A preliminary volcanic ash fall susceptibility map of Canada

Volcanic ash has the potential to impact the economy and human health in many ways, including interfering with air travel, damaging machinery, damaging crops, and contaminating water supplies. We present a semi-quantitative 1:12 million scale map of volcanic ash fall susceptibility in Canada (Figure 1), which shows the locations, types, and ages of 320 Canadian volcanoes with the potential for future eruptions, and four zones of estimated annual ash fall probability. Hazards other than volcanic ash are assumed to affect areas smaller than the symbols used to depict the volcanoes; therefore, although areas susceptible to all other volcanic hazards are encompassed by this map, the map only explicitly delineates areas susceptible to volcanic ash hazards. Four elliptical volcanic Ash Fall Hazard Zones (A, B, C, and D) are defined around Canadian and American volcanoes that pose high ash hazards for Canada, and then merged to form regional ash hazard zones. For the two outermost zones, C and D, the shape, size, orientation, and placement of ash fall hazard ellipses are based on ash distribution data compiled for the six largest ash fall events in Canada during the last 10,000 years. Zone D is defined for hypothetical eruptions of Volcanic Explosivity Index (VEI) ?6 (mp;lt;"hugemp;gt;"), while Zone C is defined for hypothetical eruptions of VEI 5-6 (mp;lt;"largemp;gt;"). Volcanoes are placed at the leftmost foci of the ellipses, which are oriented eastwest for Canadian and Alaskan volcanoes, and northeast for volcanoes in Washington and Oregon. The distance from each volcano to the far margin of the hazard ellipse is estimated based on average maximum dispersal distances (Dx) for the compiled Holocene ash data. Annual ash fall probabilities for zones C and D are based on the number of huge and large eruptions that deposited significant ash in Canada during the last 10,000 years. Since there were 2 huge events, the annual probability of ash fall at some unspecified point within zone D is 1 in 5000. Since there were 2 huge events and 4 large events, the annual probability of ash fall at some unspecified point within zone C is 6 divided by 10,000, which is approximately 1 in 1700. For Ash Hazard Zone B (mp;lt;"mediummp;gt;" events, VEI 4-5), we assume that the apparent frequency-magnitude trend for Pleistocene and Holocene eruptions of VEI?5 within or on the border of Canada (1 huge event, 2 large events) is meaningful. By this reasoning, we would expect 4 medium eruptions. Therefore, we estimate 4 medium ash fall events within Canada during the last 10,000 years. For Ash Hazard Zone B, we would thus predict a total of 10 ash fall events (including the 6 events used to define Ash Hazard Zones C and D), leading to an ash fall recurrence interval of 1000 years, or an annual probability (of ash fall at some unspecified point within zone B) of 1 in 1000. The average maximum ash dispersal distance for Zone B is crudely estimated based on ash fall data from the 1980 Mount St. Helens eruption. Because actual data about Canadian eruptions smaller than VEI 5 are scarce, we have not estimated the number of Holocene events for Ash Hazard Zone A (mp;lt;"smallmp;gt;" events, VEI<4); we simply assume that the annual probability of ash fall (at some unspecified point within zone A) is higher than for Zone B (therefore, >1 in 1000). The maximum dispersal distance, Dx, for Zone A, is speculatively extrapolated from Zone B.

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