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Canadian Underwriter, pp. 3-6, 1956-04-01
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A seismic probability map for Canada
Hodgson, J. H.
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BLDG
A SEISMIC PROBABILITY MAP
FOR CANADA
John H. Hodgson
Chicf, Division of Seismology Donzinion Obscruatory, Ottawa
HEPRIIITED FROM CANADIAN UNDERWRITER APRIL 1, 1956 EDITION
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Figure I
A SEISMIC PROBABILITY M A P FOR CANADA
reccnt months the Seismological ivision of thc Dominion Obser\ra- tory has reccivcd many lettcis ask- ing ahout c a r t h q ~ ~ a L c risks in various parts of Canada. This interest un- clo~~btcclly rcflects the fact that arch- itccts and enginecis have become dFvalc ol' the esistcnce of earthquake rids t h r o ~ ~ z h the National Building Code of Canada. T h e new Code, is- sued in 1953 by thc National Rescarch Chuncil, inakcs provision for thc forces clue to ea~thquakes and in- cludcs an Earthquake Plobability map.
This map is reproduccd as Fijiilrc 1. I t cli\.idcs Canada into zoncs, ac- cording to the damage which is to bc evpcctecl from future carthquakcs. Thcse range from Zonc 3, in which inajor daillagc is to be expected; to Zone
0.
in which no damage is anti-cipated. Zone 3 has been applied to two areas, thc coast ol' British
By JOHN H. HODGSON Chief, Division of Seismology D o m i n i o n O b s e r u a t o r ~ ~ , O t t a w a Columbia, and the Ottawa-St. Law- rence valley area.
T h e Zone designations used in the Canadian m a p are similar to those established fol the United States by the United States Coast and Geodetic Survey. Their map1 agrees with ours in the two Zone 3 areas. for this rat- ing has been applied to the adjacent sections of the United States. But the United States map also shows California and Nevada, in which so Inany serious earthquakes havc oc- curred, as Zone 3. Can it he true P~rblished b y permissioz of t h e A c t i n g De!)ut)l Minister, Defiartmcnt of M i n e s t?? Tech7lical Surveys, O t t a w a .
that the St. La~vrencc valley the coastal area of British Colunl are as potentially dangerous as C fornia? Most Canadians would qu tion this, for the total damage caused by earthquakes in Canada has been quite small, and thcre has been little loss of life.
Earthquake Intensity
If we a r e to compare t h e seismicity of different areas we must have some means of comparing the relative sizes of earthquakes. Until fairly recently it has been necessary to rate earth- quakes according to their effects, that is to say, the damage done or the alarm caused. The most recent scale2 devised for this pUSIJOSe is reproduced on the next page.
Scales based on c l a ~ ~ l a g e have n l~een completely satisfactoly. A ve larpc earthquake occurriny in a re- mote area may cause little damace, whereas a much smaller earthquake
Log E = 12
+
1.8MDalr~age sligl~t.
[aces. Lines o[ sight and level clistorted. Ol~jrrts tl~ro!\.r~ up\varcl into the air.
occurring undcr a city may be dis- a s t r o ~ ~ s , partic~~larly if thc city has illany buildings that arc poorly con- structed or that stand on poor ground. Similarly, the alarm which an earth- cl~iake causes, and the attendant amount of newspaper space dcvotecl to it, are no indication of its sizc.
T h c damage which an e a r t h q ~ ~ a k c will do to a particular ht~ilding dc~~encls on four factors:
(ii) T h c distance of the b~lilding from thc ccntrc of the carth-
the soil on .ivhich the building ) T h e type of building, and the care with lvhich it has hcc11 clesigncd and constructcd.
As a matter of interest this is about as much energy as that released by the atomic bomb dropped on Hiro- shima.
Now consider the point B, corres- ponding to a magnitude of 7.8 If a n earthquake of this magnitude oc- curs in a settled area it may be ex- pected to d o great damage. Many buildings will be damaged beyond re- pair, fissures may open in the ground, and underground pipelines such as water mains may be put out of serv- ice, with resultant fire hazard. From F i p r c 2 we see that a n earthquake of this magnitude would releasc lozG ergs, 10,000 times as much as is released by our threshold earthquake of magnitude 5.6.
T h e largest earthquake of the last 0 years occurred in Assam, in 1950. t had a magnitude of 8.6. This is quivalent to a n energy release of
ore than loz7 ergs, corresponding o the energy released by 100,000 of ur threshold earthquakes, or of a n qua1 number of atomic bombs.
T h e magnitude scale gives LIS a
method of comparing the intrinsic energy of earthquakes. T h e scale must be uscd with discretion, for it is a very compressed one. As we have seen, the difference in magnitude between a n earthquake which does only minor damage ancl the largest earthquake evcr recorded is only three scale divi- sions. Becausc the scale is logarithmic this small increase corresponds to a v c ~ y lalgc increase of 100,000 times in thc energy relcasecl.
