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Earthquake Hazards Mapping
Composite Relative Earthquake Hazard Map of
Greater Victoria - Expanded Map Legend
Geoscience
Map 2000-1.
Composite Relative Earthquake 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
Victoria is in one of the most seismically active areas of Canada. Vancouver
Island has experienced two large historic earthquakes, in 1918 (Magnitude= 7. 0) and 1946
(Magnitude= 7. 3; Rogers, 1998). The 1946 earthquake was the most damaging in western
Canada and caused minor damage in the Victoria area, which was 200 km from the epicentre
(Wuorinen, 1976). In addition, there is the potential for a very large (Magnitude ~9)
earthquake on the Cascadia subduction zone west of Vancouver Island (Rogers, 1998;
Hyndman, 1995). Clague (1996) has documented evidence for prehistoric earthquakes in
southwest British Columbia. The tectonic setting of the region and distribution of
significant historical earthquakes for the Victoria region are shown on the block diagram.
The effects of an earthquake are not only dependent upon the magnitude of the
earthquake and the distance from the source, but vary considerably due to local geological
conditions. The objective of this map is to show areas of Greater Victoria where the
earthquake hazard is likely to be increased due to the presence of potentially unstable
slopes, and soils susceptible to amplification of ground motion and/or liquefaction.
Although the timing, location and magnitude of earthquakes cannot be predicted, areas in
which the earthquake hazard is increased due to these factors can be mapped with varying
degrees of completeness using existing geological and geotechnical data.
This map (Map 1) is a composite earthquake hazard
map, and has been compiled from three other maps published as part of this investigation:
a relative liquefaction hazard map (Geoscience Map 2000-3a
Monahan et al., 2000a), a relative amplification of ground motion hazard map (Geoscience Map 2000-3b; Monahan et al., 2000b) and an
earthquake- induced slope instability hazard map (Geoscience Map
2000-3c; McQuarrie and Bean, 2000). The methodology of preparation of these source
maps is summarized below and simplified versions are included here as inset maps (Maps 2
to 4). However, for details of the assessment of earthquake hazards in the Victoria
area, and to determine the specific hazards that are likely to affect different areas, the
user should refer to the source maps and accompanying report.
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. It
should be used to help planners select areas for development, avoid geologically
vulnerable areas and prioritize seismic upgrading programs. This map does not replace
the need for site- specific geotechnical evaluations prior to new construction
or upgrading of buildings and other facilities.
This map flags areas of high hazard for planning purposes. However, the user
must refer to the more detailed liquefaction, amplification of ground motion amplification
and slope instability hazard maps noted above for details of the hazards potentially
present in an area.
A high hazard does not necessarily preclude land from a specific use. In these
areas more detailed engineering studies may be required, depending on the use proposed and
the specific hazards present, and higher costs may be incurred. A qualified professional
engineer or geologist should be consulted when making decisions related to this map. The
qualifications and limitations of this map are discussed in more detail below. |
GEOLOGICAL
MAPPING
The initial step in evaluating earthquake hazards was preparation of a
surficial geological map that shows the thickness and distribution of Quaternar
stratigraphic units (Geoscience Map 2000-2; Monahan and Levson,
2000). The map is based on data from borehole logs, engineering drawings for municipal
sewer and water lines, airphoto interpretation and large- scale topographic maps. In areas
where borehole data are sparse, the subsurface conditions had to be 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 are
shown on this map. Limited field checking was conducted. |
AMPLIFICATION
OF GROUND MOTION HAZARD MAP
(MAP 2; Monahan et al., 2000b)
Amplification of ground motion refers to the increase in the intensity
of ground shaking that can occur due to local geological conditions, such as the presence
of soft soils. The amplification hazard is estimated by assigning U.S. National Earthquake
Hazard Reduction Program (NEHRP; Building Seismic Safety Council, 1994) site classes to
each geological map unit defined above. The NEHRP site classes are defined primarily on
the basis of the average shear-wave velocity in the upper 30m of the underlying soil and
rock. This hazard is greatest in areas underlain by thick deposits of soft clay,
particularly where they are capped by peat and organic soils, and lowest where bedrock is
exposed (Monahan and Levson, 1997; Monahan et al., 1998, 2000). Consistent with
these hazard ratings, damage in the City of Victoria from the 1946 Vancouver Island
earthquake was concentrated in soft soil areas, and damage was the least where bedrock is
at or near the surface ( Wuorinen 1976).
