Re-evaluation of conditional probability of rupture of the Wellington-Hutt Valley segment of the Wellington Fault

D.A Rhoades,R.M Langridge,T.A Little,R.J Van Dissen,D Ninis,E.G.C Smith,R Robinson

doi:10.5459/bnzsee.44.2.77-86

Abstract

New information on the activity of the Wellington-Hutt Valley segment of the Wellington Fault, New Zealand, has become available from geological and modelling studies undertaken in the last several years as part of the “It’s Our Fault” project. There are now revised estimates of: 1) the timing of the most recent rupture, and the previous four older ruptures; 2) the size of single-event displacements; 3) the Holocene dextral slip rate; and 4) rupture statistics of the Wellington-Wairarapa fault-pair, as deduced from synthetic seismicity modelling. The conditional probability of rupture of this segment over the next 100 years is re-evaluated in light of this new information, assuming a renewal process framework. Four recurrence-time distributions (exponential, lognormal, Weibull and Brownian passage-time) are explored. The probability estimates take account of both data and parameter uncertainties. A sensitivity analysis is conducted, entertaining different bounds and shapes of the probability distributions of important fault rupture data and parameters. Important findings and conclusions include:  The estimated probability of rupture of the Wellington-Hutt Valley segment of the Wellington Fault in the next 100 years is ~11% (with sensitivity results ranging from 4% to 15%), and the probability of rupture in the next 50 years is about half of that (~5%). In all cases, the inclusion of the new data has reduced the estimated probability of rupture of the Wellington Fault by ~50%, or more, compared to previous estimates.

Highlights

The Wellington-Hutt Valley segment of the Wellington Fault (Figure 1), extending from offshore Cook Strait to Kaitoke, near Upper Hutt, is widely perceived to pose the greatest risk to life, property and societal infrastructure of any known active earthquake fault in New Zealand
A re-evaluation of this conditional probability was a primary goal of the Likelihood Phase of the “It’s Our Fault” (IOF) project (Van Dissen et al 2009, 2010)
The statistical method applied here is the same as that described by Rhoades & Van Dissen (2003) in a study of the conditional probability of rupture of the Alpine Fault, and applied by Rhoades et al (2004) to the major faults of the Wellington region, including the Wellington-Hutt Valley segment of the Wellington Fault

Summary

Introduction

The Wellington-Hutt Valley segment of the Wellington Fault (Figure 1), extending from offshore Cook Strait to Kaitoke, near Upper Hutt, is widely perceived to pose the greatest risk to life, property and societal infrastructure of any known active earthquake fault in New Zealand. The basic statistical method adopted is that of Rhoades et al (1994), with modifications described by Rhoades & Van Dissen (2003) and applied by Rhoades et al (2004) to the major faults in the Wellington area, including the WellingtonHutt Valley segment of the Wellington Fault. In this method, the probability of rupture of the fault in some future timeperiod of interest is expressed as a single value that accounts for both data and parameter uncertainties. The Brownian passage-time (inverse Gaussian) distribution was proposed by Ellsworth et al (1999) and Matthews et al (2002) as a physically realistic model of earthquake occurrence, and at present appears to be the most

Methods

Results

Conclusion