Presented at the
Eighth Canadian Conference on Earthquake Engineering,
Vancouver, June 13-16 1999
Original paper published in Proceedings, Eighth Canadian Conference on Earthquake Engineering, Vancouver, 1999 p 77-82
Crossing the Border:
Assessing the differences between
new Canadian and American seismic hazard maps
Halchuk, Stephen1, and Adams, John1
1 Earthquakes Canada,
Geological Survey of Canada,
7 Observatory Cres., Ottawa K1A 0Y3
NOTE: some changes have been made to this web version of the text
since the original document was published.
ABSTRACT
The Geological Survey of Canada's new suite of seismic hazard maps will form the basis of seismic design codes in the year-2001 edition of the
National Building Code of Canada. The USGS has released a similar set of maps in 1996 for the 1997 NEHRP. While there is general agreement
in relative hazard levels, as shown by comparing hazard between Canadian and appropriate U.S. cities, hazard contours do not necessarily match
across the border. Differences in the definition of source zones, choice of attenuation relations, and incorporating Cascadia subduction
earthquakes all contribute to these cross-border differences.
INTRODUCTION
The Geological Survey of Canada (GSC) and the United States Geological Survey (USGS) have both recently completed a new generation of
seismic hazard maps. At the border, the two agencies have a common set of recorded historical seismicity, share an understanding of the
seismotectonics, and agreement on the probability levels and ground motion parameters to be mapped. While there is some similarity in how the
seismic hazard model is constructed, the approaches differ in detail. For eastern Canada the GSC applied the Cornell-McGuire method to two
new seismic source models, one historical and one geological. In the eastern US the USGS employed spatially-smoothed representations of
historic seismicity (together with direct input for a few large earthquakes and a background source zone) to avoid using subjective source zones to
calculate hazard. Hence not all the hazard captured by the GSC's "geological" model (e.g. how often large earthquakes may happen in areas of
low historical seismicity) is represented in the USGS results. For western Canada the GSC used two source zone models but combined them with
a deterministic estimate for a repeat of the 1700 A.D. Cascadia subduction earthquake. This is very different from the USGS's incorporation of
Cascadia subduction earthquakes into its probabilistic model.
METHODS
The methods used by the GSC and the USGS have been well documented (e.g. Adams et al., 1995 and 1999a; Frankel et al., 1996). The GSC
applies the traditional Cornell-McGuire (e.g. McGuire, 1993) method of delineating source zones based on historic seismicity and/or regional
tectonics. Hazard is calculated with a customized version of the FRISK88 program (a proprietary product of Risk Engineering), which includes
epistemic uncertainty. The GSC has adopted four models - two sets of probabilistic source zones that attempt to capture the spectrum of
knowledge for seismicity and tectonics, a deterministic Cascadia model in southwestern Canada, and a newly-proposed probabilistic floor level for
the "stable" part of Canada (see Adams, 1999b). The hazard values from these models are combined in a "robust" fashion (Adams et al., 1995,
1999a), i.e. by choosing the highest value from the four models calculated at each point. The "robust" approach preserves protection in areas of
high seismicity while providing increased protection in regions of low historical seismicity that are geologically not unlikely to experience future
large earthquakes.
The USGS employs spatially smoothed representations of historical seismicity for different magnitude events in combination with data from
individual faults. Hazard is computed with a new suite of software developed by Frankel et al. (1996), with the assumption that earthquake
occurrence is Poissonian with time-independent probability. Probabilistic hazard is calculated without the use of subjective source zones.
Different magnitudes and completeness times were used to determine recurrence parameters in the western and eastern US. An adaptive
weighting was used to ensure that the rate of earthquakes within any calculated cell did not fall below the historic value, the final values maintain
the hazard in the areas of historic seismicity and provide some additional hazard to low seismicity regions.
Strong Ground Motion Relations
The different physical properties of the crust in eastern and western North America, and different types of earthquakes, require the use of different
strong ground motion relations. The Canadian and US choices are detailed below, with the references being given in Adams et al. (1999a) and
Frankel et al. (1996). In Canada, the GSC placed the transition from eastern to western attenuation approximately 400 km east of the Rockies
(near 106W at the border). In the United States, the USGS placed the boundary along the eastern edge of the Basin and Range province (near
114W). Hence, US sites east of this boundary are computed with eastern attenuation and can be expected to yield higher hazard estimates than
for adjacent Canadian sites.
| Region/earthquakes | Canada | United States |
| Eastern | Atkinson & Boore (1995) | Toro et al. (1993) + Frankel et al. (1996) |
| Western crustal | Boore et al. (1993, 1994) | Boore et al. (1994) + Sadigh et al. (1993) + Campbell and Bozorgnia (1994) |
| Western subcrustal | Youngs et al. (1997) @50 km depth | Youngs et al. (1997) @40 km depth |
| Cascadia subduction | Youngs et al. (1997) | Sadigh et al. (1993) + Youngs et al. (1997) |
COMPARISONS AT THE CANADA-US BORDER
In Table 1 we group selected Canadian and US cities we consider to have similar seismic hazard. Where we believe that each agency's model is
adequate, we provide both sets of results for PSA0.2 and PSA1.0 for a direct comparison. Site conditions used for the US calculations are slightly
firmer than for Canada (760 m/s vs 560 m/s). Therefore we have increased the USGS PSA0.2 values by 10% and the PSA1.0 values by 15% in
order to match their results to ours, factors we based on the NEHRP Fa/Fv ratios. The same factors were applied to the US values before we
contoured Figures 1-3. For a second comparison,