Lesson 21

Lesson 21 - Module E Planetary Engineering Mesozoic...

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Module E. Planetary Engineering: Mesozoic Tectonics Lesson 21: Earthquakes and Cascadia Introduction In Vancouver we live on an active tectonic margin, a subduction zone -- and this margin is seismically very active. As such, we are very vulnerable to the effects of both earthquakes and other related phenomena, such as tsunami. Thus it is important that we are knowledgeable about and aware of earthquakes, as they are part of our lives in this region. Figure E-81. Port Alberni Following the Tsunami Triggered by the 1964 M9.2 Earthquake in Alaska Results of Earthquakes Most earthquakes are related to displacement along a fault. In the cartoon below, stresses are acting on a portion of the Earth’s crust that cause that body of crust to first of all deform (bend) and then finally to break, with one piece moving past the other piece, along a fracture surface or fault. The release of energy when the block of crust actually breaks is given off in the form of seismic energy (the " earthquake "). (top) Movements within the Earth’s crust cause stress to build up at points of weakness, and rocks to deform. When the stress finally exceeds the strength of the rock, the rock fractures along a fault, often at a zone of existing weakness within the rock. The stored energy is suddenly released as an earthquake. (bottom) Fence parted by an earthquake at a fault in Marin County, California. Surface Rupture The photo below shows a grassy slope in New Zealand where the block on the left has been uplifted and moved away from the block on the right during a major earthquake. This disturbance of the surface is called a surface rupture .
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Note the uplift to the left and subsidence to the right. Photo by L. Homer. The cartoon on the left below shows a fault surface that is dipping, with the hanging wall block having moved down with respect to the footwall block. Recall from a previous Lesson that this would be a normal fault . The main release of energy is focused at some depth along the fault surface referred to as the focus . The point on the surface directly above the focus is called the epicentre . When you hear about an earthquake in the media you will commonly hear the location of the earthquake described in terms of where the epicenter was and what the depth to the focus was. Figure E-84. Earthquakes and Normal Faults (left) Cartoon of an earthquake caused by a slip in a normal fault. (right) Hebgen Lake, Montana following an earthquake in 1959. Photo by J.R. Stacy, USGS . The photo on the right above is of Hebgen Lake, Montana, following an earthquake in 1959. It shows the situation in the cartoon on the left. Note the Jeep standing on the road is on the footwall. We can see the dipping fault surface below the person (black arrow), and the block on the right (hanging wall) has clearly been downdropped, in this case by a few meters, with respect to the footwall block. Structural Damage
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This note was uploaded on 11/18/2011 for the course EOSC 116 taught by Professor Randell during the Winter '09 term at UBC.

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Lesson 21 - Module E Planetary Engineering Mesozoic...

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