T h c epicentres of i ccent Canadian
c a r t h q ~ ~ a k c s have been plotted on the E a ~ t h q u a k e Probability Map, with their magnitudes besicle them. How clo thcse maqnitltdcs compaie with thosc for California earthquakes? T h e maqnitudcs of both Canadian and Califolnian calthcluakcs havc hcen shown on the graph of Figure 2, the Canadian carthqi~akcs to the light, thc Califolnian carthcluakcs to thc Icft. Total damage cstilnates h a \ c becn givcn for the Califolnian shocl,s and for two of the Canadian oncs
I t is a p p a ~ c n t fiom the chait that. E~oln thc standpoint of magnitl~clc. Canadian ca1 thcluahcs have hcen about as scbcle as thosc lo1 Califolnia. T h c fact that thcy did so littlc dam- age has bccn a mattcr of good fol- t ~ ~ n c . Only thc two smallcst Canadian shoclts welc locatcd ncai citics. T h c Tcmishaming eai thclual,c \\as closc to a vely \.rcll consti~~ctccl town, built almost cntilcly on loch, and it did littlc damagc. T h c Coinwall Masscna shock, thc srnallcst of those consicle1- ed. clid $2,000.000 damage.
Only recent Canadian e a r t h q ~ ~ a k e s have been indicated on the map. However, studies by E. A. Hodgson5 show that the St. Lawrence vallev has a history of seismic activity dating back to the time of Jacques Cartier. T h e largest Canadian earthquake on record occurred in 1663; its epicentre was probably near the site of the 1925 shock. T o judge by contempor- ary rcports the earthquakc must have been at least as large as the San Francisco earthquake. Whole areas of forest were levelled by it, and landslides caused by the eaithquake dammed the Saint Maurice river be- low Shawinigan Falls, causing it to change its course.
T h e history for the west coast has been traced back to 1811 by W. G. Milne? H e finds that several earth- quakes with magnitudes of 7 or more, and a t least one with a m a g n i t ~ ~ d e probably greater than 7.5, occurred in the years prior to the installation of sensitive seismographs at Victoria. I t seems clear that large earth- quakes have been occurring in these two areas as far back as records are available. Earthquake dainage in Canada has been low, not because of the lack of earthquakes, but be- cause the earthquakes have not hap- pened close to the large cities and towns.
The Effect O f Distance
I t is obvious that the farther a building is from the centre of a n earthquake the less damage that building is likely to s ~ ~ s t a i n . I t is difficult to define this effect esactly, but the damage probably drops off about as thc sqllare of the distance. We may seek some examples from Canadian earthquakes.
T h c British C o l ~ ~ m b i a c a r t h q ~ ~ a k c of June 23, 1946', (magnitude 7.3), occurrcd under water, so that thc cffects at the ccntrc of thc shock coi~lcl not bc appraised. T h c ncarcst towns Ivcrc fiftccil miles from the c-piccntre ancl the damagc thcrc was mocleratc. Largc numbers of chim- neys \vcrc darnagccl with somc consc- cluent damage to roofs, ancl much plaster was knocked clown. Minor damagc was reported at distanccs of as much as 200 inilrs ancl the carth- quake was felt to distanccs of 500 rnilcs or morc.
T h e Queen C h a ~ l o t t e eaithcl~~ahc of August 20, 19 1-g6, (maqnitudc 8 1 \ bias fclt gcnclally a t clistances of as milch as 800 miles and alailncd iesidcnts of Plincc Georqc. n ~ o ~ c than 1-00 miles a\\ay. T h e clamaqc in the cpicentr a1 I egion \\as \,el y sc\ er c :
trees were telled a n slides occurred. Yet major damage at miles from the epice
I n thc St. Lawr of March 1, 19258, almost every buildin central zone was chimneys within epicentre were stone churches we molished. Even which normally were twisted out o
distant 80 miles from the epicentle, damage was confined to areas of bad ground. Minor damage was reported at distances in excess of 200 miles.
O n the basis of Canadian experi- ence, it appears that damage from large earthquakes is likely to be severe within 20 miles of the epicentre, a to be minor at distances of 100 mil some damagc may occur beyond t distance.
The Effect O f Soil And Site
H . F. Reid, who studied the rela- tionship between da
tion in the San Fran found that if the most solid rock was the damage on ma from 4.4 to 11.6, on 2.1 to 4.4, and o 1 to 2.4. These fi cordance with Can Particularly in easte
Inany towns are built on alluvium, soil type is the principal factor con- tlolling clamage. A seconcl important factor is the actual nature of the sitc. Espei icnce su~gcsts that a build- in? on the side of a hill is more subjcct to clamage than one built on thc level.
Earthquake-Resistant Construction
T h e f o ~ u t h fact01 - the nature of
the building and of its desiqn ancl construction - is a very important
onc. After thc southeln California c a ~ t h c l ~ ~ a h e of 1932, two enqinecls, K. V. S t ~ i n b i r ~ e g e ancl D. F . Moran of the Pacific Fiic Iiatiilg Bureau. nlade a \ ciy con~prehcnsive stitdy9 of
the damace causcd by thc eaith- cl~~akc". Thcy rcvic~v insurance prac- ticc and m a l x ~ccommendations bascd on t h c i ~ clctailccl examination of buildings of all soits.
They find that, in gcnclal, b~~ilcl- incs clcsiqned to ~csist carthclualtcs.
ith riqicl const~ uction and 1) ith
aclcquatc provision for thc cffccts of
\ cop\ o f t h e leport be p i ~ l c l ~ a s c d from thc S c c ~ e t a ~ v S r ~ > n ~ o l o q ~ c a l S o c ~ c t v of \ r n e ~ ~ c a U ~ I \ C I , I ~ ) o l C a l ~ l o ~ n ~ a , B c r b c l c ~ . C d l ~ l o ~ n ~ a
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