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