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 experienced significant damage Wellington Ground from recent Cook Strait and Kaikoura Motions 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 • LowDunedin 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: Tentative Risk rank by city: 1.Wellington 1.Wellington 2.Christchurch 2.Dunedin or Auckland 3.Dunedin 3.Christchurch 4.Auckland/Invercargill 4.Invercargill The Mw 7.8 14 November 2016 Kaikoura Earthquake Location and aftershocks Focal Mechanisms Up to Friday 25th November Mainshock thrust mechanism Two M6+ aftershocks (largest M6.3) occurred Aftershock mechanisms thrust and strike-slip within two hours of the mainshock.
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 Vertical measurements Hmax: 6m; Vmax: 2m Horizontal
Sigrun Hreinsdottir et al (preliminary 6/12/16) Ground Motions: Peak Ground Acceleration (PGA) • PGA confirmed up to 1.3g (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: Mark Stirling Photo: Julie Rowland
NE trend thrust , ~ 1m vertical 3.7m right-lateral, and 2m vertical slip at coast
Photo: Mark Stirling
Papatea Fault fault fault
Active fault trace AFDB
Papatea + NIWA 2016 Surface rupture 2012 NSHM fault source Western coastal trace (left-lateral, reverse)
Photo: Robert Langridge
Photo: Robert Langridge Photo: Julian Thomson
Kekerengu Fault fault fault
Active fault trace AFDB
Papatea + NIWA 2016 Surface rupture 2012 NSHM fault source Trench 1
Trench 1, as of Nov 20, 2016 (looking NE) Photo: Tim Little Drone Photo Courtesy Julian Thomson, GNS Coastal (Photo: Pilar Villamor) uplift
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
Source: NIWA fault fault
Active fault trace AFDB
Papatea + 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.