EPSC 185 Lecture Notes - Lecture 9: Seismic Hazard, Santa Susana Mountains, Mid-Ocean Ridge

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The San Andreas fault:
-Along much of the west coast, the plate boundary is a transform margin
-The San Andreas is a right-lateral strike-slip or transform fault
-Transform boundary between the pacific plate and the north american plate
—> pacific plate moves northwest
—> north american plate moves southeast (relative to fault)
-Some parts of the fault lock up and store energy
-Release it in abrupt motions, large earthquakes
-Other parts of the fault move smoothly
-Cause ground deformation, but only small earthquakes
-Over 1,2000 km long
-At least 16 km deep in places
-One of the most studied faults in the world
-Has produced some infamous earthquakes
Loma Prieta 1989
-The marina district of San Francisco was very hard hit
-Unconsolidated, water-saturated materials were liquefied and mobilized by the shaking
-The lower picture shows a “volcano” of liquified sand
Cascadia:
-In the pacific northwest, the tectonic regime is subduction related, rather than transform
as we have seen in California
-Here, there is evidence for a very large earthquakes over the last several thousand years.. the
most recent is 300 years ago
Quebec
-The st lawrence region has high levels of seismicity for a zone in the interior of a tectonic plate
-This seismicity may be related to old, aborted refs about 200 Ma ago
hat is an aborted or failed rift?
-Failed rifts are ancient to modern features where continental rifting began, but then
failed to continue
-Rifts are distinct from mid-ocean ridges, where new oceanic crust and lithosphere is
created by seafloor spreading
-In rifts, no crust or lithosphere is produced. If rifting continues, eventually a mid ocean
ridge may form, making a divergent boundary between two tectonic plates
-There are three main groups of theories that have been proposed to explain the spatial
occurrence of intraplate earthquakes: stress concentration, zone of weakness and high
heat flow
J- effects of earthquakes: aftershocks
-Aftershocks normally occur after a major earthquake
-There may be many thousands of aftershock events over the space for months or even
years
-Although their magnitudes generally decrease with time, aftershocks have potential to
cause significant damage to already weakened materials (e.g., rocks, soils, buildings,
power and gas lines)
Effects: liquefaction
-Occurs on sediments during earthquake shaking
—> Groundwater can move upwards due to the shaking
-Water lubricates contact between sediment grains
—> Weakens sediments
-Liquefaction has two consequences
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Document Summary

Along much of the west coast, the plate boundary is a transform margin. The san andreas is a right-lateral strike-slip or transform fault. Transform boundary between the paci c plate and the north american plate. > north american plate moves southeast (relative to fault) Some parts of the fault lock up and store energy. One of the most studied faults in the world. The marina district of san francisco was very hard hit. Unconsolidated, water-saturated materials were lique ed and mobilized by the shaking. The lower picture shows a volcano of liqui ed sand. In the paci c northwest, the tectonic regime is subduction related, rather than transform as we have seen in california. Here, there is evidence for a very large earthquakes over the last several thousand years the most recent is 300 years ago. The st lawrence region has high levels of seismicity for a zone in the interior of a tectonic plate.

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