Earthquake damage and casualties due to large earthquakes impacting Wellington Region W. J. Cousins GNS Science Report 2013/41 July 2013 BIBLIOGRAPHIC REFERENCE Cousins, W. J. 2013. Earthquake damage and casualties due to large earthquakes impacting Wellington Region, GNS Science Report 2013/41. 12 p. W. J. Cousins, GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand © Institute of Geological and Nuclear Sciences Limited, 2013 ISSN 1177-2425 ISBN 978-1-972192-87-0 CONTENTS ABSTRACT ........................................................................................................................... II KEYWORDS .......................................................................................................................... II 1.0 INTRODUCTION ........................................................................................................ 1 2.0 COMPUTATION OVERVIEW ..................................................................................... 2 3.0 RESULTS AND DISCUSSION ................................................................................... 3 4.0 PRECISION OF THE MODELLING ............................................................................ 5 5.0 ACNOWLEDGEMENTS ............................................................................................. 6 6.0 REFERENCES ........................................................................................................... 6 TABLES Table 1.1 Earthquake scenarios used in the modelling. ............................................................................... 1 Table 3.1 Estimated costs for earthquake damage, and additional costs due to subsequent tsunami damage. ....................................................................................................................................... 3 Table 3.2 Numbers of collapsed buildings caused (a) by earthquake shaking, and (b) subsequent tsunami inundation. ...................................................................................................................... 3 Table 3.3 Estimated deaths from earthquake and tsunami inundation. ........................................................ 4 Table 3.4 Estimated injuries from earthquake and tsunami inundation. ....................................................... 4 APPENDICES APPENDIX 1: STATE-BASED MODELLING OF DAMAGE AND INJURY FROM EARTHQUAKES ........................................................................................... 9 APPENDIX FIGURES Figure A 1.1 Example of the relationship between shaking intensity and the probability of being in one or other of the defined damage states. The lines are the upper boundaries to the damage states, with the red line being the upper boundary of the DS5 state, and so on. ........................ 11 APPENDIX TABLES Table A 1.1 Damage state definitions for buildings, and indicative consequences. ....................................... 10 Table A 1.2 Adopted correspondence between loss ratio and damage state. ............................................... 11 Table A 1.3 Casualty state definitions............................................................................................................ 12 Table A 1.4 Examples of the relationship between building damage state and casualty state for two types of building, URM (unreinforced masonry) and Timber. As an example of the relationship, if a URM building is in DS3, then there is 95.76% probability of an occupant being in Casualty State 1 (CS1), 4% probability for CS2, 0.24% probability for CS3, and 0% probability for CS4 and CS5. ................................................................................................ 12 GNS Science Report 2013/41 i ABSTRACT Damage and casualties have been estimated for seven large earthquake scenarios located in the Wellington Region. Four of the earthquakes, involving rupture of the Wellington, Wairarapa and Ohariu Faults, and the Hikurangi Subduction Zone, are considered to be the most costly and deadly earthquakes likely impact the Wellington Region. In the modelling they generated shaking losses ranging from $9 billion to $16 billion, death numbers ranging from 400 to 1600 for a daytime event, 140 to 470 for night-time, and injury numbers from 4000 to 9000 daytime, 3000 to 7000 night-time. The remaining three scenarios involved the Wairau and BooBoo Faults, and the segment of the Wellington Fault that runs through the Tararua Range starting about 40 km north of Wellington. They resulted in much lower losses and casualties, ranging from $1 billion to $1.6 billion for losses, 10 to 60 daytime deaths, 1 to 14 night-time deaths, 300 to 400 daytime injuries, and 100 to 300 night-time injuries. Additional losses and casualties due to subsequent