Earthquake Triggering in South Iceland :Earthquake Triggering in South Iceland Anosua Mukhopadhyay


South Iceland Seismic Zone (SISZ) :South Iceland Seismic Zone (SISZ)


SISZ Fault Mechanism :SISZ Fault Mechanism “Bookshelf” Faulting: An array of north-south faults that trend east-west Right-Lateral Slip North-South Left-Lateral Shear East-West


What do you mean “earthquake triggering”? :What do you mean “earthquake triggering”? The classic example is a pair of magnitude 6 earthquakes that shook the Superstition Hills, California in 1987. First, the left-lateral Elmore Ranch fault ruptured. 12 hours later, the right-lateral Superstition Hills fault ruptured in a second earthquake. Examples in the SISZ include pairs of magnitude ~6 earthquakes that occurred in 1784, 1896, 2000, 2008 I am focusing on Magnitude 6.6 earthquakes that occurred on June 17th and 21st 2000. Stars indicate epicenters (where fault first ruptures) on each fault. Faults are about 10-20 km. long and about 5 km apart.


Goal :Goal Create an analytic model that best describes these earthquake sequences and comparing model to two data sources: Global Positioning System (GPS) Interferometric Synthetic Aperture Radar (InSAR)


InSAR :InSAR Records the change in distance between the ground and the European ERS-2 Satellite Data is in the form of “range change” The dot product between the unit “look vector” and the displacement vector


Observed GPS Data :Observed GPS Data


Observed InSAR Data :Observed InSAR Data Interferogram T52 covers only east of June 17th fault and spans June 16th until July 21st -> recording both earthquakes Interferogram T95 covers both the June 17th and June 21st rupture areas but only spans from June 19th to July 24th -> recording only the June 21st event


Modeling Process :Modeling Process There is deformation in the earth for up to 1-2 months past intial rupture to Interseismic (June 17th earthquake only) Coseismic (June 21st rupture) post-seismic (1-2 months after second rupture) displacement time


Slide 10:Interferogram T52 covers only east of June 17th fault and spans June 16th until July 21st -> recording interseismic deformation and a fraction of the coseismic deformation Interferogram T95 covers both the June 17th and June 21st rupture areas but only spans from June 19th to July 24th -> recording only the June 21st event, but some rapid post-seismic deformation from June 17th event can be seen


Finite Element Model :Finite Element Model Take the geometry of Iceland and divide the geometry up into elements Describe each element with two elastic material properties: the Poisson’s Ratio and Young’s Modulus of rock Use FEA software to: Apply Loading Conditions (displacements along fault) Solve equilibrium equations for each element Make sure each element satisfies compatibility equations with each other and constitutive relations Loic Dubois used TECTON, I am using ABAQUS


Material Properties :Material Properties finite element model describing two different configurations: (1) an elastic homogeneous medium (2) horizontal layers with a depth-dependent gradient in rigidity


Mesh :Mesh


How do you Apply Loading Conditions? :How do you Apply Loading Conditions? Elements are defined by nodes We apply “split-nodes” to nodes along the fault for the nodes along the fault, we give them different number IDs, but they occupy the same place in space Then we define “dummy nodes” that are not attached to any elements We specify the initial displacements along the fault to the dummy nodes Node ID: 1 Coordinates: (0,0,5) Node ID: 100000 Coordinates: (0,0,5) Node ID: 1000000 Coordinates: (0,0,7) Node ID: 2 Coordinates: (0,0,0) Node ID: 200000 Coordinates: (0,0,0) Node ID: 2000000 Coordinates: (0,0,2)


What ABAQUS Sees :What ABAQUS Sees *Node 1,0,0,5 2,0,0,0 100000,0,0,5 200000,0,0,0 1000000,0,0,7 2000000,0,0,2 *Nset,Nset=Left_Fault 1,2 *Nset,Nset=Right_Fault 100000,200000 *Nset,Nset=Dummy 1000000,2000000 *Equation 3 Left_Fault,1,1.0,Right_Fault,1,-1.0,Dummy,1,-1.0 3 Left_Fault,2,1.0,Right_Fault,2,-1.0,Dummy,2,-1.0 3 Left_Fault,3,1.0,Right_Fault,3,-1.0,Dummy,3,-1.0 *Boundary, op=new Dummy,1,1,0.000710 Dummy,2,2,0.0 Dummy,3,3,0.0 Constrains Dummy’s x direction (direction 1) to 7.1 cm, and y and z (direction 2 and 3) to 0 Dummyx = 7.1 cm Dummyy = 0 Dummyz = 0 Equations Applied to All 3 Directions: Left_Fault = Dummy Left_Fault = -Right_Fault Left_Fault+Right_Fault-Dummy=0 Resulting Application: Left_Faultx = 7.1 cm Right_Faultx = -7.1 cm Left_Faulty = 0 Right_Faulty = 0 Left_Faultz = 0 Right_Faultz = 0 Defining the Nodes Putting the Nodes into Sets


Data Analysis :Data Analysis Used Loic’s model and initial displacements Homogenous Model


Range Changes :Range Changes Heterogeneous Model Homogenous Model


A Closer Look :A Closer Look Tension Quadrants Pore Pressure is Low Compression Quadrants Pore Pressure is High


After Time :After Time Pore Pressure will Increase: May increase just enough to cause second fault to fail Pore Pressure will Decrease


Wait, Everything Has Just Been Elastic? :Wait, Everything Has Just Been Elastic? Adding in the Pore Pressure Theory: Make model “poroelastic” by adding following material properties: Permeability of Water Porous Bulk Modulus


Homogenous Poroelastic Range Changes :Homogenous Poroelastic Range Changes


Conclusions :Conclusions Heterogeneous poroelastic model does not work because I’m still trying to work out compatibility and convergence issues After heterogeneous model works, I will analyze model against data and use the coloumb failure criteria to decide if second fault failed due to pore pressure