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Ministry of Energy Mines and Responsible for Core Review
 

Relative Liquefaction Hazard Map of Greater Victoria - Map Legend

Geoscience Map 2000-3a - Relative Liquefaction Hazard Map of Greater Victoria


TRIM SHEETS (92B.043, 044, 053 & 054)


Scale 1:25,000 (approximate)

 

Patrick A. Monahan, P.Geo.1, Victor M. Levson, P. Geo.2,


Paul Henderson, P. Eng.3 and Alex Sy, P. Eng.3

 

1Monahan Petroleum Consulting, 2 British Columbia Geological Survey, 3 Klohn-Crippen Consultants Ltd.

 

 

INTRODUCTION

 

This map accompanies the "Relative Liquefaction and Amplification of Ground Motion Hazard Maps of Greater Victoria (Geoscience Maps 2000-3a and 3b): Report and Expanded Legend", by P.A. Monahan, V.M. Levson, P. Henderson and A. Sy.

 

Victoria is located in one of the most seismically active regions of Canada (Rogers, 1998; Clague, 1996). The effects of earthquakes are not only dependent upon the magnitude of the earthquake and the distance from the source, but they can vary considerably due to local geological conditions. These conditions can be mapped with varying degrees of completeness using existing geological and geotechnical data. It is the objective of this map to show those areas of Greater Victoria in which the earthquake hazard is potentially increased due to the presence of soils susceptible to liquefaction. This map accompanies four other maps relevant to earthquake hazards in Greater Victoria: a map of the Quaternary geology, on which this hazard map is based (Geoscience Map 2000-2; Monahan and Levson, 2000); a map that shows areas susceptible to amplification of ground motion (Geoscience Map 2000-3b; Monahan et al., 2000b); a map that shows areas susceptible to earthquake-induced slope instability Geoscience Map 2000-3c; McQuarrie and Bean, 2000); and a composite map that shows areas susceptible to the amplification of ground motion, liquefaction, and earthquake-induced slope instability hazards (Geoscience Map 2000-1; Monahan et al., 2000a). Results of this project are also discussed by Monahan et al. (1998).

 

For the proper use of this map, the accompanying report and expanded legend should be carefully read and understood. This map is intended for regional purposes only, such as land use and emergency response planning, and should not be used for site-specific evaluations. This map can be used with other criteria to help planners select potential areas for development, avoid geologically vulnerable areas, and prioritize seismic upgrading programs. However, this map does not replace the need for site-specific geotechnical evaluations prior to new construction or upgrading of building and other facilities. The qualifications and limitations of this map are discussed in more detail below and in the accompanying report and expanded legend.

GEOLOGICAL MAPPING

 

The initial step in the evaluation of the liquefaction hazard in the Victoria area was the preparation of a geological map that shows the thickness and distribution of Quaternary stratigraphic units (Monahan and Levson, 2000). Subsurface geological data on which the geological map is based include: over 5000 geotechnical borehole logs; several hundred water well logs; and nearly 3000 engineering drawings for municipal sewer and water lines. Geological map units were defined on the basis of these data, and in part coincide with the U. S. National Earthquake Hazard Reduction Program (NEHRP) soil classes for susceptibility to amplification of ground motion (Building Seismic Safety Council, 1994). Although the relative liquefaction hazard map is colour-coded as to the level of hazard, the geological map units are shown on the map and indicated by the appropriate label in each polygon (see legend). The geological map units are described in more detail in the accompanying report and expanded legend. Map unit boundaries were interpreted on the basis of the subsurface data airphotos, large-scale topographic maps, and published soil maps. In addition, limited field checking was conducted. In areas of poor subsurface control, the subsurface conditions are largely inferred from topographic and geomorphic evidence. To assist the user in determining the accuracy of the subsurface geological mapping, sites where subsurface geological data were available to us are shown on the maps.

LIQUEFACTION HAZARD MAPPING


 

Liquefaction is the transformation that occurs when earthquake shaking (or other disturbance) causes a saturated granular soil (e.g. sand) to lose its strength and behave like a liquid. Liquefaction can be one of the major causes of damage during an earthquake. The susceptibility of a site to liquefaction is dependent upon the depth to water table and the density, grain size and age of the underlying deposits (e.g. Youd and Perkins, 1978).

