Geodetic Deformation, Seismicity and Fault Friction

Ge277- 2007

Sensitivity of seismicity to stress perturbations, implications for 

earthquakes  nucleation

Key Observation :

Seismicity rate and Geodetic strain rate are generally proportional, and both decays as t-1 following an earthquake

• Aftershocks and postseismic relaxation following the : – Mw7.6, Chichi earthquake (1999) – Mw 7.2 Landers Earthquake (1992)– Mw 8.1, Peru earthquake (2001)– Mw 8.7, Nias earthquake (2005)

• Background seismicity in the Nepal Himalaya

(Svarc and Savage, 1997)

CPA analysis show that all GPS stations follow about

the same time evolution f(t)

Postseismic Displacements following the Mw 7.2 1992 Landers Earthquake

Comparing aftershocks and afterslip following Landers EQ

(Perfettini and Avouac, 2004a)

Seismicity and Postseismic displacements follow the same relaxation curve consistent with the Omori Law.

Velocities relative to India(Bettinelli et al, 2006)

Geodetic deformation across the Nepal Himalaya

Creeping Zone17-18mm/yr

Locked Fault Zone,width 110km

Seismicity and Coulomb stress change due to interseismic stress accumulation

(Bollinger et al, JGR, 2004)

Seismicity coincides with the area where Coulomb stress increases by 4-6 kpa/a

Conceptual Model

Seismicity and Coulomb stress change due to interseismic stress accumulation

(Bollinger et al, JGR, 2004)

Seismicity coincides with the area where Coulomb stress increases by 4-6 kpa/a

Winter seismicity rate is twice as large as summer seismicity rate

Detrend GPS time-seriesSeasonal variation of Horizontal Displacements

Water level in Ganges Basin determinedfrom TOPEX-POSEIDON and GRACE

GUMBA-SIMRA

TOPEX-POSEIDON GRACE

Displacements induced by surface water level variations in the Ganges basin

Summer

Winter

Finite Element Modeling

Seasonal variations of seismicity and water level in the Ganges Basin

Seasonal variations of seismicity and water level in the Ganges Basin

Compression CompressionExtension

Strain induced by surface water level variations in the Ganges basin

Summer: Extension

Winter: Compression

Seasonal Coulomb stress variations

Coulomb stress variations are estimated to about 500 pa(<earth tides, 2-3kPa for T=12h and 14 days)

Coulomb stress rate

• Seasonal variations of geodetic displacements reflect deformation due to water level variations in the Ganges basin

• Interseismic Coulomb stress increase by 4-5 kPa/yr is modulated by seasonal variations of 500pa (corresponding to stress rate variation of 2-3kPa/yr) modulating the seismicity rate by a factor of 2.

Seasonal variation of Coulomb stress rate

6 kPa/yr

8 kPa/yr

With secular term added 10 kPa/yr

In these examples:– Seismicity rate and stress rate are

approximately proportional– The characteristic time associated with the

stress variations is of the order of 1 yr.

Standard Coulomb failure

- seismicity rate obeys :0

0 S

SRR

- so, for periodic loading :

nS

00

minmax

0

minmax 2STS

SS

R

RR m

Assuming , 0S

0S

0S

(Lockner and Beeler, 1999; Heki, 2003)

Standard Coulomb failure

For periodic loading : 00

minmax

0

minmax 2STS

SS

R

RR m

Seismicity rate should be much more sensitive to earth tides.

This is not observed, probably because rupture is a time-dependent process, as suggested by rock mechanics experiments.