COMPLEX PLATE BOUNDARY ZONE

IN SOUTHEAST ASIA

Northward motion of India deforms large region

Many small plates (microplates) and blocks

Molnar & Tapponnier, 1977

Davidson p304

COLLISION BETWEEN INDIAN AND EURASIAN PLATES: GPS MOTIONS.Mountain building by

continental collision produced boundary zone extending 1000’s of km northward from the nominal plate boundary at the Himalayanfront.

Total plate convergence taken up several ways. About half (20 mm/yr) occurs across locked Himalayan frontal faults such as the Main Central Thrust

These faults are part of the interface associated with the underthrusting Indian continental crust, which thickens crust under high Himalayas.

Larson et al., 1999Larson et al., 1999

COLLISION BETWEEN INDIAN AND EURASIAN PLATES: GPS MOTIONS.

GPS data also show along-strike motion behind the convergent zone, in the Tibetan Plateau, presumably because the uplifted and thickened crust spreads under its own weight.

Extension is part ofa large-scale process of crustal "escape" or"extrusion" in which large fragments of continental crust are displaced eastward by the collision along major strike-slipfaults.

Larson et al., 1999

COLLISION BETWEEN INDIA AND EURASIA

PLATES: EARTHQUAKES

Large destructive thrust earthquakes reflect convergence on Himalayan frontal faults such as Main Central Thrust

Normal faulting earthquakes occur behind convergent zone in the Tibetan Plateau, due to along strike extension from gravitational collapse

Strike slip earthquakes occur further north

Ni and Barazangi, 1984

Collision process is thought to involve a complex interplay between forces due directly to the collision, gravitational forces due to the resulting uplift and

crustal thickening, and forces from the resulting mantle flow

Crustal "escape" or "extrusion" in which large fragments of continental crust displaced eastward by the collision along major strike-slip faults has been modeled assuming that India acts as a rigid block indenting a semi-infinite

plastic medium (Asia), giving rise to a complicated faulting and slip pattern.

Also modelled numerically as thin viscous sheet flow

Tapponnier et al., 1982

India

Tibetan PlateauHimalayaIndia

Himalayan earthquakes are caused by the slip of India

beneath Tibet.

R. Bilham

India

Tibetcritical stress

Every few hundred years part of the Himalaya approaches failure

R. Bilham

great earthquake

TibetIndia

2-8 msurface rupture 1.5 m co-seismic

subsidence

… and a great earthquake ruptures the plate boundary.

R. Bilham

R. Bilham

R. Bilham

20001900180017001600

time

Indi

a/T

ibet

con

verg

ence

(m

)

2 mM=7.4

6 mM=7.8

4 mM=7.6

8mM=8

10 mM=8.2

convergence rate 18

mm/yr

The longer we wait the bigger the 'quake.

R. Bilham

Bilham, R., V. K. Gaur and P. Molnar, Himalayan Seismic Hazard, Science, 293, 1442-4, 2001

FUTURE EARTHQUAKE POTENTIAL

USGS

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SLIP DISTRIBUTION

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are needed to see this picture.Two members of the British Rescue and Preparedness in Disasters team, center, inspect the site in Islamabad where a 10-story apartment building collapsed a day earlier. (CNN)

The death toll now stands at 30,800 in the aftermath of a massive earthquake in Pakistan, as nations around the world try to get aid to the northern parts of the devastated country and to Kashmir. "This could get very close to tsunami levels," an Islamic Relief spokesman said. "It's horrific. It really is terrible." Many survivors have been wading through mountains of rubble in search of survivors, food and clean water.

An aerial view of the town

of Balakot. (NY Times)

STRONG GROUND MOTION DECAYS RAPIDLY WITH DISTANCE

0.2 g

Stein & Wysession, 2003

Pakistan M 7.6 earthquake?

0.2 g Damage onset for modern buildings

DAMAGE DEPENDS ON BUILDING TYPERESISTANT CONSTRUCTION REDUCES EARTHQUAKE RISKS

“Earthquakes don't kill people; buildings kill people."Coburn & Spence 1992

Pakistan earthquake?

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USGS

MODERN CONSTRUCTION WITHOUT SEISMIC STRENGTHENING

Non-ductile concrete buildings

~ 40,000 in California. ~ 8,000 are schools, including 239 in the Los Angeles Unified School District. Downtown Los Angeles has about 500 of the buildings.

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RETROFIT FOR SEISMIC STRENGTHENING

USGS

Problem: cost of retrofits can come close to that of razing a building and starting over. $24 B needed for California hospital retrofits!