Earthquake dynamics and source inversion
Jean-Paul Ampuero
ETH Zurich
Overview
The forward problem: challenges, open questions Dynamic properties inferred from kinematic models Direct inversion for dynamic properties: which
parameters can be resolved ? Perspectives
The “standard” dynamic rupture problem
Planar strike-slip fault Slip-weakening friction
Gc = fracture
energy
Initial stress 0(x,z)
Basic ingredients: linear elastic medium (wave equation) a pre-existing fault (slip plane) Friction: a non linear relation between fault
stress and slip (a mixed boundary condition) initial conditions (stress)
Planar strike-slip fault Slip-weakening friction
Gc = fracture
energy
Initial stress 0(x,z)
The “standard” dynamic rupture problem
Fault geometry and velocity model ?
Boundary element dynamic simulation of Landers earthquake, by Hideo Aochi
P-wave tomography and structural interpretation near Parkfield,
by Malin et al 2006
Initial conditions ?
SBIEM simulations by J. Ripperger (ETHZ)
Fault constitutive law (“friction law”) ?
Input: Geological field observations Geophysical boreholes Laboratory Strong motion seismology
Candidate ingredients: Dry friction Frictional heating Melting Fluid thermal pressurization Off-fault damage Compaction / porosity evolution
Fault constitutive law (“friction law”) ?
Input: Geological field observations Geophysical boreholes Laboratory Strong motion seismology
Fault constitutive law (“friction law”) ?
Rupture propagation on a multi-kinked fault, solved by SEM (Madariaga, Ampuero and Adda-Bedia 2006)
Upscaling of fault constitutive law from micro- to macroscopic scales ?
(homogeneization)
Candidate ingredients at the micro level:Dry frictionFrictional heatingMeltingFluid thermal pressurizationOff-fault damageCompaction / porosity evolutionGeometrical roughness
Inferring fault dynamic properties from
seismograms
Kobe earthquake Ide and Takeo (1997)
Kinematic inversion
Elastic wave equation
Seismograms
Slip (x,z,t)
Stress (x,z,t)
Stress / slip relation
Plot
Interpretation
Inferring fault dynamic properties from
seismograms
Kobe earthquake Ide and Takeo (1997)
Stress / slip relation
Space-time resolution problems
Effect of time filtering the initial data at cut-off period Tc
(Spudich and Guatteri 2004)
Inferring fault dynamic properties from
seismograms
Non-linear dynamic inversion of the Tottori earthquake, with neighborhood algorithm, by Peyrat and Olsen (2004)
Required 60 000 forward simulations
One model 19 models with low residuals
Fracture energy Gc controls dynamic rupture
Inversion of dynamic friction parameters with frequency band-limited data suffers from strong trade-off
Same Gc same strong motion <1Hz
A B
Dynamic source inversions of the Tottori earthquake by Peyrat and Olsen 2004
Scale contraction issue
Displacement
Rupture growth
Scale contraction issue
Slip velocity snapshot
Problem: The process zone shrinks affecting numerical
resolution
Energy dissipation and high gradients concentrated within a process process zonezone
Linear elastic fracture mechanics (LEFM) predicts a stress singularity at the tip of an ideal crack.
crack
K-dominant
regionThe stress concentration must be physically accommodated by nonlinear material behavior (damage, plasticity, micro-fractures)
Inelastic process zone
The view from classical fracture mechanics
Kostrov, Freund, Husseini, Kikuchi, Ida, Andrews (60-70s)
Gc controls dynamic rupture: theory
Classical fracture mechanics +Griffith criterion local energy balance at the rupture front:
Gc = G(vr, L, )
crack tip equation of motion relates rupture speed to Gc
Gc = f(vr) Gstatic(L,)
Gc = f(vr) K2(L,)/2
where: stress intensity factor = K ≈ √Land f(vr) is a universal decreasing function
fracture energy
energy release rate, energy flow towards the crack tip
Crack
Size =
L
Summary So far:
The development of dynamic source inversion methodologies is in its infancy
Parameterization issue Resolution limited by:
Data band-pass filtering Attenuation Inaccurate Green’s functions, poor knowledge of the crust Scarce instrumentation Coarse parameterization, computational cost
Ideal wish-list: Reach higher frequencies Understand the meaning of the inferred macroscopic parameters Faster, better forward solvers
2.5D dynamic inversion
Dynamic source inversion = from seismograms +GPS +InSAR to spatial distribution of initial stress and fracture energy along the fault
Computationally expensive and low vertical resolution
Reduce the problem dimensionality: solve rupture dynamics averaged over the seismogenic depth (3D wave equation 2D Klein-Gordon equation)
M7.9 Denali earthquake from inversion of GPS data (Hreinsdottir et al, 2006)