The silent aftermath of Tibetan
normal fault earthquakes
Isabelle Ryder, Roland Bürgmann, Eric
Fielding, Jianbao Sun
Coseismic InSAR
Funning et al. 2007
Yutian
2008 (7.2)
Manyi
1997 (7.6)
Nima-Gaize
2008 (6.4)
Zhongba
2008 (6.6)
Kokoxili
2001 (7.8)
Damxung
2008 (6.3)
Ryder et al. 2009
Lasserre et al. 2005
Postseismic InSAR
Manyi
1997 (7.6)
Yutian
2008 (7.2)
Nima-Gaize
2008 (6.4)
Zhongba
2008 (6.6)
Kokoxili
2001 (7.8)
Damxung
2008 (6.3)
YUTIAN
Yutian earthquake
• 20 March 2008
• M 7.2
• West-dipping normal fault
forming half-graben
• Rupture beneath Guliya
ice cap
AL
15
OS T 5
T514
ALOS
t
Envisa
T155
Coseismic InSAR
ALOS
ALOS
Postseismic time series (Envisat)
coseismic
ALOS-Envisat coseismic inversion
N
Poly3D (Thomas, 1993)
S
Envisat
ALOS
InSAR
Model
Residual
Envisat postseismic inversion
COSEISMIC:
POSTSEISMIC:
InSAR
Model
Residual
ZHONGBA
InSAR data
25 August 2008
M 6.6
2005
2008
?
?
DAMXUNG
Damxung InSAR
Damxung slip inversions
COSEISMIC:
POSTSEISMIC:
NIMA-GAIZE
Nima-Gaize earthquake
• 9 January 2008
• M 6.4 mainshock
• M 5.9 aftershock
1 week later
• Normal fault source
mechanisms
Qiangtang terrane
ALOS
T509
a
uo f
ei-C
p
n
a
Rig
Gaize
back Siling Co
thrus
t
z
uture
s
g
on
Bang
Lhasa terrane
one
ult
Nima InSAR
Similarity of spatial
deformation fields
associated with coand post-seismic
phases
coseismic
Æ afterslip occurred
on both fault planes
Æ initial slip and
afterslip
approximately
coincident
postseismic
ALOS-Envisat joint inversion
• 2 faults required to explain LOS
displacement field
• Peak slip on mainshock fault
plane: 1.04 m at 6-8 km depth
• Peak slip on aftershock fault
plane: 67 cm at 2-4 km depth
Ryder et al. JGR 2009
Moment release function
Total moment
release ~10%
of coseismic
value
Ryder et al. JGR 2009
lower crustal
viscosity
Constraints on lower crustal viscosity
NIMA:
viscoelastic model prediction
elastic
Maxwell
Poly3D (Pollitz, 1992)
Ryder et al. JGR 2009
Constraints on lower crustal viscosity
YUTIAN:
elastic
Maxwell
Poly3D (Pollitz, 1992)
Final time series image:
Burgers rheology modelling
elastic
Burgers
Poly3D (Pollitz, 1992)
Uplift of graben?
Summary (i)
Afterslip signal observed for all recent normal
faulting events
Varying spatial relationship between initial slip
and afterslip:
Approx. same area slips in both
Afterslip concentrated at ends of rupture
Slip on subsidiary faults
Afterslip releases moment up to 10% of
coseismic value
Can afterslip on these faults be used as
analogues for e.g. larger, strike-slip earthquakes?
Summary (ii)
≥1x1019
Pa s
7x1018 Pa s (Maxwell)
3x1018 Pa s | 3x1019 Pa s (Burgers)
>3x1017 Pa s
?
?
?