Identifying active submarine faulting
beneath Pegasus Bay in the aftermath of
the Canterbury earthquakes:
Geophysical imaging and 3-dimensional
analysis
Philip Barnes
National Institute of Water & Atmospheric Research,
Wellington, New Zealand
ALGIM 2014 GIS Symposium, 7-8th April, Auckland
Outline
1. Tectonic faulting and plate boundary deformation in NZ
2. Canterbury earthquake sequence of 2010-2012, & regional
setting
3. 2011 NIWA marine geophysical survey: Geoscience response
coordinated by the NZ Natural Hazards Research Platform
(NHRP)
4. Below the ocean floor: Identifying geological surfaces and
faulting
5. Characterising earthquake sources for hazard assessment
An Earthquake Happens When a Fault in the Earth Breaks
Three Types of Faulting..…in a simple world
Thrust
Normal
Strike-Slip
Crustal shortening
Crustal Extension
Wrench tectonics
…but pre-existing rock weaknesses (old faults) often add complexity
source USGS
What happens when a region formerly under tectonic
extension is put under compressional stresses?
…..…….tectonic structural inversion
Uplifted islands squeezed by PAC – AUS convergence
70 Myr
Zealandia
AUS
5.5 cm/yr
PAC
3.5 cm/yr
50 Myrs of crustal
extension (110-60 Ma)
Zealandia
Widespread
normal faulting
The obliquely convergent continental Pacific – Australia plate boundary zone
Photo c/o Rob Langridge
South Island
active faults
Litchfield et al. 2013
Sibson et al., 2012
Canterbury earthquake sequence of 20102012 & regional geological structure
The 2010-2012 Canterbury Earthquake Sequence
Magnitude vs Time
NHRP Geoscience Response
Magnitude
Seismic reflection
Gravity & Magnetic s
Seismology, GPS, InSAR
Lidar
Aerial photography
Geol & Geotech- field work
All material sourced from GNS Science
At least 10 faults involved…………..incl. several different types
Strike-slip
Thrust
Oblique-slip reverse
Greendale Fault rupture
Max. 5m displ.
Ristau et al., 2013
December 23rd EQ
Max. ~1.2m displ.
Beavan et al., 2011
Quigley et al., 2012
Villamor et al., 2012
February 22nd EQ
Max. 2.5m displ.,
~1m at surface
sourced from GNS Science
Synoptic Geological Map of the Canterbury Plains and Foothills
Map of Gravity
gradients
Davy et al., 2012
Jongens et al. 2012
> 60 Myr extensional fault basins buried beneath plains
Ground surface
Widespread
normal faults
Hinds Graben
Inherited faults and structural inversion
Depth km
Jongens et al. 2012
Ghisetti et al., 2012
Fault scarp
Springbank
Structure
Photo c/o Jarg Pettinga
Active submarine faults identified before the Sept 2010 Darfield Earthquake
Barnes 1996
……..already incorporated into previous assessments of seismic hazard
2011 NIWA marine geophysical survey:
Geoscience response coordinated by the
NZ Natural Hazards Research Platform (NHRP)
Responding to coastal earthquake disaster: Development of crossinstitutional decision making and coordinated geoscience operations through the NHRP
Typical land-based field geoscience responses:
• GEONET instrument deployments, seismic reflection, gravity & magnetic surveys
• Geotechnical/EQ engineering field teams
• Coastal geological field teams (fault?, tsunami, landslides…)
• Societal impacts
Marine geo response? Factors to consider:
•
•
•
•
•
•
Coastal/marine fault involved?
Earthquake magnitude and mechanism?
Seafloor rupture expected?
Tsunami?
Severity of damage, loss of lives, impacts?
What could we learn and contribute?
Understanding the cause and science
of the event,
………informing authorities for
response & recovery
Image courtesy of Ian Chan
Various marine survey options:
•
•
•
•
•
Seismic reflection profiling
Multibeam bathymetry
Sediment cores
Seafloor video/photography
Vessel availability
Philip Barnes
Claire Castellazzi
Steve Wilcox
Submarine Faulting
Beneath Pegasus Bay,
Offshore Christchurch
National Institute of Water & Atmospheric Research
Andrew Gorman
University of Otago
Research to Inform the
Canterbury Earthquake Recovery.