 

This map was prepared by assigning a hazard rating or range of hazard ratings to each geological map unit based on these criteria and a suite of quantitative analyses using a modified version of PROLIQ2 and similar analyses (Monahan et al., 1998). PROLIQ2 (Atkins et al., 1986) estimates the probability that liquefaction will occur at a site by combining Seed's method of determining liquefaction susceptibility (Seed et al., 1985) with the probabilistic seismic model developed for the National Building Code of Canada (National Research Council of Canada, 1995). However, the severity of surface disruption caused by liquefaction is a function of the depth and thickness of the liquefiable units. Consequently, Klohn- Crippen Consultants introduced the term "probability of liquefaction severity" (PLS), in which a depth weighting function is applied to the layer by layer probabilities of liquefaction calculated in PROLIQ2 (Levson et al., 1996a, b, 1998). PLS is defined by:

 

PLS=

S(WiHiPLi)
S(WiHi)
 

where Pli  is the probability of liquefaction at depth i (calculated from 0 to 20 metres), Hi is the layer thickness, and Wi is the weighting function that decreases linearly from 0.1 at the surface to 0 at 20 metres. Hazard ratings for specified PLS ranges are summarized in the following table.

 

Liquefaction Hazard Ratings

PLS (in 50 years)

Hazard Rating

>25%

very high

15-25%

high

5-15%

moderate

2-5%

low

0-2%

very low

 

Holocene sands (map units O5, S1, S2, S3 and S4) and modern anthropogenic fills (map units F, FR2, F T, FG, FC1 and FC2) are assigned high to very high hazard ratings. Consistent with these assignments, many sandy shoreline deposits on the east coast of Vancouver Island liquefied during the 1946 Vancouver Island Earthquake (Hodgson, 1946; Rogers, 1980), and non-engineered fills historically perform very poorly in earthquakes. The larger fills in the Victoria area are associated with port facilities and reclaimed gravel pits. For further details on the hazard assessment of fills, refer to section 5 of the qualifications and limitations of this map.

 

Map units with Capilano age sands and a typically shallow water table (map units G2, G3, O1, O3, O3a and O4) are assigned hazard ranges up to the moderate level. The liquefaction hazard in the other map units is very low to low.

 

QUALIFICATIONS AND LIMITATIONS OF THIS MAP

  1. This map is intended for regional purposes only, such as land use and emergency response planning, and should not be used for site specific evaluations.

  2. The map is based on interpretations of borehole records, the approximate locations of which are shown on the map. Where borehole data are scarce, subsurface conditions had to be inferred from topographic and geomorphic evidence.

  3. The boundaries of most map units are gradational, particularly in the Victoria area due to the extreme irregularity of the bedrock surface. For these reasons, map unit boundaries are approximate, may enclose smaller occurrences of other map units, and are subject to revision as more borehole data become available. Furthermore, geological materials are variable, and deposits of a map unit may locally have unusual properties. Consequently, the hazard at a specific site may be higher or lower than shown on the map.

  4. This map does not fully address man-made alterations to ground conditions whether the changes decrease or increase the hazard at a site. Poor soil sites may have been improved during construction, which will change the hazard from that shown on the map.

  5. Only the larger fills of which the authors were aware are shown on this map. Other areas of fill are present, and new areas of fill will be developed in the future. The properties of fills vary from dense engineered fills with a very low liquefaction hazard to loose fills with a very high liquefaction hazard. Because these could not be distinguished on a regional basis with the data available, all fill units were assigned a high to very high hazard, to indicate that such a hazard could be present. Non-engineered fills historically perform very poorly in earthquakes.

  6. The stability of dams under earthquake shaking, and hazards due to the failures of dams or other man-made structures have not been addressed.

  7. This map shows areas where the earthquake hazard is potentially increased due to liquefaction only. The amplification of ground motion and earthquake-induced landslide hazards are addressed on accompanying maps (Monahan et al., 2000b, and McQuarrie and Bean, 2000, respectively). However, a low hazard on these maps does not mean freedom from earthquake hazards, because all areas could be subjected to significant ground shaking during an earthquake. Furthermore, other earthquake hazards, such as tsunamis, land subsidence and ground rupture are not addressed on this or any companion maps published as part of this investigation.
  1. This map can not be used to directly predict the amount of damage that will occur at any one site because many other factors, such as building design and construction details, must be considered. The map in no way shows how different types of buildings or other man-made structures will perform during earthquakes. This map can be used to estimate the relative natural hazard due to liquefaction susceptibility alone.

ACKNOWLEDGMENTS

 

This project received funding from the Capital Regional District, the Geological Survey of Canada, the British Columbia Resources Inventory Committee, Corporate Resources Inventory Initiative, and the Joint Emergency Preparedness Program. The authors also acknowledge the wealth of geological and geotechnical data and other assistance provided by the numerous agencies and individuals listed by Monahan et al. (2000). In particular, the authors acknowledge the assistance of G. C. Rogers, J. Cassidy, R. Lloyd, M. Williams, R. Gibbs, B. Harding, B. Kerr and J. Valeriote. Cartography by C. Spicer and G. Letham at AXYS Environmental Consulting Ltd.