However, several important qualifiers must be added to these hazard ratings:
The intensity of amplification on soft soils diminishes as the strength of
ground shaking (i.e. acceleration) on bedrock increases (Building Seismic Safety
Council, 1994). Consequently, amplification by soft soils may be minimal in the event of a
large earthquake in close proximity to the city (i.e. all areas will be shaken
strongly), but could be significant for a large earthquake tens of kilometres distant and
generating moderate shaking on bedrock in the city. However, a moderate shaking event is much
more likely to occur than a strong shaking event, so that areas assigned a high
amplification hazard on Maps 1 and 2 will be subjected to potentially damaging ground
motion much more often than areas with a very low hazard (see Maps 5 to 8 and
adjoining text, and the relative amplification of ground motion hazard map (Monahan et
al., 2000b) for more details).
The map does not address amplification of ground motion due to resonance.
The specific periods of ground motion that match the natural periods of a site can be
greatly amplified, and can be particularly destructive to structures whose natural periods
match those of the site* (Reiter, 1990; Rial, 1992).
The map does not address amplification due to topography, which can
exceed amplification due to soil conditions in some cases. High amplification is commonly
experienced on hills, ridges and the tops of cliffs (Finn, 1994; Somerville, 1998), which
are generally underlain in the Victoria area by dense soils and bedrock. Consequently, the
very low and low hazard ratings normally expected on bedrock may not apply on such
topographic features.
The map does not address amplification due to three-dimensional effects,
such as the focussing of energy due to the structure of the earth’s crust in the
region, which can be as great as amplification due to soil conditions (Somerville, 1998).
The amplification of ground motion hazard map reflects variations in
earthquake hazard due to soil conditions, which are applicable to most earthquakes that
will affect the region. Topographic and three-dimensional effects are more dependent on
the earthquake location and direction of seismic energy. |
LIQUEFACTION
HAZARD MAP
(MAP 3; Monahan et al., 2000a)
Liquefaction is the transformation that occurs when earthquake shaking causes a
sand to lose its strength and behave somewhat like a liquid. It commonly is one of the
major causes of damage in 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). In the Victoria area, the liquefaction
hazard is greatest in geologically young beach sands and in artificial fills. The latter
are common in port facilities and other shoreline areas (Monahan et al., 1998,
2000). Many sandy shoreline deposits along the east coast of Vancouver Island liquefied
during the 1946 Vancouver Island earthquake (Rogers, 1980) and non-engineered fills
perform poorly in earthquakes. However, the liquefaction hazard is generally not high in
the Victoria area. |
EARTHQUAKE-INDUCED
SLOPE INSTABILITY HAZARD MAP
(MAP 4; Monahan et al., 2000)
The slope instability hazard was assessed by estimating the intensity of seismic
motions that would cause a given slope to fail, considering the slope angle and typical
strengths of the geological units present. The slope instability hazard is greatest along
sea cliffs where sediments are exposed and along valleys and gullies deeply incised into
these deposits. Most rockslopes appear to be relatively stable, although the potential for
boulder ravelling or very small rock falls exists, and some areas of less stable bedrock
occur in the Mount Finlayson/Malahat/Goldstream River area (McQuarrie and Bean, 2000). |
COMPOSITE
RELATIVE EARTHQUAKE HAZARD MAP
(MAP 1)
Map 1 was prepared by combining the amplification of ground motion (most likely
cases), liquefaction and earthquake- induced slope instability maps (Monahan et al.,
2000a, b, and McQuarrie and Bean, 2000). The amplification of ground motion hazard is the
most widespread in the Victoria area. Consequently, the colour coding on this map
represents the amplification hazard. This map is simplified from the source amplification
map (Monahan et al., 2000b) by combining the very low and low hazard ratings, and
the high and very high hazard ratings to produce a three-class hazard rating system –
low, moderate and high. Areas of moderate and high liquefaction and slope instability
hazards are shown by cross hatched and shaded areas superimposed on the amplification map.
As with the amplification hazard ratings, high and very high hazard ratings have been
combined for the liquefaction and slope instability hazards.
Similarly, the three inset maps for each hazard (Maps 2 to 4) have been
simplified from the source maps by using a three-class rather than the five-class system
used on the source maps.
