Geog 312: Natural Hazards
(return to tutorial assignments)
Earthquake exercise
"Are we prepared for the next great Cascadia earthquake?"

I. Review the Cascadia subduction zone (CSZ) earthquake scenario outlined in the "Province" for July 25th, 2000.

II. How much ground shaking? For how long?

If we assume that the CSZ is segmented as shown in Fig. 1 (a hypothetical situation), rupture of a single segment (Fig. 2a), will produce an earthquake with a magnitude (Mw) of about 8. If two neighbouring segments rupture, we might expect Mw~ 8.5 (Fig. 2b). If the entire CSZ ruptures, we might expect Mw ~ 9.1 (Fig. 2c).

The critical threshold for strong ground motion during an earthquake is usually considered by structural engineers to be a peak-ground acceleration (PGA) of 0.2 g (1 g = gravitational acceleration). Above this threshold structural failure of buildings is common. Frictional effects cause a rapid attenuation of earthquake energy and shaking with distance from the epicentre (Fig. 3). Different substrates cause variable amplification of these surface waves, and therefore differential shaking

Utilize the three earthquake ruptures shown in Fig 2 and the attenuation relationships shown in Fig. 3 to estimate the PGA in Tofino (on the west coast of Vancouver Island, about 50 km from the plate margin), and the Vancouver area (about 300 km from the plate margin) on:

a) bedrock
b) “soil” (i.e. unconsolidated deposits such as. raised beaches or deltaic sediments)
.

If sites on landfill are amplified X2 compared to sites on unconsolidated deposits, what would be the predicted PGA in the Vancouver area?
General conclusions?
III. What can individuals do? Start by thinking about possible scenarios….

The next CSZ quake (Mw=9.1) strikes at 7:30 a.m. on January 26, 2011. The shaking lasts for three minutes. Put yourself in the place of one of the following twenty-somethings, and BRIEFLY describe their main questions and actions from 7:33 - 7:35 a.m. - 2 minutes for each person. (Choose four characters from this list)

PAUL is a BC Hydro employee who works in a substation in Delta which lies on the main transmission line from the Lower Mainland to Vancouver Island. At 7:30 a.m. he is cycling to work from Ladner.

ANGELA, who is in her first semester at SFU, lives on campus. At 7:30 a.m. she is asleep.

ART lives in a trailer at the mouth of the Capilano River in West Vancouver. At 7:30 a.m. he is asleep.

ROD is an SFU Geography student who is skipping class to take advantage of the record snowfalls at Whistler to go snowboarding. At 7:30 a.m. he is 10 km south of Whistler village.

AMY is a nurse and single mother who works in Chilliwack and lives in Mission. At 7:30 a.m. she is leaving the hospital and heading home after a 12-hour shift.

MARY works in computer operations at Skytrain, which she uses to get to work. At 7:30 a.m. she is cooking breakfast in her new 13th-floor apartment in Yaletown.

BART is a train driver on the West Coast Express. At 7:30 a.m. his train is leaving the station at Pitt Meadows.

ROSE is having breakfast on the ferry from Swartz Bay to Tsawwassen. At 7:30 a.m. the ferry is entering Active Pass.

XENA is an RCMP constable in Richmond. At 7:30 a.m. she is re-routing traffic round an accident at the northern entrance to the Deas Island Tunnel.