2. Hypothesis: Faults interact by the transfer of stress
WINCH SPRING BRICK
EARTHQUAKE! (only if stick-slip)
Force Balance – Brick will not move until:
Force on spring Force resisting motion
(its length change x its stiffness) (the weight of the brick x friction on surface)
Modeled after Ross Stein’s Coulomb Training I
-Lisa Walsh
3. Source fault Receiver fault
WINCH SPRING BRICK
Modeled after Ross Stein’s Coulomb Training I
SPRING BRICK
EARTHQUAKE! EARTHQUAKE!
• Add another spring & brick
• If you start cranking winch, PURPLE will move first.
Then tension on spring will move GREEN.
Coulomb stress calculation
= shear stress change + (coefficient of friction x normal stress change)
ΔCFS = Δτs+μ' Δσn
Coulomb
stress change
+ ΔCFS = closer to failure
- ΔCFS = farther from failure
-Lisa Walsh
4. Key concepts:
•Source faults
•Receiver faults
•Optimally oriented faults
•Assume receiver faults
are close to failure
•Triggering lag time is a
problem
5. Change of coulomb stress on faults
of specified orientation
Can change spatially
Remote: Sremote
Induced: Sinduced
Total:Sremote+Sinduced
Can change spatially
14. Stress changes are permanent but seismicity is not
from Todal et al (JGR, 2005)
15. Los
Angeles
Big Bear M = 6.5
(2nd - 3 hrs later)
LLaannddeerrss MM == 77..33
((11sstt))
First 3 hr of
Landers
aftershocks
from Stein
(Nature, 2003)
plotted
1992 - Landers earthquake triggered Big Bear earthquake 3 hrs later!
Stress
trigger
zone
Stress
Shadow
16. Los
Angeles
Hector
Mine
First 7 yr of
aftershocks
plotted
from Stein
(Nature, 2003)
…and promotes the M=7.1 Hector Mine shock 7 years later (1999).
Coulomb theory is based on the hypothesis that faults interact by the transfer of stress. This can be understood simply by thinking about stress transferred between a brick and spring. If you have a winch winding up the spring, the brick will not move until it exceeds the “force balance” or equilibrium between the force on the spring (its length change x its stiffness) and the force resisting motion (the weight of the brick x friction on the surface). Once the force of the spring is great enough, the brick will zip forward, spring will relax. This is like what happens when we have an earthquake! Then you just have to wait another couple 100 years for the winch to keep winding, stress to accumulate and another earthquake to occur.
Until we get to force balance, brick will just sit there. Once we get to equilibrium (or just past) brick will zip forward, spring will relax, causing an earthquake! Then we will wait another couple 100 years for winch to keep winding, stress to accumulate and another earthquake to happen again.
If you add another spring and brick and start cranking the winch, the BLUE brick will move first, then tension will move the red brick.
This is like the what happens with stress is transferred from one fault to another. For example, you could think of a long strike slip system as a series of adjacent bricks and springs.
Transfer of Coulomb stress from one fault to another
Release of stress from fault plane with earthquake will transfer stress to another fault
Example: Long strike slip fault system = series of adjacent bricks and springs
This is as complex as Coulomb thinking gets!
Stresses affect very little of the area
Stresses affect very little of the area
Trigger lobes are large & continuous, but you’ve seen that seismicity is blotchy (aftershocks do not fill red zones): why?
Stress changes are permanent but seismic response is not.
How do we go from stress change to seismicity?
STEIN quake prediction in magazine- MAGNITUDE 7.3 SHOCK in the southern California desert near Landers in 1992 increased the expected rate of earthquakes to the southwest, where the magnitude 6.5 Big Bear shock struck three hours later (top). Stresses imparted by the combination of the Landers and Big Bear events coincided with the regions where the vast majority of tremors occurred over the next seven years, culminating with the magnitude 7.1 Hector Mine quake in 1999 (bottom).
Figure 5 (a) The Landers and Joshua Tree earthquakes increased Coulomb stress
(red regions) where the Big Bear earthquake occurred three hours later. Coulomb
stress calculations in (a) are for left-lateral strike-slip faults aligned with the Big
Bear rupture surface. (b) Stress changes induced by the Joshua Tree, Landers, and
Big Bear earthquakes increased Coulomb stress in regions where the vast majority
of aftershocks occurred over the next seven years, culminating in the 1999 Hector
Mine earthquake. Coulomb stress calculations in (b) are for right-lateral strike-slip
faults aligned with the Hector Mine rupture surface (also reasonable for most aftershocks
in the region).