The different hazards have been shown together in this way because the hazard
rating scales are not directly comparable. The liquefaction and slope instability hazard
ratings both reflect the probability of liquefaction or slope failure – the stronger
the ground shaking required to cause failure, the lower the hazard rating. A moderate
rating indicates that liquefaction or slope failure could occur at the level of ground
shaking used for building design under the current building code. The amplification hazard
reflects the frequency that an area could be subjected to damaging ground motions.
However, the intensity of amplification on soft soils diminishes as the strength of ground
shaking (i.e. acceleration) increases. Consequently, at the level of ground shaking used
for building design under the current building code, amplification could be minimal, and
all areas could be shaken equally strongly. Furthermore, the three hazards mapped are not
cumulative in all cases. Although amplification may increase the probability of
liquefaction, liquefaction inhibits amplification, so that in areas with high liquefaction
and amplification hazards, ground motions will not be amplified if liquefaction is
triggered. |
APPROXIMATE
AMPLIFICATION FACTORS FOR
DIFFERENT GROUND MOTIONS
The relative amplification hazard ratings shown on Maps 1 and 2 are
generalized ratings and do not reflect the amplification hazard in all cases. In
particular, the amount of amplification due to soil conditions diminishes as the strength
of ground shaking (i.e. acceleration) increases. Maps 5 to 8 show how amplification
factors (not the actual amount of earthquake ground motion) can vary with different
strengths and periods of ground motion. See text below and amplification of ground motion
hazard map (Monahan et al., 2000b) for more details. |
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Click
on images below to view Maps 5 to 8 |
MODERATE EARTHQUAKE SHAKING
(0.1 g on bedrock - approximate onset of damage in buildings not designed to be
earthquake resistant) |
STRONG EARTHQUAKE SHAKING
(0.4 g on bedrock - current building code design acceleration for Victoria) |
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SHORT PERIOD GROUND MOTIONS
(typically affecting short buildings*) |
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LONG PERIOD GROUND MOTIONS
(typically affecting tall buildings*) |
QUALIFICATIONS
AND LIMITATIONS OF THIS MAP
- This map is intended for regional purposes only, such as land use and
emergency response planning, and cannot be used for site specific evaluations.
- Because of the techniques used to prepare the regional map, the uneven
distribution of the data on which the map is based, and the commonly gradational nature of
geological boundaries, all geologic map unit boundaries shown are approximate. The
geological units often include smaller occurrences of other map units, and unit boundaries
may change as more borehole data become available. Furthermore, the characteristics of
geological materials are variable and, therefore, parts of a particular map unit may
behave differently than the rest of the unit during an earthquake. Consequently, the
hazard at a specific site may be higher or lower than shown on the map.
- This map does not consider the effects of subaqueous failures that could occur
along the coastline or along the shores of lakes and might affect the slope above the
shoreline.
- Except where noted, this map does not consider man-made alterations to ground
conditions, whether the changes lower or increase the hazard at a site. For example, poor
soil sites may have been improved during construction, which will change the hazard rating
from that shown on the map. The assessment of slope instability hazard does not consider
artificial cuts and fills or other man made changes to the natural terrain. Nor does it
consider damage caused by construction procedures, settlements, failures of retaining
walls or instabilities caused by water, storm or sewer lines that could rupture during an
earthquake.
- Neither the stability of dams under earthquake shaking, nor the hazards related
to failure of dams or other man- made structures have been addressed.
- Only the larger fills of which the authors were aware are shown on the map. Other
areas of fill are present, and new areas of fill will be developed in t he future. The
properties of fills vary widely, from dense engineered fills with a very low liquefaction
hazard to loose fills with a very high liquefaction hazard. Insufficient data were
available to distinguish these, so to be conservative all fill units were assigned a high
liquefaction hazard, to indicate that such a hazard could be present. Non-engineered fills
historically perform very poorly in earthquakes.
- The National Earthquake Hazard Reduction Program (NEHRP) site classes for
susceptibility to amplification of ground motion (Building Seismic Safety Council, 1994)
which are based on the average response of various types of soils, have been used to
estimate the amplification of ground motion hazard. This approach has the following
limitations:
 | The map does not address amplification of ground motion due to resonance. If the
periods of a specific earthquake match the natural periods of a site, ground motions can
be greatly amplified, and this can be particularly destructive to structures whose natural
periods match those of the site Rial et al., 1992).
