Earthquakes Flashcards
Plate Boundaries: Trench
- Ocean-Ocean
- Ocean-Continent
Plate Boundaries: Collision
- Continent-Continent
- Ocean-Ocean only when thick collides with thin
Plate Boundaries: Transform
- Continent-Continent
- Ocean-Ocean
- Ocean-Continent
Plate Boundaries: Ridge
- Ocean-Ocean
Plate Boundaries: Rift
- Continent-Continent
Shallow focus:
< 70km depth
- all plate boundaries
Intermediate focus:
70-300km
Deep focus:
> 300km
- Only convergent
Wider bands of seismicity are typically what type of boundary?
- Convergent
Seismicity deepening from trench
Wadati-Benioff zone
What is a possible explanation for deep EQ’s on a collision margin (Himalayas)
- Remnant ocean slab under Himalayas
Why does the Cascade zone show only shallow EQ’s unlike other subduction zones (deep)?
- Locked for a very long time frame, no data collected before 1975
Why do extensional EQ’s occur in a compression zone (subduction)?
- Not enough material on surface to cover the bending/bulging area of subducting slab
What is sense of motion on fracture zones that connect MOR’s?
Transform
Elastic Rebound Theory
- 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
Types of Faults
- Strike-slip
- Normal
- Thrust (Reverse)
- Oblique
Hypocentre (focus) vs. Epicenter
- Focus - Fault surface location (at depth)
- Epicenter = Earth surface above focus
Which is the first seismic wave to arrive? Second? Last?
- P-wave
- S-wave
- Surface (Love and Rayleigh)
What does the s-wave minus p-wave arrival time indicate?
- Distance to focus
- Further apart = further from focus
- Only for local/regional, outside this the linear model (dist/time) falls apart
Concepts of seismicity
- Seismicity defines plate boundaries
- Seismic wave arrivals at different stations provide info on location (S minus P arrival times)
P-wave first motions
- Up/down = Compression/Dilation
- Fault plane
- Mechanism
- Slip direction
Seismic waves: Body waves
P-waves, S-waves
P-waves
- Primary (fastest), push-pull
- Elastic deformation by compression/dilation in propagation direction
- Travel through solids, liquids, gasses
S-waves
- Secondary, shear
- Elastic deformation by shearing perpendicular to propagation direction
- Only travels through solids
Leading Push
Compress then extend
- Compression
- Upward first motion
- Towards receiver
Leading Pull
Extend then compress
- Dilation
- Downward first motion
- Away from receiver
Ground motion at an observed receiver
Direction related to fault motion
What are the 2 nodal planes?
- Fault plane
- Auxiliary plane 90 degrees to fault plane
For a vertical fault strike-slip example, what are the 1st motions at stations along nodal planes?
Zero, neither up or down
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
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
Fault plane solutions
- Usually in map view
- Lower hemisphere
Pressure axis P
- Max compressive stress direction
- In tensional quadrant at 45 degrees to both nodal planes
- Sigma 1
Tension axis P
- Minimum compressive stress direction
- In compressional quadrant at 45 degrees to nodal planes
- Sigma 3
P and T axis in vertical strike-slip fault
Both horizontal
P and T axis in thrust fault dipping 45 degrees
- P-axis is horizontal
- T-axis is vertical
P and T axis in normal fault dipping 45 degrees
- P-axis is vertical
- T-axis is horizontal
Focal Sphere
Unit sphere centered on EQ focus
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
Nisqually EQ
- 2001
- Mw 6.8
- 58km depth
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
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
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
What happens if motion occurs along auxiliary plane for Normal or Thrust EQ’s?
The same compressional/dilational pattern will occur
Motion
- Direction of motion/slip on fault plane
- Vector made up of strike component and dip component
- Pole of auxiliary plane
Horizontal
- Direction of slip projected to the horizontal
Plunge
- Angle between Horizontal and Motion
- Does not lie on fault plane
Rake/pitch
- Angle btwn Motion and Strike
- In the fault plane
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
Great Success of modern plate tectonics era
- Motions of continents predicted from paleomagnetic data and confirmed w/ geophysics and geochronology
- Explanation of seafloor magnetics, mechanism is seafloor spreading
- Magnetic stripe prediction of ocean crust ages confirmed by sedimentology/paleontology
- Motions at transform faults confirmed w/ seismology
Transcurrent vs. Transform
- Markers offset
vs. - Fault btwn ridge segments
