Download Surface rupture of the 14 Nov 2016 Kaikoura Earthquake .. and more

The Seismic Hazardscape of New Zealand
Mark Stirling
Professor of Earthquake Science
Introduction
• Plate tectonic setting of New Zealand
• Seismic hazards for University of Otago campuses
• Kaikoura earthquake investigations
Plate Tectonics
• New Zealand lies on the
boundary of the Pacific and
Australian plates
• The plate boundary is
expressed by a zone of
active earthquake faults,
seismicity, and crustal strain
Seismicity
• Moderate to high rates of crustal
(shallow) seismicity along the plate
boundary, and at depth under the
North Island and Fiordland
• Relatively low levels of seismicity in
Auckland/Northland and
Otago/Southland
• Low seismicity rates in the Canterbury
Plains pre 2010
• High seismicity rates in Wellington
Earthquake Faults
Geodetic strain
Log Shear Strain
max(-2.5,log10 (shear strain))with Principal Contraction
Directions
Wellington
• The greater Wellington region has had two major earthquakes (M>7)
in historic time, but there are many other faults that could produce
major earthquakes close to the city
• Wellington Fault
• Ohariu Fault
• Pukerua Fault
• Hikurangi subduction zone
Wellington area faults
Wairarapa Fault and M8.1-8.2 1855 Wairarapa earthquake
Wellington Ground
Motions
Wellington experienced significant damage
from recent Cook Strait and Kaikoura
earthquakes
The city is not earthquake resilient (cf
Christchurch)
A major local earthquake will have
catastrophic impacts
Anna Kaiser, GNS Science
Christchurch
• Seismically quiet until the M7.1 2010 Darfield earthquake and Canterbury
earthquake sequence.
Christchurch
• Christchurch has probably had it’s worst-case earthquake: Direct hit on the city
in the form of the M6.2 February 2011 Christchurch earthquake
• Other major earthquakes are expected to occur at greater distances from the
city: Southern Alps foothills and north Canterbury (c.50km distance), and
Alpine Fault (>100km distance)
• The new Christchurch will be a more earthquake resilient city than before:
building stock, hazard guidelines, and community
• Good ground conditions in vicinity of Otago University campus
Dunedin
• Low seismicity area. Akatore and Titri Faults are located close to the city.
• May have had it’s worst case events in the time period 750-1300 yrs BP: Akatore
Fault study
• However, building stock is old and fragile: regional earthquakes will be damaging
Dunedin
• 13 earthquake faults within 100 km of
Dunedin city
• Fiordland and Alpine Fault earthquakes could
also produce significant damage to the city’s
old built environment
Auckland and Invercargill
• Lowest seismicity areas of the country, and closest active faults >50km distant.
• Hard ground conditions at Auckland campus: good news for seismic hazard
• Soft ground conditions in Invercargill: susceptible to strong shaking and ground
damage from major Alpine Fault and Fiordland earthquakes
Auckland and Invercargill
Earthquake Hazard
Hazard rank by city:
1.Wellington
2.Christchurch
3.Dunedin
4.Auckland/Invercargill
Tentative Risk rank by city:
1.Wellington
2.Dunedin or Auckland
3.Christchurch
4.Invercargill
The Mw 7.8 14 November 2016 Kaikoura Earthquake
Location and aftershocks
Up to Friday 25th November
Two M6+ aftershocks (largest M6.3) occurred
within two hours of the mainshock.
Focal Mechanisms
Mainshock thrust mechanism
Aftershock mechanisms thrust and strike-slip
John Ristau
Epicentre
M7.8
Stephen
Bannister
Observed fault
ruptures and
amount of
displacement (m)
InSAR
Radar satellites orbiting at 700 km measure the displacement of the Earth’s surface
by measuring the change in the path length between two radar acquisitions.
Each contour represents 11.5 cm of ground motion either towards or away from the
satellite
Ian Hamling
Coseismic geodetic
measurements
Vertical
Hmax: 6m; Vmax: 2m Horizontal
Sigrun Hreinsdottir et al (preliminary 6/12/16)
Ground Motions: Peak Ground
• PGA confirmed up to 1.3g
Acceleration (PGA)
(Ward station)
• Ground shaking significantly
lower in Christchurch than
Wellington due to northward
rupture from epicentre and
distribution of fault slip
Anna Kaiser
Humps-Leader Fault Fault
Active fault trace AFDB
+Barrell et al 2012
2016 Surface rupture
2012 NSHM fault source
The Woodchester
Wall
Photo: Kate Pedley
Photo: Andy Nicol
Hundalee Fault
Active fault trace AFDB
+Barrell et al 2012
2016 Surface rupture
2012 NSHM fault source
Photo: Julie Rowland
Photo: Mark Stirling
NE trend thrust , ~
1m vertical
3.7m right-lateral, and
2m vertical slip at coast
Photo: Mark Stirling
Papatea fault
Papatea Fault
Active fault trace AFDB
+ NIWA
2016 Surface rupture
2012 NSHM fault source
Western coastal trace (left-lateral, reverse)
Photo: Robert Langridge
Photo: Robert Langridge
Photo: Julian Thomson
Papatea fault
Kekerengu Fault
Active fault trace AFDB
+ NIWA
2016 Surface rupture
2012 NSHM fault source
Trench 1
Photo: Tim Little
Trench 1, as of Nov 20, 2016 (looking NE)
Drone Photo Courtesy Julian Thomson, GNS
Coastal
uplift
(Photo: Pilar Villamor)
High tide
Seal snoozing in former subtidal zone
Stranded crayfish
Low tide tide: subtidal kelp exposed
Photo: Pilar Villamor
Photos: Kate Clark
Needles Fault
Fault rupture
Fault rupture
Papatea fault
Source: NIWA
Active fault trace AFDB
+ NIWA
2016 Surface rupture
2012 NSHM fault source
Landslides
• Initial estimates of 80,000 - 100,000
landslides triggered
• ~5 >1,000,000 m3
• Identified ~50 landslide dams
Association with huge landslides close
to the coast
Photo: Robert Langridge/Julie Rowland
Seaward Landslide
(previously known as “cow
island” or “stranded cow”
landslide)
Photo: Robert Langridge/Julie Rowland
Conclusions
• Ranking of Otago campuses in order of decreasing earthquake hazard:
1=Wellington, 2=Christchurch, 3=Dunedin, 4=Auckland/Invercargill
• Tentative earthquake risk ranking: 1=Wellington, 2=Dunedin or Auckland,
3=Christchurch, 4=Invercargill
• Kaikoura earthquake was a complex event that ruptured multiple faults;
some fully, and some partially
• Kaikoura earthquake will challenge our modelling of future earthquake
hazard
Acknowledgments
• The landowners for generously allowing us to undertake the reconnaissance
work during difficult times
• GeoNet, EQC, GNS Science and LINZ for essential data
• Images and funding from GNS Science, University of Otago, University of
Canterbury, University of Auckland, Victoria University of Wellington, NIWA,
University of Southern California, Humboldt State University, and PG&E.