REFERENCES

 

Atkinson, G.M., Finn, W.D.L. and Charlwood, R.G. (1986): PROLIQ2 - A computer program for estimating the Probability of Seismic Induced Liquefaction including both Areal and Fault Zones; Department of Civil Engineering, University of British Columbia.

 

Building Seismic Safety Council (1994): NEHRP recommended provisions for seismic regulations for new buildings Part I - Provisions; Federal Emergency Management Agency, Washington, D.C., 290 pages.

 

Clague, J.J. (1996): Paleoseismology and seismic hazards, southwestern British Columbia; Geological Survey of Canada, Bulletin 494, 88 pages.

 

Hodgson, E.A. (1946): British Columbia earthquake; Journal of the Royal Astronomical Society of Canada, Volume 40, pages 285-319.

 

Levson, V.M., Monahan, P.A., Meldrum, D.G., Matysek, P.F., Gerath, R.F, Watts, B.D., Sy, A., and Yan, L. (1996a): Surficial geology and earthquake hazard mapping, Chilliwack, British Columbia (92G/ 1& H/ 4); in Geological Fieldwork 1995, B.M. Grant and J.M. Newell, Editors, British Columbia Geological Survey, Ministry of Energy, Mines and Petroleum Resources, Paper 1996-1, pages 191-203.

 

Levson, V.M., Monahan, P.A., Meldrum, D.G., Sy, A., Yan, L. Watts, B.D., and Gerath, R.F (1996b): Preliminary Relative Earthquake Hazard Map of the Chilliwack Area showing areas of relative potential for liquefaction and/or amplification of ground motion; British Columbia Geological Survey, Ministry of Employment and Investment, Open File, 1996-25.

 

Levson, V.M., Monahan, P.A., Meldrum, D.G., Watts, B.D., Sy, A., and Yan, L. (1998): Seismic microzonation in the Pacific Northwest, with an example of earthquake hazard mapping in southwest British Columbia; in A Paradox of Power: Voices of Warning and Reason in the Geosciences, C.W. Welby and M.E. Gowen, Editors, Geological Society of America, Reviews in Engineering Geology XII, pages 75-88.

 

McQuarrie, E.J. and Bean, S.M. (2000): Seismic slope hazard map for Greater Victoria; British Columbia Geological Survey, Ministry of Energy and Mines, Geoscience Map 2000-3c.

 

Monahan, P.A., and Levson, V.M. (2000) Quaternary Geological Map of Greater Victoria British Columbia Geological Survey, Ministry of Energy and Mines; Geoscience Map 2000-2.

 

Monahan, P.A., Levson, V.M., McQuarrie, E.J., Bean, S.M., Henderson, P., and Sy, A. (1998): Seismic microzonation mapping in Greater Victoria, British Columbia, Canada; in, Geotechnical Earthquake Engineering and Soil Dynamics III, P. Dakoulas, M. Yegian, and R.D. Holtz, Editors, American Society of Civil Engineers, Geotechnical Special Publication No. 75, pages 128-140.

 

Monahan, P.A., Levson, V.M., McQuarrie, E.J., Bean, S.M., Henderson, P., and Sy, A. (2000a): Relative Earthquake Hazard Map of Greater Victoria, showing areas susceptible to amplification of ground motion, liquefaction and earthquake- induced slope instability; British Columbia Geological Survey, Ministry of Energy and Mines, Geoscience Map 2000-1.

 

Monahan, P.A., Levson, V.M., Henderson, P., and Sy, A. (2000b): Relative Amplification of Ground Motion Hazard Map of Greater Victoria; British Columbia Geological Survey, Ministry of Energy and Mines, Geoscience Map 2000-3b.

 

National Research Council of Canada (1995): National Building Code of Canada - 1995; National Research Council of Canada, Ottawa, 571 pages.

 

Rogers, G.C. (1980): A documentation of soil failure during the British Columbia earthquake of 23 June, 1946; Canadian Geotechnical Journal, Volume 17, pages 122-127.

 

Rogers, G.C. (1998): Earthquakes and earthquake hazard in the Vancouver Area; in Geology and Natural Hazards of the Fraser River Delta, British Columbia, J.J. Clague, J.L. Luternauer, and D.C. Mosher, Editors, Geological Survey of Canada, Bulletin 525, pages 17-25.

 

Seed, H.B., Tokimatsu, K., Harder, L.F., and Chung, R.M. (1985): Influence of SPT procedures in soil liquefaction resistance evaluations Journal of Geotechnical Engineering, Volume 111, pages 1425-1445.

 

Youd, T.L. and Perkins, D.M. (1978): Mapping liquefaction-induced ground failure potential. Journal of Geotechnical Engineering, Volume 104, pages 433-446.