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| This map does not specifically address amplification of ground motion due to
topography. For example, topographic amplification of ground motions can occur on hills,
ridges and the tops of cliffs (Finn, 1994; Somerville, 1998).
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| Amplification due to three-dimensional effects, such as the focusing of energy by
buried bedrock structures is not considered (Somerville, 1998). |
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- This map shows the areas where the earthquake hazard varies due to
amplification of ground motion, liquefaction and earthquake -induced slope instability. However,
a low hazard rating on this map does not mean that there are no earthquake hazards because
all areas could be subjected to significant ground shaking in a strong earthquake. Furthermore,
the degree of amplification on soft soils diminishes as the intensity of ground shaking on
bedrock increases, so that in the case of a strong earthquake close to the city, little variation
in ground shaking may occur due to local soil conditions at short period ground
motions. However the city will be affected more often by more distant earthquakes that
generate moderate shaking on bedrock, so that areas with a high amplification hazard will
be subjected to potentially damaging ground motions more often than sites with a
low amplification hazard. This subject is discussed in more detail in the amplification
hazard map Monahan et al., 2000b).
- 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.
- For further information on the types of hazards that might affect specific areas, the
user should refer to the companion earthquake hazard maps by Monahan et al. (2000)
and McQuarrie and Bean (2000).
- This map cannot 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 shows
the relative natural hazard due to geological factors 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. |
OTHER SOURCES OF INFORMATION
For information on earthquake activity in British Columbia contact the Pacific
Geoscience Centre of the Geological Survey of Canada at P. O. Box 6000, Sidney, B. C., V8L
4B2. For more information on earthquake hazards in Western Canada see the references
listed below or visit the following web sites: http://www.em.gov.bc.ca/geology
(and click on surficial mapping) or http://www.pgc.nrcan.gc.ca.
For information on earthquake preparedness contact the B.C. Provincial Emergency Program
at P.O. Box 9201, Stn Prov Govt, Victoria, B.C., V8W 9J1 [phone (250) 952-4913 or
1-800-663-3456] website http://www.pep.bc.ca or
Emergency Preparedness Canada at P.O. Box 10000, Victoria, B.C., V8W 3A5 [phone (250)
363-3621]. |
REFERENCES
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.
Finn, W.D.L. (1994): Geotechnical Aspects of the Estimation and Mitigation of
Earthquake Risk; in Issues in Urban Earthquake Risks, Tucker, B.E., Erdik, M. and
Wang, C.H., Editors, Kluwer Academic Publishers, pages 35-77.
Hyndman, R.D. (1995): Giant Earthquakes of the Pacific Northwest, Scientific
American, Volume 273 Number 6, pages 50- 57.
McQuarrie, E.J. and Bean, S.M. (2000): Seismic Slope Hazard Map for Greater
Victoria; British Columbia, Ministry of Energy and Mines, Geoscience Map 2000- 3c.
Monahan, P.A. and Levson, V.M. (1997): Earthquake Hazard Assessment in Greater
Victoria, British Columbia: Development of a Shear-Wave Velocity Model for the Quaternary
Sediments; in Geological Fieldwork.
1996, Lefebure, D.V., McMillan, W.J. and McArthur, J.G., Editors, British
Columbia Geological Survey, Ministry of Employment and Investment, Paper 199 -1, pages
467-479.
Monahan, P.A., and Levson, V.M. (2000): Quaternary Geological Map of Greater
Victoria; British Columbia 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., Henderson, P., and Sy, A. (2000a): Relative
Liquefaction Hazard Map of Greater Victoria; British Columbia Geological Survey,
Ministry of Energy and Mines, Geoscience Map 2000-3a.
Monahan, P.A., Levson, V.M., McQuarrie, E.J., Bean, S.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.
Reiter, L. (1990): Earthquake hazard analysis, issues and insights; Columbia
University Press, New York, 253 pages.
Rial, J. A., Saltzman, N.G. and Ling, H. (1992): Earthquake-induced resonance in
sedimentary basins; American Scientist, Volume 80, pages 566-578.
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.
Somerville, P. (1998): Emerging art: earthquake ground motion; 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 1-38.
Wuorinen, V. (1976): Chapter 5; Seismic microzonation of Victoria: A social
response to risk; in Victoria: physical environment and development, Foster, H.D.,
Editor, Western Geographical Series, Volume 12, pages 185-219.
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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