tsunami were also estimated for four of the scenarios. The necessary tsunami data were not available for the Ohariu and Wairau Fault cases, and the Tararua segment of the Wellington Fault was not considered for tsunami because it is entirely on-land. The tsunami made relatively modest contributions to the overall losses, but large contributions to the numbers of deaths. For the scenario involving rupture of Hikurangi Subduction Zone, the numbers of tsunami deaths greatly exceeded those caused by shaking damage (3200 vs. 370 for a daytime event, and 2500 vs. 140 for a night-time event). KEYWORDS Earthquake, fault rupture, tsunami, casualties, deaths, injuries, damage costs GNS Science Report 2013/41 ii 1.0 INTRODUCTION The purpose of this report is to present preliminary results from current work on estimating damage and casualties due to large earthquakes impacting the Wellington Region of New Zealand. It is intended to focus on the results, with only very brief comment on the earthquakes, the buildings and people models, and the methodology. Seven earthquake scenarios have been modelled (Table 1.1). Four were located within about 25km of Wellington City (Wellington Fault, Wairarapa 1855, Ohariu South, Subduction Zone), and three more than 25 km away (Tararua, Wairau, BooBoo). For further information on the earthquake sources see Stirling et al. (2012) [16]. With the exception of the Tararua scenario, all of the earthquake ruptures extended offshore, and so were expected to be accompanied by tsunami. Inundation data were available for four of the scenarios (Wellington Fault, Wairarapa 1855, Subduction Zone and BooBoo) so that combined shaking and tsunami impacts could be estimated for those four events. Table 1.1 Earthquake scenarios used in the modelling. Source Name Magnitude Earthquake Name Wellington Fault (Wellington-Hutt Valley segment) 7.5 Wellington Fault Wairarapa Fault (last ruptured in 1855) 8.2 Wairarapa 1855 Hikurangi Subduction Zone (rupture into Cook Strait) c. 8.9 Subduction Zone Ohariu Fault (southern section) 7.4 Ohariu South Wellington Fault (Tararua segment) 7.3 Tararua Wairau Fault 7.8 Wairau BooBoo Fault 7.6 BooBoo Detailed assets models had previously been developed for most of Wellington Region area as part of RiskScape, a risk modelling package being developed jointly by GNS and NIWA (Institute of Geological and Nuclear Sciences, www.gns.cri.nz, and National Institute of Water and Atmospheric Research, www.niwa.co.nz) (King & Bell [11]). Attributes attached to each building in the models included the location, replacement value, structural type, age (i.e. era of building code) and quality (i.e. with or without structural deficiencies), and the numbers of occupants for night-time and work-daytime scenarios. The final model contained 194,000 buildings, occupied by 460,000 people. It was on a building-by-building basis for most of the region, the exception being residential buildings in the Wairarapa which were in the model but aggregated to Area Unit scale. Ground hazard ratings for shaking amplification, liquefaction and landsliding were also attached to the building locations. The hazard ratings were taken from GNS’s in-house databases. They consisted of five-point ratings for each of the three phenomena, A to E for amplification (paralleling the A (strong rock) to E (very soft soil) ground classifications defined in the New Zealand Loadings Standard [15], and 1 (zero hazard) to 5 (very high hazard) for each of liquefaction and landsliding [2,13]. GNS Science Report 2013/41 1 2.0 COMPUTATION OVERVIEW For earthquakes, the computation procedure was as follows: • Select an earthquake model (magnitude, location, mechanism of rupture). • Estimate the MM (Modified Mercalli) intensity at each asset location using the Dowrick and Rhoades attenuation model for New Zealand [9]. • Adjust the intensity to allow for ground hazards [3,11]. • Estimate the repair cost using a damage ratio method [3]. • Estimate the damage state for each building using a damage state vs. damage ratio relationship developed by Spence et al. [4,5,14]. See Appendix 1 for brief further information. • Estimate the injury state for each person using damage state to injury state models developed by Spence et al. [4,5,14]. See Appendix 1 for brief further information. Post-earthquake tsunami impacts have previously been modelled [6] for four of the above earthquake scenarios, viz. Wellington Fault, Wairarapa 1855, Subduction Zone and BooBoo. Of the three remaining scenarios, the Wellington Fault (Tararua segment) is entirely on land and so will not generate a tsunami. Both the Ohariu Fault (southern section) and the Wairau Fault extend offshore into Cook Strait, by about 25 km for the Ohariu Fault and 40 km for the Wairau Fault. For