Earthquake dynamics and source inversion Jean-Paul Ampuero

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

Fault geometry and velocity model ?

Initial conditions ?

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”) ?

Inferring fault dynamic properties from seismograms

Inferring fault dynamic properties from seismograms

Fracture energy Gc controls dynamic rupture

• Inversion of dynamic friction parameters with frequency band-limited data suffers from strong trade-off

Scale contraction issue

Scale contraction issue

The view from classical fracture mechanics

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 ≈ √L

• and f(vr) is a universal decreasing function

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)