A historical summary of Geological Survey of Canada studies of earthquake seismology and geomagnetism

Abstract

The solid-earth geophysical sciences of seismology and geomagnetism have roots in Canada that predate the founding of the Geological Survey of Canada (GSC) in 1842. These sciences matured in the Dominion Observatory, which was formed in 1904, and came to the GSC when it merged with the Earth Physics Branch in 1986. For most of the past three decades, these two sciences have been closely linked in the federal government by a common administration and by jointly housed and staffed observatories.Knowledge of Canadian earthquakes dates from the time of first European settlement, in the 17th century in eastern Canada and in the 19th century in western Canada. Instrumental recording began at the end of the 19th century, but the early instruments were unable to detect most of the earthquakes occurring in Canada. An ability to locate and study Canadian earthquakes followed the installation of sensitive seismograph stations in the east in the late 1920's and in the west in the early 1950's; the impetus for these installations came from large damaging earthquakes in both regions. Assessments of seismic hazards were first made in the 1940's, were displayed on three successively more detailed seismic zoning maps up to 1985, and are currently under revision for 1995. The Standard Seismograph Network, installed in the 1960's as the Canadian contribution to a worldwide program and supplemented by regional station networks in the 1970's and 1980's, has greatly advanced our understanding of Canadian earthquakes. These networks are now being refurbished to form a modern, satellite-based, digital network that should stand the GSC s seismology program in good stead into the next century.Magnetic observations in Canada date from the time of the earliest European explorers, Cartier and Champlain. Other explorers made measurements of magnetic declination during expeditions in search of the Northwest Passage. A Toronto magnetic observatory was established in 1840, and the observations begun there started a series of secular change measurements that continues today as part of the repeat station network. Major magnetic surveys made by the Toronto observatory staff formed the basis of the magnetic charts of British North America until the 20th century. Shortly after its founding, the Dominion Observatory began a comprehensive magnetic survey of Canada, which has evolved into the 60-station repeat station program in the GSC today. The Dominion Observatory also expanded the permanent magnetic observatory network to a total of 13 stations by the late 1970's, which, under the current GSC program, are being refurbished with the latest generation of digital technology. A breakthrough in regional magnetic field modelling was made in the 1980's with the development of spherical cap harmonic analysis, which is now used for revisions of the Canadian Geomagnetic Reference Field. Magnetic disturbance forecasting was begun in the 1970's, and has recently taken on greater importance as the complexity of the technology at risk has increased.