Module 11 Flashcards
Lower Mainland BC
Refers to lowlands surrounding Vancouver, bounded by the Coast Mountains to the north, the Cascade Mountains to the east, the Chuckanut Hills to the south and the Strait of Georgia to the west
-Home to the Fraser River floodplain, delta, and estuary
-Canada’s third largest urban region; settled for at least 9,000 years (Groulx & Mustard 2004)
In 2011, 2,590,921 people lived in this region (59% of total population in B.C.)
Home to 16 of 30 of the province’s largest population centres
-does not count mountain systems
Lower mainland soil type
unconsolidated and saturated, very young
Possible hazards in Lower Mainland:
Endogenic:earthquake, tsunami
Exogenic: river flooding, mass movement processes, rock slides, avalanches
Endogenic processes
originate within the Earth and are driven by heat from pressure and radioactive decay
Tectonic processes responsible for earthquakes, volcanoes, tsunami
Exogenic processes
operate at the Earth’s surface and obtain their energy from the Sun and from gravity
Glacial, fluvial, and atmospheric processes contribute to mass wasting (landslides, avalanches), flooding
Geological History of Lower Mainland
Tectonic: the current tectonic regime (subduction) has been stable over the last 200 million years; volcanic activity dates back to 10,000 years
Glacial: maximum extent of glaciation was achieved 15,000 years ago; large ice sheets disappeared by 10,000 years ago
Fluvial: deposition of the Fraser River delta and floodplain has occurred over the past 10,000 years
Tectonically active coastline
-many moderate level magnitude
. Vancouver island is rising because of tectonic compression and setting the stage for a megathrust earthquake
-subduction environments also associated with volcanoes(strato)
-mount baker biggest in area(ash fall would effect Vancouver area)
Glacial:
.steep, deepended, valley slopes from Pleistocene glaciation
.progradation of shoreline creates delta
Tectonic Processes
Big subduction one is Juan De Fuca Plate subducting under North American Plate
Earthquake Hazards (3) crustal settings
Earthquakes in the Lower Mainland occur in three different crustal settings:
Continental crust earthquakes: occur at shallow depths down to 40 km; relatively common and usually of lower magnitude but can be in the range of 6-7.7 magnitude
Oceanic crust earthquakes: at greater depth within the subducting oceanic crust; relatively common and usually are relatively high-magnitude (6.5-7 magnitude)
Megathrust earthquakes: occur on the subducting boundary are rare and large, usually magnitude 8-9; last subduction zone earthquake occurred on January 26, 1700
The Lower Mainland is primed for ______ ground shaking
amplifying
-made mostly of unconsildated sediments
Largest secondary hazard of an earthquake in the lower mainland for the urbanized areas is…
Not many Exit/escape routes from the alpine valleys
- earthquakes can initiate mass movements on these escape routes
- high chance of debris flows because of torrential rainfall swells as clouds move over mountains
The Lower Mainland is comprised of thick, soft, deltaic sediments prone to ground-motion amplification in certain frequency ranges
Amplification is estimated to be up to six times greater in the Fraser River sediments than in surrounding bedrock (Rogers et al. 1998)
Liquefaction is also possible due to the loose, water-saturated sands and muds of the Fraser River
When sediments liquefy the ground may subside irregularly causing buildings to tilt and/or collapse
Risk of Avalanche?
- coastal location means wet snow which makes avalanches worse
- high prevalence of slab like avalanches
Volcanic Hazards?
Most risk comes from ASHFALL
-Mount baker is the most dangerous because of prevailing winds
Pay attention to volcanoes at northern string above
- HIGH risk of Lahars
- Mount Garibaldi and Mount Meager(just north of fraser lowland)
Almost _____ of the urbanized region of the Lower Mainland is built on a floodplain
Almost one-half of the urbanized region of the Lower Mainland is built on the floodplain and delta of the Fraser River
These sediments were continuously deposited over the past 10,000 years following deglaciation
The sediments include fine sand, silty sand, and silt (as a result are associated with seismic hazards, and form rich agricultural lands)
Problem with dykes?
Dykes have been used to prevent flooding
BUT
-Once you separate the river from its floodplain/sediment supply
-Little sediment is now being deposited at delta front because of dykes along the Fraser River, leading to erosion.
-cutting it off means the delta subsides instead of rises bringing it farther into flood zones
Even a __ rise in sea level would have a catastrophic effect
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