Geodetic Insights into the Growth and Evolution of Thrust...
Transcript of Geodetic Insights into the Growth and Evolution of Thrust...
Geodetic Insights into the Growth and Evolution of
Thrust Belts: Examples from East Iran
Alex Copley, Kirsty Reynolds, Romain JolivetCOMET, Bullard Labs, Department of Earth Sciences, University of Cambridge, UK
Key Questions:
• Which parts of thrust belts slip in earthquakes, and which
aseismically?
• How do each of these processes contribute to the growth of
topography and geological structures?
• What is the rheology of the faults?
2. The Shahdad thrust belt
1. The 1978 Mw 7.3 Tabas-e-
Golshan earthquake
1978 Mw 7.3 Tabas earthquakeBerberian (1978,1982)
Walker et al (2003,2013)
11,000 dead
13,000 population
Manuel Berberian
1978 Mw 7.3 Tabas earthquake
Blind thrusting. Minor and discontinuous surface slip on Neogene anticlines
Coseismic fault plane is low-angle (~15 degree dip), with 9km centroid depth
Berberian (1978,1982)
Walker et al (2003,2013)
Postseismic InSAR results
Stacks of 7 ERS and 11 Envisat
interferograms (cumulative
observation times of 21.7 and
46.2 years). Time span 1996 to
2009.
Signals visible in the individual
interferograms, and not
correlated with topography
Signal width ~ 5 km
Model of fault slip
• Creep on high-angle thrust
ramp
• Significantly faster than
time-averaged rate (~1—2
mm/yr; Walker et al 2013,
Walker et al 2009)
Approximate location of
low-angle fault plane at
depth (from aftershocks
and body-waveform
modelling; Berberian 1982,
Walker et al 2003)
Cumulative Mw from 1996 to
2009 = 5.7
Conceptual model of the seismic cycle at Tabas
This structural style is common in the geological record; here produced by afterslip
Walker et al 2013
The Shahdad
thrust belt and
Gowk fault
1998 Mw 6.6
Fandoqa
earthquake
Coseismic and first 6
months of postseismic
Right-lateral strike-slip with normal
component (displacements
saturated on this scale)
~8cm slip on low-angle thrust
(probably postseismic)
(Berberian et al 2001; Fielding
et al 2004)
5 to 11 years post-
earthquake
Ele
va
tio
n (
m)
LOS
disp
lace
me
nt (m
m)
Black: topography
Green: co- and early post-seismic
Red: 5 to 11 yrs postseismic
9 descending-track interferograms, 30 yrs cumulative time
Descending-track
interferogram stack
(ascending track unusable due to atmospheric effects)
Model of fault slip
Field photos (in 1998) from James Jackson
Transient or steady-state motion?Earthquake stress-changes would lead to
NORMAL-faulting
Pre-earthquake interferogram
Variation of slip rate through time
Driving forces for the steady-state creep
1.5 MPa
Lith
ost
ati
cp
ress
ure
Slip-rate vs shear stress
Inconsistent with: Diffusion or pressure-solution creep
Dislocation creep
Consistent with: rate and/or state dependent friction, e.g.
(if so, ‘a’ ~ 10-3)
> 40 mm/yr
1.6 to 1.9 MPa
3 to 8 mm/yr
1.5 MPa
Conclusions
• Creep on two ramp-and-flat fault systems
• Postseismic (both examples) and steady-state creep (Shahdad)
• Fault rheological law at Shahdad (rate-dependent friction)
• Contrasts: - postseismic timescales (< 6 months to > 30 yrs)
- ‘Flat’ behaviour (Mw 7.3 earthquake vs creeping)
-> possible effects of depth and stress pertubation?
• Construction of geomorphology and shallow geological structures
by aseismic creep
• Implications for hazard assessment (stress-shadowing and creep)
Ascending-track stack: 6 interferograms, 17.1 years