Earthquakes in Canada

Transcription

1 Earthquakes in Canada Maurice Lamontagne, Ph.D., ing. Geological Survey of Canada Natural Resources Canada 1

2 What is an Earthquake? P S P S P PS 2 2

3 Movement on a fault plane causes vibrations The larger the area on which there is slip (rupture) the larger the magnitude of the earthquake; the longer the duration of rupture/ the more intense the vibrations/ the longer shaking lasts 3

4 Earthquake Magnitude (M) Depends on the size of the reactivated fault surface; Large subduction-related earthquakes can exceed M8, almost all M8+ earthquakes occur at plate boundaries; Earthquakes below M2.5 may not be felt and can be only detected by nearby seismographs. For M4+, ground vibrations can be felt over large areas; If near the epicentre, M5 is the minimum magnitude to make light objects fall, M5.5 can cause some damage to masonry; In eastern Canada: Largest event was M7.0 (1663 Charlevoix earthquake, magnitude is estimated from historical records); 4 Almost all earthquakes are weaker than M5, which is below the threshold that might cause damage to engineered facilities.

5 Global Seismicity over 20 years: concentrated at plate boundaries Mw 6.0 Mw

6 Major earthquakes are related to movements at Plate Boundaries 6 6

7 Three types of Plate Boundaries TYIM 01 M.rgin Diy.rgenl Conv.. gent Tr'IInatorm,_ Motion Sp-... SlIbductlon ulle.'.1 $Iiding --, Destruct... (oceanic Coo """'" lithosphere Cfeatlld) (oceanic I/IhosptI&ra dastro,.ad) No map Iltect TCIP09"'phy \101(:,"1<:.alvity? RIdgelRif\,.. ''''''''. Volcanic arc '0 Comeivative (lithosphere neilher created 0+' No

8 Tectonic Context of Canada s West Coast Convergence 8 8

9 NRCan Seismograph Network detects earthquakes in Canada High density of stations in eastern Canada 9

10 Canadian National Earthquake Catalogue Authoritative inventory of earthquake location, magnitude, depth, etc; Based on written historical accounts and instrument records; NRCan seismograph network can detect all earthquakes greater than M3.0 anywhere in Canadian territory; Near some populated areas, the denser station network allows us to detect earthquakes as small as M1.0; If the earthquake can be felt, we can record and measure it. 10

11 Record of M6+ earthquakes in Canada and smaller events that were notable Significant earthquakes of Canada ,

12 Earthquakes in eastern Canada Earthquakes that can be felt in eastern Canada are rare; They occur mainly in well-defined zones, characterized by many tens of small (<M3) earthquakes annually; Most earthquakes that have caused any damage have occurred in known seismically active zones; Most earthquakes occur at depths between 5 and 25 km, and represent reactivation of old faults deep in the Precambrian Basement; Only one earthquake is known to have ever caused a surface fault rupture (Ungava); Faults mapped at the surface are not necessarily seismically active; The Canadian National Earthquake Catalogue and our knowledge are the basis for Seismic Hazard Maps. 12

13 Earthquake Distribution of Eastern Canada Hudsol1 Btl)' o ~M' 8ank. 0 Zone 199,7.2) 0, 00 o 1935 t'.2)' ~ Atlalltic Oceall o o Canadian Earthquakes kno'hl to hive caused $OITl' damage o Magnitude 4.5 to 8.6 MagnlbJde 3.0 to 4.5

14 Seismic Hazard Map Historical seismicity + Geological Model Québec Robuste Estimate of shaking that new buildings are required to withstand (under the National Building Code of Canada) Ottawa Montreal Toronto Increasing Hazard 14 Sa(0.2)

15 Map of all M5 and greater earthquakes recorded in the region (Jan Mar 2011) -- '... ~ * " {J ~/-./ ~.~'Z ~.Darllngton...- * km :, I' o 100' ~ oo Magnitude * * * ~ Natural Resources Ressources naturelles C,aIlada Canada Canada 15

16 Conclusions Large earthquakes (M8+) are expected near plate boundaries such as subduction zones; Eastern Canada is an "intraplate environment, well away from plate boundaries; Generally low-level of earthquakes in eastern Canada, but some well-defined zones are recognized as being more active (i.e. Charlevoix); Historically, most eastern Canada earthquakes occur in these well-defined zones, but have produced only minor damage; The region around Lake Ontario is one of low- to moderate seismicity, earthquakes in the Canadian Shield are even more rare; Historical observations and more recent continuous records of earthquake activity are used to develop seismic hazard maps that define the seismic provisions of 16 the National Building Code.