The subaerial and submarine extent of the delta are each over 1000 km² in area. The slope break, marking the modern transition from the delta plain to the foreslope, lies in about 10 m water depth. The western delta slope is inclined 1-23° (average ~2-3°) towards the marine basin of the Strait of Georgia and terminates at about 300 m water depth, 5-10 km seaward of the edge of the tidal flats.

The stratigraphy of the Fraser River Delta has been studied through onshore and offshore seismic and drilling programs (Tiffin, 1969; Hamilton, 1991; Clague et al., 1991; Hart et al., 1992a&b, 1993, 1995; Luternauer et al., 1993; Monahan et al., 1993 a&b). Postglacial deltaic sediments have a maximum reported thickness of 305 m (Dallimore et al., 1994, 1996) but are known to be extremely variable. They overlie Late Pleistocene till, stratified proglacial sediments and interglacial deposits (Clague et al., 1991). The deltaic package includes bottomset, foreset and topset facies. Clays, silts and sands which constitute the bottomset facies of the delta unconformably overlie Pleistocene glacial and glaciomarine deposits. Sandy and silty foreset beds, dipping up to about 7°, conformably overlie the bottomset facies, or unconformably overlie Pleistocene deposits. Topset deposits are composed of distributary channel sands, channel-fill sands and silts, and intertidal and overbank silts, sands and peats. These deposits thin westward from 40 m at the apex of the delta to 20 m or less at the western margin of the diked delta plain.

As mentioned in the introductory section, the purposes of the present study are to identify potential geohazards and refine assessments of potential earthquake ground surface response on the Fraser River delta, including its on- and offshore portions. For these purposes, a number of technologies have been employed to better understand the geology and geotechnical properties of the sediment constituting the delta. The following is a brief discussion of some of these technologies and the methods of application.

Geological

The surface and subsurface geology provide the framework for understanding geohazards and ground surface responses, and potentially document paleoseismic "events". Geologic mapping of the Fraser River Delta has included standard geologic field mapping techniques, i.e. studying outcrop and delta morphology (c.f. Clague et al., 1983), as well as taking advantage of urban development opportunities, e.g. examining trenches and construction excavations, and many engineering boreholes and water well data (c.f. Clague et al., 1992, in press; Monahan et al., 1993a&b). A number of boreholes have been drilled for the express purpose of studying the geology and geotechnical properties of the sediment and it is from these boreholes that the most relevant information has come (e.g. Christian et al., 1994a, 1995; Dallimore et al., 1994, 1996). These dedicated boreholes have generally been drilled with the intent of continuous core recovery and have included traditional split-spoon sampling, retractor core-barrel Shelby tube sampling, as well as vibra sonic drilling. These techniques have been applied onshore and from a barge in the shallow offshore. Sediment samples from deep water in the Strait of Georgia have been acquired through surface sediment grab sampling and gravity, piston and vibro-coring from surface vessels.

Geophysical

Various types of surface and borehole geophysical techniques have been applied to the on and offshore portions of the survey area. Surface morphology and sediment type can be studied in detail in the offshore with such technologies as sidescan sonar and swath bathymetry. Much of the foreslope of the Fraser River Delta has been surveyed with sidescan sonar (Hart et al., 1991; Collins et al., 1996). The recent advent of swath bathymetry, combined with computer imaging, opens a new dimension in seafloor mapping (Currie and Mosher, 1996).

To obtain the configuration of the buried bedrock surface and stratigraphic subsurface relationships, seismic reflection techniques have been employed. Over 300 line-km of combined industry and GSC seismic reflection data have been collected for the onshore (Luternauer and Hunter, 1996; Pullan et al., 1989, 1998). In addition, the GSC has collected thousands of line-km of seismic reflection data in the offshore (Hamilton, 1991, Hart et al., 1992 a&b, 1993; Mosher et al., 1995, Mosher, 1995, Mosher and Hamilton, 1998). Routine velocity analysis on multichannel data yield important sediment velocity information for earthquake response modeling (e.g. Hunter et al., 1996). In many areas, high resolution compressional (P-wave) reflection techniques are limited because of the presence of small quantities of interstitial gas (methane). The presence of gas results in high attenuation of the seismic signal, either obliterating any coherent signal from the subsurface, or resulting in low frequency content of reflection events (Hart and Hamilton, 1993; Judd, 1995).

