Earthquakes Flashcards
(51 cards)
1
Q
Plate Boundaries: Trench
A
- Ocean-Ocean
- Ocean-Continent
2
Q
Plate Boundaries: Collision
A
- Continent-Continent
- Ocean-Ocean only when thick collides with thin
3
Q
Plate Boundaries: Transform
A
- Continent-Continent
- Ocean-Ocean
- Ocean-Continent
4
Q
Plate Boundaries: Ridge
A
- Ocean-Ocean
5
Q
Plate Boundaries: Rift
A
- Continent-Continent
6
Q
Shallow focus:
A
< 70km depth
- all plate boundaries
7
Q
Intermediate focus:
A
70-300km
8
Q
Deep focus:
A
> 300km
- Only convergent
9
Q
Wider bands of seismicity are typically what type of boundary?
A
- Convergent
10
Q
Seismicity deepening from trench
A
Wadati-Benioff zone
11
Q
What is a possible explanation for deep EQ’s on a collision margin (Himalayas)
A
- Remnant ocean slab under Himalayas
12
Q
Why does the Cascade zone show only shallow EQ’s unlike other subduction zones (deep)?
A
- Locked for a very long time frame, no data collected before 1975
13
Q
Why do extensional EQ’s occur in a compression zone (subduction)?
A
- Not enough material on surface to cover the bending/bulging area of subducting slab
14
Q
What is sense of motion on fracture zones that connect MOR’s?
A
Transform
15
Q
Elastic Rebound Theory
A
- Following 1906 San Fran EQ fault offset
- ‘Stretch over a time (short or long) until rupture and slip
- Dip doesn’t necessarily tell you type of slip
16
Q
Types of Faults
A
- Strike-slip
- Normal
- Thrust (Reverse)
- Oblique
17
Q
Hypocentre (focus) vs. Epicenter
A
- Focus - Fault surface location (at depth)
- Epicenter = Earth surface above focus
18
Q
Which is the first seismic wave to arrive? Second? Last?
A
- P-wave
- S-wave
- Surface (Love and Rayleigh)
19
Q
What does the s-wave minus p-wave arrival time indicate?
A
- Distance to focus
- Further apart = further from focus
- Only for local/regional, outside this the linear model (dist/time) falls apart
20
Q
Concepts of seismicity
A
- Seismicity defines plate boundaries
- Seismic wave arrivals at different stations provide info on location (S minus P arrival times)
21
Q
P-wave first motions
A
- Up/down = Compression/Dilation
- Fault plane
- Mechanism
- Slip direction
22
Q
Seismic waves: Body waves
A
P-waves, S-waves
23
Q
P-waves
A
- Primary (fastest), push-pull
- Elastic deformation by compression/dilation in propagation direction
- Travel through solids, liquids, gasses
24
Q
S-waves
A
- Secondary, shear
- Elastic deformation by shearing perpendicular to propagation direction
- Only travels through solids
25
Leading Push
Compress then extend
- Compression
- Upward first motion
- Towards receiver
26
Leading Pull
Extend then compress
- Dilation
- Downward first motion
- Away from receiver
27
Ground motion at an observed receiver
Direction related to fault motion
28
What are the 2 nodal planes?
- Fault plane
| - Auxiliary plane 90 degrees to fault plane
29
For a vertical fault strike-slip example, what are the 1st motions at stations along nodal planes?
Zero, neither up or down
30
What are the possible motions on the nodal planes for vertical fault strike-slip? Is this the same for Normal and Thrust slip?
- Either dextral motion on one plane
- or Sinistral on the other plane
- Can't tell which is FP or AP
- Normal and Thrust remain the same no matter which plane is the FP/AP, just orientation changes
31
How do you tell which nodal plane is the fault plane?
- Can't tell without geological or geophysical evidence
- Surface trace, bathymetry
- Maybe GPS data
32
Fault plane solutions
- Usually in map view
| - Lower hemisphere
33
Pressure axis P
- Max compressive stress direction
- In tensional quadrant at 45 degrees to both nodal planes
- Sigma 1
34
Tension axis P
- Minimum compressive stress direction
- In compressional quadrant at 45 degrees to nodal planes
- Sigma 3
35
P and T axis in vertical strike-slip fault
Both horizontal
36
P and T axis in thrust fault dipping 45 degrees
- P-axis is horizontal
| - T-axis is vertical
37
P and T axis in normal fault dipping 45 degrees
- P-axis is vertical
| - T-axis is horizontal
38
Focal Sphere
Unit sphere centered on EQ focus
39
1st motions recorded at stations provide what info?
- Pattern of P-waves leaving the lower half of focal sphere
- Only lower sphere b/c they bend upwards towards surface by refraction since velocities increase downwards while upper sphere bends downwards into Earth
40
Nisqually EQ
- 2001
- Mw 6.8
- 58km depth
41
Stereonets
- Projections of lower hemisphere onto horizontal plane
- Intersection of a line w/ hemisphere = point
- Intersection of a plane w/ hemisphere = Great circle
- Line normal to a great circle is a point 90 degrees away from centre of circle = pole (line w/ plunge 90 degrees from dip of plane
- Pole of fault plane is a point on stereonet in the auxiliary plane and vice versa
42
How to find angle btwn 2 lines on stereonet
- Given trend and plunge of 2 lines
- Mark both ends of strike on circle circumference
- Rotate overlay to line strike up w/ equator, count in plunge and mark point, repeat for 2nd point
- Rotate overlay so points lie on common GC, count degrees along circle
- Rotate N back to top, read off strike and dip of plane on which both lines lie
43
Given a pole, find strike and dip of a plane on a stereonet
- Rotate pole until it lies on equator
- Count off the dip from the centre of the net
- The GC lies 90 degrees away
- Mark strike of the plane at the ends of the N-S meridian
- Rotate overlay to N and measure the strike
44
What happens if motion occurs along auxiliary plane for Normal or Thrust EQ's?
The same compressional/dilational pattern will occur
45
Motion
- Direction of motion/slip on fault plane
- Vector made up of strike component and dip component
- Pole of auxiliary plane
46
Horizontal
- Direction of slip projected to the horizontal
47
Plunge
- Angle between Horizontal and Motion
| - Does not lie on fault plane
48
Rake/pitch
- Angle btwn Motion and Strike
| - In the fault plane
49
Why is M the pole of the aux plane?
- No velocity component in Aux plane
- Only 2 nodal planes, one is fault, parallel to motion
- Therefore pole to aux must be M
50
Great Success of modern plate tectonics era
1. Motions of continents predicted from paleomagnetic data and confirmed w/ geophysics and geochronology
2. Explanation of seafloor magnetics, mechanism is seafloor spreading
3. Magnetic stripe prediction of ocean crust ages confirmed by sedimentology/paleontology
4. Motions at transform faults confirmed w/ seismology
51
Transcurrent vs. Transform
- Markers offset
vs.
- Fault btwn ridge segments
