11: Earthquakes (Exercises)

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Q11.1 Earthquakes in British Columbia

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

Most of the earthquakes between the Juan de Fuca (JDF) and Explorer Plates are related to transform motion along that plate boundary,
The string of small earthquakes adjacent to Haida Gwaii are likely aftershocks of the 2012 M7.7 earthquake in that area.
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.
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