Adv Advanced anced Worksho shop p on n Ea Earthquake Fa Fault - - PowerPoint PPT Presentation

Adv Advanced anced Worksho shop p on n Ea Earthquake Fa Fault Mechanics: The Theory, , Simulation

• n and Observation
• ns

ICTP, Trieste, Sept 2-14 2019 Lecture 5: 3D rupture effects Jean Paul Ampuero (IRD/UCA Geoazur)

Pulses on faults with finite seismogenic depth

! ∝ # $ ∝ # ! ∝ % $ ∝ % #

Arrest of long ruptures

Weng and Ampuero (2019) Rupture length/width Rupture width Earthquake magnitude Runaway condition: !" = $Δ&'( ) > !+ ( > (

+ = )!+/$Δ&'

Savage and Brodsky (2011) mature immature

What limits the thickness of damage zones ?

Damage zone thickness saturates at large fault displacement

Mitchell and Faulkner (2009)

< 1 #$

Ampuero and Mao (2017), Upper Limit on Damage Zone Thickness Controlled by Seismogenic Depth

Inner damage zone thickness depends on seismogenic width

W=9 km W=12 km W=15 km

Inelastic strain Inelastic strain

Fracture mechanics theory

Map view: Stress near crack tip: ! ≈

# $ + !&

where ' is the stress intensity factor, '~ ) Δ! Δ! is stress drop and ) the shortest rupture size: ) = , (radius) for circular ruptures, ) = - (width) for elongated ruptures (- ≪ /)

Damage zone size: distance at which ! = !0 (stress=yield strength) 1

2 ∼

Δ! !0 − !&

5

) < ~0.01 -

Seismogenic zone depth(W)

3D view: Rupture length L>> W Relative stress drop

Seismogenic zone depth control on the likelihood of fault stepover jump

Examples of rupture complexity in large strike slip earth quakes

Meng et. al 2012 Sieh et. al 1993 Mw 8.6 Indian Ocean earthquake Mw7.3 Landers Earthquake 20km

Stepover jumps in past earthquakes

• Surface fault traces of 22

continental strike slip faults are studied

• 2/3 of the rupture end points are

associated with stepover or fault trace termini.(Wesnousky 2006)

• Rupture rarely jump through

stepovers wider than 5km. And below this limit ,stepover only stop 40% of the total events.

(Wesnousky 2006)

(Wesnousky 2006) Shaw and Dieterich (2009)

Simulations Observations

Critical stepover distance ≈ 5 km

Critical stepover distance from static stress analysis

prestress stressdrop r

• 1/2

) ( ) ( 2

ij

r O r K

ij II

+ + S = s q p s

2

÷ ÷ ø ö ç ç è æ

• D

µ D × µ s s s s

yield c II

W H W K

Bai and Ampuero (2017) Critical stepover distance proportional to seismogenic depth

Bai and Ampuero (2017) Dynamic rupture simulation Evolution of the rupture front

Relative stress ratio: = (strength excess) / (stress drop) High S = low initial stress ! = ($% − $')/($' − $*)

ß Critical initial stress Compressional stepovers Dilatational stepovers ! = ($% − $')/($' − $*) Bai and Ampuero (2017)

ß Critical initial stress Compressional stepovers Dilatational stepovers ! = ($% − $')/($' − $*) Bai and Ampuero (2017)

ß Initial stress ß Initial stress Compressional stepovers Dilatational stepovers ! = ($% − $')/($' − $*) ! = ($% − $')/($' − $*) Critical stepover distance Bai and Ampuero (2017)

Summary

Effects of seismogenic width W:

• Pulse-like rupture
• Changes the energy balance: limits the energy flux, introduces rupture inertia

à implications on rupture arrest size

• Limits the thickness of damage zones
• Limits the stepover distance that ruptures can jump
• Allows for rupture at “unstable” and “forbidden” speeds