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Ministry of Energy Mines and Responsible for Core Review
 

Relative Amplification of Ground Motion Hazard Map of Greater Victoria - Expanded Map Legend

Geoscience Map 2000-3b.

Relative Amplification of Ground Motion 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


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

 

INTRODUCTION

 

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 the Relative Amplification of Ground Motion Hazard Map of Greater Victoria (Map 1) to show those areas of Greater Victoria in which the earthquake hazard is increased due to the presence of soils susceptible to amplification of ground motion.  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 liquefaction (Geoscience Map 2000-3a; 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 and Levson (1997) and Monahan et al. (1998).

 

This expanded legend and the accompanying report should be carefully read and understood to properly interpret the Relative Amplification of Ground Motion Hazard Map of Greater Victoria (Map 1). The 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 can be used with other criteria to help planners select potential areas for development, avoid geologically vulnerable areas, and prioritize seismic upgrading programs. However, the map does not replace the need for site-specific geotechnical evaluations prior to new construction or upgrading of buildings and other facilities. The qualifications and limitations of the map are discussed in more detail below and in the accompanying report.

GEOLOGICAL MAPPING

 

The initial step in the evaluation of the relative amplification of ground motion 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) site classes for susceptibility to amplification of ground motion (Building Seismic Safety Council, 1994). Although the relative amplification of ground motion 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. The geological map units are described in this expanded legend and in the accompanying report. 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 with little or no subsurface data, 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.

AMPLIFICATION OF GROUND MOTION HAZARD MAPPING

 

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. In the Victoria area, the amplification hazard rating for each geological map unit is estimated primarily on the basis of the NEHRP site classes for susceptibility to amplification (see Table 1) and are shown on the legend of this map. The NEHRP site classes are defined primarily on the basis of the average shear-wave velocity in the upper 30 metres (Building Seismic Safety Council, 1994). Shear-wave velocity data were derived from 15 seismic cone penetration tests (SCPTs) and 4 spectral analysis of surface wave tests (SASW) in the Victoria area. These techniques are described by Robertson et al., 1992 and Stokoe et al., 1994, respectively. The shear-wave velocity data were used to develop a shear-wave velocity model for the principal Quaternary geological units, so that the average shear-wave velocity in the upper 30 metres could be estimated at other sites where such data were not available (Monahan and Levson, 1997).

 

On the basis of these criteria, the amplification hazard varies from very low, where bedrock is exposed (unit R1*), to high where soft clay is present (units C2, C5, O1 and O5). The assigned hazard rating extends to very high in units where peat more than 3 metres thick occurs at the surface (Map 1; Monahan et al., 1998). Consistent with these hazard ratings, most damage experienced inVictoria during the 1946 Vancouver Island earthquake was concentrated in areas underlain by soft soils, and damage was the least where bedrock is near or at the surface (Wuorinen, 1974, 1976).

 

TABLE 1

Categories for Soil Susceptibility to Amplification (definitions from Building Seismic Safety Council (1994); descriptive susceptibility ratings from Klohn-Crippen Consultants Ltd., 1994)

Site Class

General Description

Definition (Vs30=average shear-

 

wave velocity in upper 30 m,

 

m/sec)

Susceptibility Rating

A

Hard rock

Vs30>1500

 

Nil*

B

Rock

760<Vs30<1500

 

Very Low*

C

Very dense soil and soft rock

360<Vs30<760; or Vs30>760 m/sec

 

where >3 m of soil over bedrock

Low

D

Stiff soils

180<Vs30<360

 

Moderate

E

Soft soils

Vs30<180; or >3 m silt and clay with

 

 plasticity index >20, moisture content

 

 >40%, and undrained shear strength

 

<25 kPa

High

F

Peats or highly organic clays

Peat thickness > 3 m

Very High (?)

* Amplification on rock sites due to topography can be significant, and can exceed amplification due to soils in some cases.

