Moment balance and stress evolution in eastern Tibet: Moment balance and stress evolution in eastern Tibet: Implications for the Great Implications for the Great WenchuanWenchuan earthquakeearthquake

Mian Liu1, Gang Luo1, and Hui Wang1,2

1. Dept. of Geological SciencesUniversity of Missouri

2. China Earthquake Administration

Beichuan County (Science 23 May 2008)

USGS

Earthquakes in Eastern TibetEarthquakes in Eastern Tibet

oM SAμ=Seismic Moment:

Seismic moment is a measure of seismic energy released, or could be released, by earthquakes.

Part 1: Balancing seismic momentPart 1: Balancing seismic moment

Balancing seismic moment:

From fault slip rate, calculate the accumulation of seismic moment.

From earthquake catalog, estimate moment released by earthquakes.

Accumulated – Released = Moment Deficitthat can be used for future earthquakes

GPS Velocities and the Block Model GPS Velocities and the Block Model

GPS data: Gan et al. (2007)

Residual Velocities (= Predicted Residual Velocities (= Predicted –– GPS)GPS)

Mean RMS =1.09 mm/a, smaller than the error in the GPS data (1.62mm/a)

Predicted Fault Slip RatesPredicted Fault Slip Rates

Earthquake records in eastern TibetEarthquake records in eastern Tibet

Records of M≥6.0 events are likely complete since 1879.

Balancing Seismic Moment: 1879 Balancing Seismic Moment: 1879 -- 20072007

accumulation deficit

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Numbers in 1019 Nm

Seismic moment was largely balanced along the E. Xiangshuhe Fault, butthe accumulated moments on the Longmenshan Fault is insufficient for a Mw 8.0 earthquake.

No large earthquakes have occurred on the Longmanshan Fault in the past 1000 years or even longer before 12 May 2008…

Moment deficit in 2007Moment deficit in 2007

?

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(Mw 8.0)

100 years from now100 years from now……

Part 2: Stress evolutionPart 2: Stress evolutionHow did the great Wenchuan earthquake change the Coulomb stress in eastern Tibet?

Did previous earthquakes in eastern Tibet triggered the Wenchuan earthquake?

Do faults in eastern Tibet interact with each other?

A 3D viscoelasto-plasticfinite element model

Predicted co-seismic slip

Chen Ji et al., 2008

Co-seismic Coulomb stress change

Did recent large earthquakes in E. Tibet triggerthe Wenchuan earthquake?

LMS F.

XSH F.1923

1933

2007

2008

Dynamic Fault Interaction

Locking on the XSH F. Clusters of earthquakes on the XSH F.

Interseismic locking on the XSH fault increases loading on the LMS Fault

LMS

XSH

Compared with long-term average

ConclusionsConclusionsLow slip rates and infrequent earthquakes on the Longmenshan Fault does not mean low earthquake risks; Seismic history is important for studies of stress triggering;Eastern Xianshuhe F. may not be the most dangerous faults in eastern Tibet now;Seismic activity on one fault may impact stress evolution on the neighboring faults.

Thank You!