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Geology LibreTexts

11: Earthquakes (Exercises)

Q11.1 Earthquakes in British Columbia

[SE from Earthquakes Canada at http://www.earthquakescanada.nrcan.g.../index-en.php]

  1. Most of the earthquakes between the Juan de Fuca (JDF) and Explorer Plates are related to transform motion along that plate boundary,
  2. The string of small earthquakes adjacent to Haida Gwaii are likely aftershocks of the 2012 M7.7 earthquake in that area.
  3. Most of the earthquakes around Vancouver Island (V.I.) are related to deformation of the North America Plate continental crust by compression along the subduction zone.

  4. Earthquakes that are probably caused by fracking are enclosed within a red circle on the map.

Q11.2 Moment Magnitude Estimates from Earthquake Parameters

Length (km) Width (km) Displacement (m) Comments MW?
60 15 4 The 1946 Vancouver Island earthquake 7.3
0.4 0.2 .5 The small Vancouver Island earthquake shown in Figure 11.13 4.0
20 8 4 The 2001 Nisqually earthquake described in Exercise 11.3 6.8
1,100 120 10 The 2004 Indian Ocean earthquake 9.0
30 11 4 The 2010 Haiti earthquake 7.0

The largest recorded earthquake had a magnitude of 9.5. Could there be a 10?

A possible solution is 2500 km long and 300 km wide with 55 m of displacement. (Other solutions are possible.) These are unreasonable numbers because subduction zones don’t tend to fail over that length (typically not much more than 1200 km), rupture zones cannot be that wide because that takes us into the asthenosphere, and displacements are never likely to be that great.

Q11.3 Estimating Intensity from Personal Observations

Building Type Floor Shaking Felt Lasted (seconds) Description of Motion Intensity?
House 1 no 10 Heard a large rumble lasting not even 10 s, mirror swayed II
House 2 moderate 60 Candles, pictures & CDs on bookshelf moved, towels fell off racks IV
House 1 no   Pots hanging over stove moved and crashed together III
House 1 weak   Rolling feeling with a sudden stop, picture fell off mantle, chair moved IV
Apartment 1 weak 10 Sounded like a big truck then everything shook for a short period III
House 1 moderate 20-30 Teacups rattled but didn’t fall off III
Institution 2 moderate 15 Creaking sounds, swaying movement of shelving III
House 1 moderate 15-30 Bed banging against the wall with me in it, dog barking aggressively IV

Q11.4 Creating Liquefaction and Discovering the Harmonic Frequency

No answer possible

Q11.5 Is Your Local School on the Seismic Upgrade List?

Answers will vary