Page 441 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

100 10
1 0.1 0.01
Queen Charlotte Fault
Cascadia Subduction Zone
◀Figure 13.1.2 Seismic activity in western North America. The nature of seismic activity along the Cascadia subduction zone and the Queen Charlotte fault. Note that seismic activity along the Queen Charlotte fault is characterised by a wide range of intensities—low-magnitude events (M = 3) occur frequently while large-magnitude events (M > 8) rarely occur. In contrast, the Cascadia subduction zone exhibits a distinctly different level of seismic activity characterised by infrequent large-magnitude (M > 8) events. The box (red colour) in the bottom-right corner of the graph indicates that earthquakes in the Cascadia subduc- tion zone exhibit magnitudes in the 8 to 9 range. [Reproduced
with the permission of Natural Resources Canada, 2011. Courtesy of the Geological Survey of Canada.]
Earthquakes per year M3
0.0012 3 4 5 6 7 8 9 Earthquake magnitude, M
Queen Charlotte Islands
Vancouver Island
Fault Break
Victoria
Seattle
Portland
(a) 1946, M 7.3
0 250 500 KILOMETRES
Anchorage
Fault Break
Prince Rupert Epicentre
◀Figure 13.1.3 Earthquake intensity and damage
in British Columbia. The magnitude of seismic activity generated by the two greatest earthquakes recorded
in British Columbia in the twentieth century: (a) 1946 and (b) 1949. Both earthquakes resulted in structural damage to buildings with little loss of human lives. [Used by permission of the Minister of Public Works and Government Services Canada; Natural Resources Canada, Geological Survey of Canada.]
Prince Rupert
Epicentre
Vancouver
S
n
a
k
e
R
.
Think of the fault plane as a surface with irregu- larities that act as sticking points to prevent movement, similar to two pieces of wood held together by drops of glue of different sizes rather than an even coating of glue. These small areas are points of high strain, known as asperities—when these sticking points break, they re- lease the sides of the fault.
If the fracture along the fault line is isolated to a small asperity break, the quake will be small in magnitude. As some asperities break (perhaps recorded as small fore- shocks), the strain increases on surrounding asperities that remain intact. Thus, small earthquakes in an area may be precursors to a major quake. However, if the break involves the release of strain along several asperities, the
Roman numerals refer to intensity of shaking on the Modified Mercalli Intensity Scale.
quake will be greater in extent and will involve the shift- ing of massive amounts of crust. The latest evidence sug- gests that movement along the fault occurs in a wavelike pattern, as rupturing spreads along the fault plane, rather than the entire fault surface giving way at once.
Human activities can exacerbate the natural pro- cesses occurring along faults in a process called induced seismicity. For example, if the pressure on the pores and fractures in rocks is increased by the addition of fluids (called fluid injection), then earthquake activity may ac- celerate. The extraction of fluids, especially at a rapid
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Victoria
Vancouver Seattle
Portland
VII VI
V
– Shaking very frightening
– Structural damage possible
– Shaking frightening to most – Damage rare
– Felt by all
– No damage
– Felt by some
(b) 1949, M 8.1
IV
Chapter 13 Tectonics, Earthquakes, and Volcanism 405