DaveE said:
Help! What does this phrase mean: "rupture occurred on either a left-lateral northeast-striking fault or a right-lateral southwest-striking fault"?
DaveE said:
For strike-slip faults, isn't a 'northeast-striking fault' the same as a 'southwest-striking fault'?
Yes it is, other than the direction of slip. Which could be either but is dependant on the focal mechism derived from first motions. I wonder if there was a typo there ?
I think ( but cant confirm) they really meant to say NE striking and SE striking as per the beachball.
They also seem to be using opposite colouring to everyone else. everone else uses the dark colour ( blue in this case) as the quadrant being compressed (P = Pressure) but they are using the light colour ( white) as the P = Pressure quadrant.
Astronuc said:
I was wondering that myself. I expect it depends on which mass if moving in whatever direction.
Seems I need to do a thread on focal mechanisms "beachballs".
here is the beachball for that event .....
This focal mech. is as almost as pure strike slip as can be obtained.
there are two possible fault plane solutions for this event.
If we use the dark colour as the compressional area and the white as the tensional areas,
we would have .....
2 possible solutions (fault planes)
Solution 1 (in red arrows) A left lateral strike-slip along a fault trending NE to SW.
That is Q1 and Q4 are moving towards the SW relative to Q2 and Q3.
Left Lateral means, if you stand on one side of the fault and look across the fault,
which direction did the ground move on the opposite side of the fault.
Standing in Q3 or Q2 quadrants, the other side (Q1 and Q4) moved to the left = Left Lateral.
Note, it doesnt matter what side you stand on, you see the same left lateral motion.
Solution 2 ( in black arrows) would be a fault trending NW to SE.
So if you stand in Q1 or Q2 (Quadrants), the other side (Q3 and Q4) has moved towards the NW.
That is towards the right - therefore a right lateral fault.
Once again, it doesnt matter which side of this fault you stand on, looking across the fault shows
movement to the right.
Clear as mud ?

hopefully not too bad
Looking at the focal mechanism, we do not know which of the 2 fault planes is the correct one.
Choosing the correct one comes from knowledge of the tectonics region where the event occured.
I will continue this in another following post. looking at how the P and T quadrants are defined
I need to save this much and head home from work
cheers
Dave