Shear wave reflection profiling and shear wave velocities are proving to be important in understanding the structure of the delta and its geotechnical properties (Hunter et al., 1993; Christian et al., 1994b; Harris et al., 1996). Shear wave velocities are being used to predict ground surface response through modeling and empirical relationships to known liquefaction events. As shear waves are less affected by interstitial gas, shear wave reflection profiling tests have recently been conducted, both on- and offshore (Hunter et al., 1993; Davis et al., 1993), with good results. Seismic refraction methods have been in routine use in engineering and environmental studies for many years. Interstitial gas in the deltaic sediments precludes the compressional (P) wave refraction approach as it does in reflection; however, shear wave refraction methods have been tested and shown to work well. Approximately 100 shallow penetration shear wave refraction sites have been occupied in the delta area to obtain shear wave velocity-depth profiles to at least 50 m subsurface. Seismic surface-to-borehole logging techniques have been routinely applied at over 40 boreholes drilled by GSC on the Fraser River delta. These seismic experiments have included both compressional and shear wave velocity methods. Down hole seismic velocity data have been collected in a similar manner with a seismic cone penetrometer (see below).

Boreholes have also been used to collect passive geophysical logs such as natural gamma, magnetic susceptibility and electrical conductivity. These data have been utilized to characterize the Quaternary sediments in the survey area. Boreholes have been logged with these technologies, mostly to depths between 30 and 120 m below surface. Natural gamma generally gives high count rates in association with fine grain sediments, thus is useful for discriminating between sands, silts and clays. Magnetic susceptibility is a measure of the ferrimagnetic mineral content (mainly magnetite) of the overburden material. The Holocene deltaic sands and silts are characteristically low in ferrimagnetic mineral content, whereas Pleistocene materials usually contain higher amounts. Electrical conductivity is mostly a function of the pore fluid and the porosity, and to a lesser degree the sediment type. It is very useful, therefore, in identifying aquifers and aquitards, and salinity of the pore water.

Surface electromagnetic (EM) sounding have also been employed on- and offshore. Offshore, conductivity data from EM results have been used to create porosity-depth profiles, yielding physical property information to about 30 metres below sea floor (Mosher and Law, 1996). Onshore, EM methods have been used to detect the presence of a low conductivity layer associated with the top of Pleistocene. It lies beneath a 80-90 m conductive layer dominated by Holocene deltaic saline pore water.

Geotechnical

Given the extreme difficulty in obtaining representative samples of soil in the undisturbed state, it was recognized that in situ testing provided an alternative means of characterizing soil state and thereby afforded a better approach to identifying liquefiable deposits. In situ soil-profiling tests such as the cone penetration test (CPT), the seismic cone penetration test (SCPT), spectral analysis of surface waves (SASW) and crosshole, and downhole shear-wave tests allow detailed and cost effective investigation of soil characteristics.

Cone testing of soils provides data on the bearing capacity, pore pressure response and sleeve friction. Empirical correlations have been developed relating tip resistance (qc) and cyclic resistance ratio (CRR), which defines the threshold of cyclic liquefaction. It is compared to the cyclic stress ratio (CSR) imposed by the earthquake shaking (Seed and Idriss, 1971; Seed et al., 1983; Robertson and Campanella, 1985; Robertson et al., 1992; Christian et al., 1997). The CPT, however, is unsuitable for evaluating cyclic liquefaction potential in fine-grained soils because the empirical database is for sands with less than 35% fines.

Empirical methods have also been developed to evaluate liquefaction resistance directly from shear wave velocity (Bierschwale and Stokoe, 1984). In addition to conventional downhole studies, as discussed above, significant advancement in the measurement of seismic wave velocities has been through the development of the seismic cone penetration test (SCPT) (Robertson et al. 1992). Shear-wave velocity (Vs) is influenced by many of the variables that influence liquefaction, such as soil density, confining stress, stress history, and geologic age. Thus, shear-wave velocity can be used as a field index in the evaluation of liquefaction susceptibility. Analogous to downhole electric logging, as discussed above, cones have also been outfitted with resistivity meters to measure conductivity/resistivity of the soils. This is known as a resistivity cone penetration test (RCPT) (Campanella and Weemees, 1990; Woeller, 1993

Geology

Over 4500 km²; within the Georgia depression, including the Fraser lowland and the Strait of Georgia, has been affected by sedimentation from the Fraser River in the last 10,000 years. The modern delta morphology reflects a component of the underlying structure and stratigraphy, as well as effects of modern sedimentary and anthropogenic processes. Morphological, structural and stratigraphic data provide the information necessary for understanding the Quaternary history, processes of delta formation and present-day geohazards associated with the Fraser River delta.

