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The 2003 Bam, Iran earthquake: what we knew, what we didn’t know and what we expect in the future
Gareth Funning (University of California, Berkeley) with Eric Fielding (JPL), James Jackson (Cambridge), Barry Parsons & Richard Walker (Oxford), Morteza Talebian (Geological Survey of Iran)
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26th December 2003, Mw 6.6
Death toll 26,000
• 26th December 2003, 5.26 am local time
• Mw 6.6
• Official death toll 26,000 (some estimates 40,000+)
• Peak ground acceleration 0.97 g in city centre
• S-P time 1.9 s in city centre (i.e. 14 km from hypocentre)
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Tectonic setting
SRTM shaded-relief topography
Bam
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Tectonic setting
SRTM shaded-relief topography
Nayband fault
Gowk fault
Sabzevaran faultJebal Barez
mountains
Dasht-e Lut
Bam
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Tectonic setting
SRTM shaded-relief topography
Nayband fault
Gowk fault
Sabzevaran faultJebal Barez
mountains
Dasht-e Lut
2003, Mw 6.6
1989, Mw 5.8
1981, Mw 6.6
1998, Mw 6.61981, Mw 7.1
2005, Mw 6.3
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Tectonic setting
SRTM shaded-relief topography
Nayband fault
Gowk fault
Sabzevaran fault
Dasht-e Lut
NEIC
Jebal Barez
mountains
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Tectonic setting
SRTM shaded-relief topography
Nayband fault
Gowk fault
Sabzevaran fault
Dasht-e Lut
Jebal Barez
mountains
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The Bam fault
LANDSAT-7 ETM 541 false colour green=vegetation
Bam
10 km
Baravat
The Bam fault is a prominent ridge running between Bam and Baravat
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Walker & Jackson, 2002
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The Bam fault
Post-earthquake field surveys found only minor cracking at the foot of the ridge…
Bam
Baravat
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Preliminary InSAR data
Bam
10 km
Baravat
First Bam interferogram (each colour cycle=2.8cm of deformation)
Constructed from Envisat ASAR data released for free by ESA
There is a prominent band of incoherence running S of Bam
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The Bam earthquake main fault
Interferometric coherenceRed = highBlue = low
Constructed from Envisat ASAR data released for free by ESA
Bam
10 km
Baravat
Low coherence indicates vegetation and surface damage
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The Bam earthquake main fault
Surface rupture found in the field – right-lateral offsets of ~20 cm
Bam
10 km
Baravat
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The Bam earthquake main fault
Surface rupture found in the field – right-lateral offsets of ~20 cm
Bam
10 km
Baravat
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The Bam earthquake main fault
LANDSAT-7 ETM 541 false colour green=vegetation
Bam
10 km
Baravat
There are no surface features corresponding to this fault – we didn’t know it existed
Main fault is ~ 4km west of the mapped Bam fault
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Coseismic interferograms
There is an ‘extra’ amount of displacement in the SE quadrant
Ascending interferogram Descending interferogram
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Single fault model
Strike 354 dip 84 rake -177 slip 2.2m length 12km top 1.1km b’m 9.3km
Ascending model Descending modelFunning et al., 2005
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Single fault model
Large residuals, especially in SE quadrant (rms = 25 mm)
Ascending residual Descending residualFunning et al., 2005
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Two fault model (uniform slip)
A ‘teardrop’ feature is reproduced in the SE
Ascending model Descending modelFunning et al., 2005
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Two fault model (uniform slip)
Improved fit in SE quadrant (rms = 17 mm)
Ascending residual Descending residualFunning et al., 2005
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Two fault model (uniform slip)
LANDSAT-7 ETM 541 false colour green=vegetation
Secondary fault appears to be a southward continuation of the Bam fault
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P
SH
Jackson, Bouchon, Fielding, Funning et al., 2006
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P
SH
Jackson, Bouchon, Fielding, Funning et al., 2006
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two sources
one source
P SH
Jackson, Bouchon, Fielding, Funning et al., 2006
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Aftershock distribution
Seismogenic crust may extend ~10 km below the rupture
Jackson, Bouchon, Fielding, Funning et al., 2006
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Strong motions & directivity
Rupture velocity of 2.8 km/s ≈ Rayleigh wave velocity
Jackson, Bouchon, Fielding, Funning et al., 2006
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Deformation is continuing
Fault zone collapse and postseismic uplift resolved with InSAR
Eric Fielding
Jan 2004-Mar 2006
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The Bam earthquake: findings
• The mainshock occurred on a blind fault; not the mapped fault
• Average slip was 2 m at 2–10 km
• The pattern of surface displacements suggests secondary oblique motion on a W-dipping fault
• Seismic data are consistent with 2 sub events
• Significant unruptured fault area may remain