in presentation i discuss the tibetan plateau seismology

1. Presented by :NAILA ISRARPresented by :NAILA ISRAR
SEICMICITY OF TIBETAN PLATEAU

2. Can you remember
how Tibet / The
Himalayas were
formed?

7. 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

8. 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 Himalayan
front.
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., 1999
Larson et al., 1999

9. 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 of
a large-scale process of
crustal "escape" or
"extrusion" in which large
fragments of continental
crust are displaced
eastward by the collision
along major strike-slip
faults.
Larson et al., 1999

10. 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

11. • (a) Map of earthquakes with accurately constrained depths, from this study (Tables S1 and S2)
and others (see supplementary material). Depths are indicated by colour, and earthquakes deeper
than 20 km are scaled by magnitude. Black arrow indicates the India–Asia convergence vector.
The dashed blue line (A) outlines the area in central southern Tibet where there is no deep
seismicity
• (b) Normal (purple) and strike-slip (green) focal mechanisms across the Tibetan Plateau
• c) GPS velocities in southern Tibet, relative to India (Banerjee et al., 2008)

13. Regional earthquake distributions in this study and those of the Hi Climb project [Liang et‐
al.
, 2008].
(a) Crosssection of seismicity across (left) the southern Tibet [Liang et al., 2008]) and (right)
the northern Tibet.(b) Statistics of the focal depths in the (first panel) Himalayan orogen [Liang et al., 2008],
(second panel) the southern Tibet [Liang et al.,2008], and (third and fourth panels) the northern
Tibet.
(c) Distributions of the events located in this study (black dots)and the events recorded by Hi
‐CLIMB project (cyan dots). The white lines show the profiles in Figure. The events in the
white dashed rectangles are projected to the cross profiles.
(d) Statistics of the focal depths and the crustal strength envelope.(left) For southern Tibet,
the strength envelope of the wet quartz (the green curve), the wet anorthite (the red curve),
the
southern Tibetan crust (the black curve), and the Byerlee’s Law (the black line) were
produced with the thermal gradient of34 °C/km. (right) For northern Tibet, the strength
envelope of the wet quartz (the green curve), the dry anorthite (the bluecurve), the northern
Tibetan crust (the black curve), and the Byerlee’s Law (the black line) were produced with
the thermal gradient of 32 °C/km.

14. India
Tibetan PlateauHimalayaIndia
Himalayan earthquakes are
caused by the slip of India
beneath Tibet.
R. Bilham

15. India
Tibet
critical stress
Every few hundred years part of the
Himalaya approaches failure
R. Bilham

16. great earthquake
TibetIndia
2-8 m
surface rupture 1.5 m co-seismic
subsidence
… and a great earthquake
ruptures the plate boundary.
R. Bilham

17. R. Bilham

18. 20001900180017001600
time
India/Tibetconvergence(m)
2 m
M=7.4
6 m
M=7.8
4 m
M=7.6
8m
M=8
10 m
M=8.2
convergence rate 18
mm/yr
The longer we wait the bigger the 'quake.
R. Bilham

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

20. CONCLUSION
The well located 400 regional earthquakes in northern Tibetan Plateau‐
were distributed not only at the boundaries of major tectonic provinces
but also within these provinces , supporting the hypothesis of continuous
deformation within the Tibetan plateau. All well located earthquakes in‐
northern Tibet occur within the upper crust, mostly at depths of 0 15 km,‐
and there is no event deeper than 30 km. Com- bining data from previous
studies, the depth distribution of regional earthquakes reported in this
study strongly supports the existence of a weak and hot lower and middle
crust in the northern Tibetan Plateau. Furthermore, the depth of the crustal
seismogenic zone in northern Tibetan Plateau is apparently thicker than
that in southern Tibet, which reflects the difference in temperature depth
profile and rheological structure of the crust between southern and
northern Tibet. Finally, there are no mantle earthquakes that would
suggest continental subduction along this part of the northern Tibet
margin

21. THANK YOU