Figure 3. Schematic of Fraser River delta morphology, structure, and stratigraphy (modified from Turner et al., 1996) larger image [GIF, 22.7 kb, 468 X 344, notice]

The stratigraphic sequence of the Fraser River Delta consists of: (1) thrusted, folded and faulted Tertiary and older sedimentary rocks, (2) ice-sculpted Pleistocene glacial deposits, (3) Glaciomarine turbidites of the latest deglaciation, and (4) modern Fraser River delta sediments which include all aspects of delta sedimentation processes (e.g., turbidity currents, levee, levee-overspill, debris flows, river bedload, hemipelagic deposition, tidal reworking) (Fig. 3). The modern delta can be grossly divided into bottomset, foreset and topset sediments. Some of the more significant surficial morphologic elements of the Fraser River Delta are: (1) extensive deltaic plains, underlain almost continuously with channel-fill deposits of sand (Monahan et al., 1993a&b), (2) active river channels constrained in their locations by dikes and jetties, (3) a 200 km² foreslope, ranging in water depths from 10 to 250 m and with slopes up to 23°, (4) erosive channels, gullies and sea valleys transecting the foreslope and maintained by sediment mass flow processes, and 5) an extensive prodelta; the entire southern strait is affected by sedimentation from the plume of the Fraser River, which includes an area probably well in excess of 1500 km² in addition to the delta proper.

A number of features have been identified from the geologic record which are interpreted to represent liquefaction and slope failure events, including, for example, sand boils, dykes, slumps and slides (Hamilton and Luternauer, 1983; Hamilton and Wigen, 1987; Pullan et al., 1989, 1998; Clague et al., 1992, 1998; Hart, 1993; Hart el al., 1992c, 1993; Mosher et al., 1994; Mosher, 1995; Mosher and Law, 1996; Mosher and Hamilton, 1998).

Ground motion amplification

Figure 4. Frequency spectra of ground response from Duvall M5.3 earthquake in May, 1996, comparing bedrock, firm ground (till) and northern edge of the Fraser River delta responses. The delta site shows amplification almost 6 times.

It has long been known that thick "soft soil" sites, such as the deltaic sediments of the Fraser River delta, are prone to ground motion amplification in certain freqield first order estimates of ground motion amplification effects and resonance ground frequencies (Shearer and Orcutt, 1986). It is possible that sites resonance effects may result from large shear wave velocity contrasts within the unconsolidated overburden, such as the Holocene-Pleistocene boundary (Hunter et al., 1998). As a rule of thumb, the natural period of a building structure is 0.1N, where N= the number of stories. For the Fraser River Delta, ground resonance periods due to large velocity contrasts associated with the Holocene-Pleistocene boundary would be in the range of multistory buildings, depending on overburden thickness. Such resonance effects would be added to an overall impedance contrast amplification effect of horizontal shear motion which might be attributed to shear wave velocity contrasts between ground surface and bedrock (Shearer and Orcutt, 1986).

Recent evidence of ground motion amplification in the Fraser River delta has come from seismograph recordings made of a moderate earthquake centered in Washington State, south of the survey area (Rogers et al., 1998) (Fig. 4). These data have shown that for thick soil sites, horizontal ground motion amplification in the range of 0.5 to 4 Hz can be 3 to 5 times that of bedrock, with even larger amplification over limited frequency ranges where Quaternary sediments are thinning (<50m) towards the northern edge of the delta .

Liquefaction

If a cohesionless soil is sufficiently loose (ie. contractive) and the static shear stress is greater than the large-strain undrained shear strength, flow-liquefaction is possible, if failure occurs. Flow liquefaction can be initiated with either dynamic or static loading and can result in rapid flow slides. If a cohesionless soil is exposed to rapid undrained cyclic loading, excess pore pressures can develop resulting in cyclic liquefaction which can lead to ground oscillations or lateral spreading. The channel-fill sediment facies, which underlies much of the surface of the modern delta, comprises the most liquefiable sediments (Watts et al., 1992).

Figure 5. Crossplot of vertical effective stress (v') vs coneresistance (qc) for four cpt sites from the southwestern margin of the Fraser River Delta. For comparison, empirical correlations showing the threshold boundaries between flow liquefaction and no flow liquefaction, as compiled by several authors, are shown.