NHRP Short Term Project: Offshore Faults
Funding:
Natural Hazards Research Platform,
EQC & NIWA
Time (seconds two-way-travel)
Marine Seismic Reflection Techniques
Very strict permitted procedures
required for seismic surveying:
Marine Mammal Observations
Hector’s dolphin
Kaharoa Seismic Survey KAH1105
Acknowledgements:
Geosphere Ltd
High-Resolution Subsurface Profiler Data
Identifying geological surfaces and faulting
beneath Pegasus Bay
…..using seismic reflection data tied to boreholes,
submarine geological samples, and sea-level cycles
Cretaceous Rift Basins and Cover Sequences
Seismic Imaging
of
Banks Peninsula
Volcanics
A
B
C
Post-Volcanic
L. Miocene-Quaternary Gravel-rich
Cover Sequence (7 Myr – Present)
Late Quaternary Climate-Cycle Sedimentation
Interglacials: Present day
Glacials: last culmination 20 kyrs ago
Lowstand
Glacial shoreline
at canyon rim
Lowstand
Coastal
plain
Active (Holocene)
Kaikoura Canyon
Glacial age
(Lowstand) activity
in Pegasus, Pukaki,
and Okains
Canyons:
Presently relatively
inactive
Low s.l. High s.l.
Max. 120 m s.l. variations
Cox et al,. 2012; Brown & Webber 1992; Brown & Naish, 2003
Reactivated faulting & inversion tectonics
Mapping faults at NIWA in ArcGIS
Fault tip
Fault structure beneath Pegasus Bay
Young, immature inversion, & low slip rates
No Holocene
Displacement
0.13 - 0.17 mm/yr
L. Quat. faulting south of
the Pegasus Bay Fault?
Yes, but relatively short
structures, & very low
displ. faults
V. small (<20 m) vertical
displacements on
Late Miocene horizon
east of Kaiapoi
Incl. normal separation:
Possibly strike-slip
Late Pleistocene fault
with normal separation
east of Kaiapoi:
Central Pegasus Bay Fault
Possibly strike-slip
Simplified Geological Map of the Ashley River Area, West of Rangiora
• Cust Anticline
• Ashley Fault
• Springbank Fault
Campbell et al. 2012
Drawn by J.K. Campbell
Beavan et al 2011
Can we see the 22nd Feb 2011 fault offshore?
Weak folding of the cover sequence
off the Heathcote Estuary
L. Quat. faulting or velocity artifacts?
L. Quat. faulting off Banks Peninsula:
Fault geometry not constrained
Dec 23 Mw5.8
CHCH
June 13 Mw5.9
Beavan et al., 2012
Ristau et al., 2013
Ongoing research work funded by the NZ
Natural Hazards Research Platform:
Structural modelling of Inversion tectonics
1. Building a 3D subsurface geological model
2. Structural restorations of fault development & evolution
Building a 3D subsurface
geological model in depth
domain
Attributed stratigraphy and rock physical properties for
depth conversion and restoration workflows
Converting post-stack time migrated seismic to depth,
using interval velocities, to construct geological sections
Depth surfaces
G. Reynolds
How the data will be used to characterise
earthquake sources to improve hazard assessment
Canterbury earthquakes in the national tectonics & hazard context
Earthquake Source model
Ground shaking hazard maps
e.g., Peak ground accelerations
expected at 500 year return times
(informs NZ Loadings Standards)
Historical seismicity
2010 Model
500-yr PGA(g) shallow soil
Fault source domains
Greendale
Fault
Stirling et al. 2012
National Seismic Hazard Model Fault Sources
Stirling et al. 2012
1000 yr Probabilistic PGA
(Stirling et al 2012)
Fault source examples
Type Length
Dip
Type Index (km) Dip dir
Boo Boo
Ohariu Sth
Shepards Gully
WellingtonHV
ss
ss
ss
ss
3
3
3
3
90
53
51
74
90
75 NW
90
80 SE
Depth Top
15
15
15
15
Sr
0 10.00
0 1.50
0 0.50
0 6.60
Width Area
Mw SED
15 1350 7.6
16 823 7.3
15 765 7.2
15 1127 7.5
5.5
3.2
3.1
4.5
RecInt
545
2141
6180
679
Conclusions
• Canterbury earthquake is structurally complex
• Available marine seismic reflection data, incl. 2011 NIWA
survey: best insights into basement faulting
• Widespread Cretaceous-Paleogene (>60 Ma) extensional
faults
• Young (~1 ± 0.4 Ma) tectonic overprint: immature basin
inversion (~10%) + new faults
• Oblique-slip reverse + strike-slip: Ongoing studies in progress
to flesh out structural complexity
• Very low L. Quat. displacement rates; decreasing to SE
• Long EQ recurrence intervals (10 3 - 104 yrs)
• Faults already in hazard models: new results won’t
significantly change models outcomes