Seismic regionalization in North America based on Average Regional Seismic Hazard Index

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Abstract
Cumulative Seismic Hazard Index (CSHI) is the logarithm of the sum of the seismic energies experienced at any location. Average Regional Seismic Hazard Index (ARSHI) is the regional average of Cumulative Hazard Index values normalized to 100 years. Both can be expressed in the same units as Mercalli intensity. ARSHI differs from previously proposed measures of seismicity in that it compensates for variations in the completeness of the historic record and for variations in the rate of attenuation of seismic energy with distance. More rapid attenutation of intensity with distance in the West than in the East reduces the difference in hazard between these regions. In North America between 30° and 50° N latitude, ARSHI varies from 8.44 in the Western Nevada to 5.28 in the Canadian Shield and Plains provinces. Since ARSHI is measured on a logarithmic scale, this is a range of a factor of over 1,000 in average energy flux. A trough of low seismicity between areas of high seismicity roughly coincides with areas of recent volcanic activity and high heat flow in parts of the Great Basin, the Columbia Plateau and adjoining areas. This low seismicity is interpreted as possibly being due to weakening of the crustal rocks by higher than normal temperatures. This interpretation would mean that low seismicity in this area is not evidence of a lack of tectonic activity. It is consistent with either a rising convection cell, a branch of the East Pacific Rise system underlying this area, or with igneous upwelling over an old subduction zone.

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