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Earthquake and Tsunami Losses from Major Earthquakes Affecting the Wellington Region

Jim Cousins, William Power, Umut Destegul, Andrew King, Richard Trevethick, Russell Blong, Brad Weir and Ben Miliauskas

Following the Boxing Day tsunami of 2004, the Minister of Civil Defence for New Zealand released the report “Review of Tsunami Hazard and Risk in New Zealand”. The findings of this report surprised many - for a 500-year return period event, forecasts of property damage ranged from $12 to 21 billion. These values were multiples of most estimates of earthquake shaking damage. Recognising the importance of the findings, Benfield commissioned GNS to advance the research using state-of-the-art modelling techniques. The specific aim was to model the combined shaking and tsunami losses from large earthquakes near Wellington.

The work required significant original research on potential tsunami sources, newly compiled elevation and bathymetry data, experience with the recently developed Australian ANUGA tsunami wave propagation model, a detailed building assets model, and a new assessment of tsunami damage to buildings.

We modelled tsunamigenic earthquakes on the Wellington Fault (the source of the present earthquake PML), the Wairarapa Fault (a historic tsunami source, second largest earthquake shaking loss), the BooBoo Fault (a large fault in Cook Strait) and the Subduction Zone (a major tsunami source). The results showed that the Wellington Fault earthquake still represents a reasonable benchmark for risk assessment purposes, with the combined earthquake and tsunami losses generated for four key earthquake scenarios either adding little to the Wellington event itself, or accumulating to significantly less than that earthquake on a stand alone basis.

Paper P24: [Read] [Presentation]

Borehole Instrument Centre for Eden Park: Development and Research

Liam Wotherspoon, Catherine Kenedi and Peter Malin

This paper provides an overview of the characteristics of the Borehole Instrument Centre for Eden Park (BICEP), a 383 m deep instrumented borehole beneath the new South Stand of Eden Park Stadium in Auckland, New Zealand. The borehole was permanently instrumented using borehole seismographs at 26 m and 383 m depth, providing a three dimensional view of the dynamic characteristics of the strata beneath the highest point of the new stand. The intent is also to install recording equipment within the stadium structure itself. Sources of dynamic excitation include seismic events from below and excitation of the overlying stadium from wind and crowd movements. Details of the borehole construction, instrumentation setup, and initial recordings are provided in this paper.

The BICEP site is part of the Strata to Structure project, which focuses on the development of a better understanding of the seismic and dynamic characteristics of the Auckland region from deep in the rock/soil strata up to the ground surface and into overlying structures. It provides opportunities for both earth science and engineering research and encourages collaboration between these fields. Multiple paths of research can be followed including soil structure interaction, seismology and geologic characterisation.

Paper P25: [Read] [Presentation]

Seismic Intensity Measures from Strong Motion Records

Peter Davenport

The concept of seismic felt intensity has been used to classify the severity of the ground motion at a given location on the basis of effects observed either during the earthquake or afterwards. With widespread use of strong motion recorders, it is possible to obtain engineering parameters such as peak ground acceleration, velocity and displacement, spectral values and other measures of instrumental intensity. Many studies have compared these strong motion parameters to the seismic felt intensity but have found the correlation is usually poor and the relationships are highly nonlinear. In this paper, a study of the nonlinear response of a set of stylized buildings of various strengths and periods to recorded strong motion records from New Zealand is reported. Artificial Intelligence methods are used to recognize patterns in the resulting large volume of data. Conclusions are drawn about the relationship of the patterns of response to the observed felt intensity.

Paper P26: [Read] [Presentation]

Precis of Natural Seismoscope Studies in the Southwestern United States and New Zealand

Mark Stirling and Albert Zondervan

Unstable bedrock landforms can be regarded as natural low resolution seismoscopes that have been in operation for time periods well beyond those of historical records. This class of landform includes unstable “unravelling” cliff faces, buttresses, pinnacles (“unstable outcrops”), and precariously-balanced rocks (PBRs). By virtue of the instability and prehistoric age of these landforms, they have the ability to provide information on non-exceedance of ground motions for long return periods (10⁴-10⁵ years), and therefore have potential for testing probabilistic seismic hazard (PSH) models. A recent study carried out near the Yucca Mountain (YM) proposed high level nuclear waste repository in Nevada shows unstable outcrops to have survived 24-40kyrs of regional earthquakes. This is inconsistent with ground motion predictions from PSH models developed for YM a decade ago, but consistent with a newer and greatly simplified PSH model that incorporates the state-of-the-art next generation attenuation (NGA) models. A parallel study in the more humid New Zealand environment shows PBRs to generally be 10³-10⁴ years old, considerably younger than their desert counterparts, and therefore more limiting as a criterion for testing long-return-period ground motions. In light of these results, future New Zealand-based efforts will focus on studying unstable outcrops near major active plate boundary faults (e.g. Alpine Fault), where relatively young landforms will have been subjected to multiple near-field earthquakes.

Paper P27: [Read] [Presentation]

Preliminary Results of Ground Motions Simulation for a Subduction Earthquake

Caroline Francois-Holden, John Zhao and Hiroe Miyake

As part of the “It’s Our Fault” project, we are working on estimating ground motions from large plate boundary earthquakes at specified locations in the Wellington region in terms of response spectra and acceleration time histories. These motions may provide synthetic strong-ground time histories for a future major earthquake. For engineering applications in NZ, considering the high frequency content of a synthetic accelerogram is a vital part of any dynamic loading analysis. To do this we need to produce broadband accelerograms for which we use an empirical Green’s function technique that was developed by overseas researchers. First we characterize the fault parameters using waveform inversion. Then, we define a suitable set of source parameters, such as the area of fault plane, moment magnitude, slip distribution (fault heterogeneity) within the fault plane and the propagation pattern of the rupture. With these parameters, we can generate synthetic accelerograms that contain the signature of all parameters for the specific fault. The method can also be used for a future earthquake using fault model parameters derived from empirical scaling functions. The synthetic records will not only contain the required response spectra but also appropriate duration of strong ground shaking specifically for a given fault. We will present preliminary results from our initial trial using the strong motion dataset from the 2003 Fiordland earthquake. The work presented here may have far-reaching effect for selecting accelerograms for a particular site.

Paper P28: [Read] [Presentation]

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