Several important qualifiers must be added to these hazard ratings:

 

1. Variation in amplification levels:  The intensity of amplification on soft soils diminishes as the strength of ground shaking (i.e. acceleration) increases (Building Seismic Safety Council, 1994). This decrease is more pronounced for short period ground motions, which typically affect short buildings, than for long period ground motions, which typically affect tall buildings such as high rises (Note: The critical period of ground motion for a specific building or building type should be determined by a qualified structural engineer.) (see Maps 2, 3, 4 and 5 and adjoining text, and the accompanying report and expanded legend for more details). For example, at ground shaking levels of 0.1 g on bedrock (0.1 g is acceleration equal to 10% of the force of gravity, and approximately the onset of damage in buildings not designed to be earthquake resistant; bedrock refers to NEHRP site class B), short period ground motions can be amplified by a factor of 2.5 on soft soils (i.e. 0.25 g; Map 2). However, at ground shaking levels of 0.4 g on bedrock (0.4 g is the current building code design acceleration for Victoria; National Research Council of Canada, 1995), amplification of short period ground motions due to soft soils is minimal, and all areas will be shaken strongly but more or less equally (i.e. ~0.4 g; Map 4). Consequently, amplification on soft soils in Victoria may be minimal for short period ground motions 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. For example in the Victoria area, shaking of 0.1 g on bedrock is more than ten times as likely to occur as shaking of 0.4 g on firm ground. Thus, areas assigned a high amplification hazard on Map 1 will be subjected to potentially damaging ground motions much more often than areas with a very low hazard. For long period ground motions, amplification due to soil conditions also diminishes as the strength of ground motions increase, but can still be significant at 0.4 g (Maps 3 and 5).

 

This 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 (Note: The critical period of ground motion for a specific building or building type should be determined by a qualified structural engineer.) (Reiter, 1990; Rial, 1992).

 

2. Topographic effects:  This map does not show areas susceptible to 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; Sommerville, 1998), which are generally underlain in the Victoria area by thin and/or dense soils and bedrock (units R1, R1/2, R2, and T). Consequently, the very low and low hazard ratings assigned to these map units may not apply on such topographic features. Amplification due to topography is poorly understood and not readily quantified at this time.

 

3. Three-dimensional effects:  This map does not consider 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).

 

This 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.

 

Click on images below to view Maps 2 to 5

MODERATE EARTHQUAKE SHAKING

STRONG EARTHQUAKE SHAKING

SHORT PERIOD GROUND MOTIONS

LONG PERIOD GROUND MOTIONS

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. 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 include 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 the 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 to loose fills with a potentially high amplification hazard.
  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 increased due to amplification of ground motion. However, a low hazard on this map does not mean freedom from ground shaking due to earthquakes, because all areas could be subjected to significant ground shaking during an 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 (i.e. acceleration) 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 shown with a high amplification hazard here will be subjected to potentially damaging ground motions more often than sites with a low amplification hazard. This subject is discussed in more detail above under "Variation in amplification levels" and illustrated in Maps 2, 3, 4 and 5.
  8. The amplification of ground motion hazard has been estimated on the basis of 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. Thus, variation in the amplification hazard should be expected within in any geological map unit. This map does not address:
    • amplification of ground motion due to resonance, which can be particularly destructive to structures whose natural periods match those of the site (Reiter, 1990; Rial, 1992);
    • amplification of ground motion due to topography, by which ground motions can be amplified on hills, ridges and the tops of cliffs (Finn, 1994; Somerville, 1998); and
    • amplification due to three-dimensional effects, such as the focussing of energy due to the structure of the earth’s crust in the region (Somerville, 1998).
  1. This map addresses only the amplification of ground motion hazard. The liquefaction and earthquake-induced landslide hazards are addressed on accompanying maps (Monahan et al., 2000b, and McQuarrie and Bean, 2000, respectively). Other earthquake hazards, such as tsunamis, land subsidence and ground rupture are not addressed on this or any other maps published as part of this investigation.

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 can be used to estimate the relative natural hazard due to the susceptibility to soil amplification alone.

ACKNOWLEDGMENTS

 

This project received funding from the Capital Regional District, the Geological Survey of Canada, the British Columbia Resources Inventory Committee and 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 numerous agencies and individuals, a complete listing of whom is provided in the accompanying report and expanded legend. 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

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.

 

Klohn-Crippen Consultants Ltd. (1994): Preliminary seismic microzonation assessment for British Columbia; Prepared for Resources Inventory Committee, Earth Sciences Task Force, 109 pages.

 

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. (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 1997-1, pages 467-479.

 

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 Liquefaction Hazard Map of Greater Victoria; British Columbia Geological Survey, Ministry of Energy and Mines, Geoscience Map 2000-3a.

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

 

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.