What Causes Strike-Slip Fault Earthquakes? New Study Says The ‘Lazy Earth’
A recent study has revealed extensive data on how strike-slip faults develop over time and eventually cause earthquakes at the Earth’s surface. Researchers coined the movement of two plates in a strike-slip motion to follow the ‘Lazy Earth’ hypothesis.
To step back a bit, strike-slip faults are how geoscientists describe the motion of two plates in contact with one another. In this specific fault, the two plates move parallel but in opposite direction from one another. This can be demonstrated by putting two pieces of sandpaper in each hand and rubbing your right hand forward and left hand backward.
Strike-slip faults produce little to no vertical offset between the two plates but significant lateral onset. In fact, type of faulting is the predominant fault type in western California such as the San Andreas Fault system. The cause of strike-slip fault earthquakes is due to the movement of the two plates against one another and the release of built up strain. As the larger plates are pushed or pulled in different directions they build up strain against the adjacent plate until it finally fails.
The recent study was published in the Journal of Structural Geology by Dr. Alexandra Hatem at the University of Massachusetts Amherst. The motivation behind the research is to focus specifically on what happens beneath the Earth’s crust when strike-slip faults form and break.
What Governs Strike-Slip Fault Movement
Geologists understand the mechanics of strike-slip faulting at the plate scale but up until now, the specifics of faulting on a very small scale haven’t been as thoroughly studied. Often times, we see faults as they currently exist but don’t get to look at the development of that fault from incipient stages.
To address this, Dr. Hatem built a miniature model of the Earth’s crust using kaolin clay. The team made sure the length to depth was scaled appropriately to mimic that on Earth and with the correct viscosity. After creating two slabs of this kaolin clay, the team setup several boundary conditions on which to test the development of the strike-slip fault. In one boundary condition, there is a pre-existing fault along the two slabs, in another there is a pre-existing displacement beneath the clay slabs, and in the last example the displacement is in a wider shear zone.
After the models were setup, the team moved the two clay slabs in opposite directions in order to measure minute changes as the strike-slip faults developed. The team found that the faults develop through a “Lazy Earth” hypothesis, whereby the fault propagation takes the easiest path. This is a similar trait we see in many systems on Earth, from rivers finding the easiest path to lower elevations to mammals taking the easiest path from point A to point B.
As the faults propagate, the team measured how strain is transferred to different parts of the fault, a process that in real life takes millions of years and across many miles. As opposed to the idealized linear movement of a strike-slip fault, the team demonstrated what geologists knew in theory, that shear strain has several stages before final movement along the fault.
Initially, the strain along the fault is distributed across the fault zone. Through further strain and development of the faults, en echelon faults begin to form, lengthen, interact and propagate along the wider fault zone. Lastly, the strain is released by movement along the dominant through-going strike-slip fault.
Another interesting find is that built-in irregularities in faults are persistent, without the fault ‘fixing’ the irregularities and forming a straight and more effective fault. This general evolution of a strike-slip fault appears to be the case no matter what boundary conditions exist.
This study provides one of the most in-depth analysis on a small scale of the formation of strike-slip faults. This allows geologists to better understand faults like the San Andreas fault, how it formed, and how it develops over time. This is one more piece in helping geologists better understand and predict earthquakes.