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 25 th 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
Sigrun Hreinsdottir et al (preliminary 6/12/16) Coseismic geodetic measurements Hmax: 6m; Vmax: 2m Vertical Horizontal
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 Papatea fault Active fault trace AFDB + NIWA 2016 Surface rupture 2012 NSHM fault source
Western coastal trace (left-lateral, reverse) Photo: Robert Langridge Photo: Julian Thomson Photo: Robert Langridge
Kekerengu Fault Papatea 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 (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 Papatea fault Fault rupture Fault rupture 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 m 3 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.