Figure 5 shows qc data from a SCPT and three adjacent CPT's on the southwestern margin of the delta, plotted against effective overburden stress v'. The penetration resistance profiles generally fell well within the zone wherein flow-liquefaction is likely, based on empirical correlations developed by Sladen and Hewitt (1989), Robertson et al. (1992) and Ishihara (1993). Over the last ten years, research has related the shear wave velocities of cohesionless water saturated soils to liquefaction resistance during cyclic (earthquake) loading. A summary of recent findings and recommended shear wave measurement methodologies is given by andrus and Stokoe (1996). Empirically-developed assessment charts are available which relate surface earthquake shaking parameters to measured shear wave velocities of soils. Figure 6a shows the suggested potential liquefaction boundary in terms of peak horizontal ground acceleration and soil shear wave velocity (after andrus and Stokoe, 1996) for magnitude 6.9 7.0 earthquakes based on observational data. Shown also are guidelines indicating threshold shear wave velocities for 0.1, 0.2, 0.3, and 0.4g peak horizontal accelerations; cohesionless water-saturated soils with shear wave velocities less than theses values have high liquefaction potential. Figure 6b is a compilation of shear wave velocities vs depth for 75 regional sites in the survey area, from measurements made by surface refraction, borehole, and SCPT techniques.

Shown also are the threshold guidelines from figure 6(a) for the four values of peak horizontal site accelerations. Measured shear wave velocities plotting to the left of a guideline indicate liquefaction potential for that particular peak acceleration condition. These data suggest that liquefaction potential is high in the near-surface materials throughout the area. The depth extent of the potential liquefiable zone will depend on site amplification effects (amax), as well as soil type and condition

Christian et al. (1997) modeled the post liquefaction stability of the present foreslope of the delta using infinite slope stability analysis coupled with digital bathymetry and using engineering parameters determined from CPT and borehole results. In their most non-conservative analysis, the post-liquefaction large-strain shear strength is taken as 20% of the effective overburden stress. In this case, the static driving stresses exceeded the post liquefaction strength over much of the foreslope. The zone of likely mass-movement is shown in Figure 7, where the grey region depicts the zone possessing a factor of safety against sliding of less than 1.0.

Figure 6 Suggested liquefaction boundary from shear wave velocity vs. ground acceleration for a M6.9-7.0 earthquake (after andrus and Stokoe, 1996). Shear wave velocity measurements vs depth below ground surface with 4 example ground accelerations at which point liquefaction is likely to occur. larger image [GIF, 11.3 kb, 371 X 302, notice]

The Fraser River delta is a young, rapidly sedimented basin that occupies a significant area of the lower mainland of British Columbia. It lies in the highest seismic risk zone in Canada and has been and will continue to be subject to intense urbanization and industrialization. State-of-the-art geological, geophysical, and geotechnical technologies have been applied to investigate the stability of the delta in terms of its liquefaction potential and ground surface response in the event of an earthquake. This work includes understanding the geology, collecting evidence of past liquefaction and slope failures, measuring physical properties and modeling ground surface response and liquefaction potential.

The ultimate goal of scientific involvement with these data is to successfully forecast the consequences of an earthquake and not have to rely on hindcasting. In general, it is realized that the delta is complex and response to ground shaking will be determined by local factors as much as by the characteristics of the earthquake itself. Sediment thickness and local bedrock and Pleistocene surface geometry will affect ground shaking amplification. Amplification likely will be most significant where the unconsolidated sequence is less than 100 m thick, such as on the lateral margins of the delta, hence the need to understand the subsurface geology. Cone penetration and shear wave velocity analysis suggest that much of the delta is susceptible to liquefaction within the top 10 to 20 m below surface, given design earthquake parameters (in the absence of amplification and local effects). In areas of a free-face, such as at the break-in-slope of the delta, slope angles significantly increase the likelihood of post-failure large-scale deformations. The existence of large slump deposits at the base of the delta slope and noted in seismic sections of the foreset beds reflects the cause for concern regarding the cyclic ground performance, should a large earthquake strike the region.

Figure 7. Infinite slope stability analysis for post liquefaction conditions of the SW margin of the Fraser River Delta. Zones with a factor of safety (FS) of less than 1 are highlighted with dark grey. larger image [GIF, 69.6 kb, 432 X 334, notice]

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