7.2 Engineering Seismology
Comparison of Attenuation Characteristics between the Data from Two Distant Regions: New Zealand and Japan
John X. Zhao & Matt C. Gerstenberger
ABSTRACT: Empirical prediction equations for response spectra are critical for seismic hazard analysis and models have been developed for many regions. However, the number of high-quality strong-motion records varies enormously based on the region. For example, as many as 90% of the globally-available high quality strong-motion records from subduction earthquakes are from Japan and New Zealand. Even with over 3000 strong-motion records, the distribution with respect to magnitude, source distance and focal depth of the New Zealand data is poorer than that of the dataset from Japan. For shallow earthquakes, the effect of possible Moho reflections, which were observed in the dataset from Japan, are also prominent in the New Zealand data. For deep New Zealand earthquakes, the variation of geometric attenuation with depth and source distance, which was found in the Japanese data, was not recognized because of the narrow distance range of the records for each deep earthquake. A careful re-examination revealed that the effect of wave-propagation paths was prominent in the New Zealand data set, but that the ‘error’ migrated into the inter-event error and an upwardly estimated depth term from deep New Zealand earthquakes. Our results suggest that the attenuation characteristics between New Zealand and Japan are remarkably similar but differ in anelastic attenuation rates.
A Procedure for Interactively Processing Digital Acceleration Records to Extract Permanent Displacement and a Comparison with GPS Data from the 2010 Darfield Earthquake
John X. Zhao, John Beavan, X.W. An, X.D. Yang & T.S. Song
ABSTRACT: A valuable set of digital strong-motion records was obtained from the 2010 Darfield, New Zealand, Mw=7.1 earthquake. We have derived permanent displacements from some of the near-source records using an interactive processing method. Most such methods involve subjective selection of processing parameters and the results can vary significantly if different but all seemingly reasonable values for the processing parameters are used. We have developed a set of rules for the interactive processing using Microsoft Excel. This procedure is easy to use and the rules can narrow the range of values for the processing parameters. The processing procedure does not alter the acceleration time histories unreasonably and does not produce sharp acceleration spikes. We processed a number of strong-motion records from the 2010 Darfield, New Zealand, earthquake “blindly”, i.e. without any other references, and the permanent displacements presented were generally very similar to the displacements obtained from GPS data.
Site-Effect Terms as Continuous Functions of Site Period and Vs30
G.H. McVerry
ABSTRACT: New site-effect terms that are a continuous function of site period Tsite are proposed to replace the spectral shape factors of the New Zealand structural design standard NZS1170.5. The new terms eliminate the 63% jump in shape factor between Class C Shallow Soil and Class D Deep or Soft Soil for spectral periods T beyond the peak of the Class D spectrum at about 0.6s. The proposed changes involve a gradual transition from the current shallow soil to deep soil factors over the site-period range 0.25s to 1.5s.
The new model represents the site-effects with respect to rock of the 5% damped spectrum by the simple form lnSiteEffect(T)= a(T) + b(T)*Tsite, where a(T) and b(T) are period-dependent fitted coefficients. NGA-type site-effect terms based on Vs30, the average shear-wave velocity to 30m depth, were also considered.
This site-period model fits the New Zealand attenuation model dataset better for spectral periods of 0.5s and longer than the original site-class model and the Vs30 model. For New Zealand earthquake records, Vs30 is a poor site-effects parameter at long spectral periods because long-period sites of the New Zealand strong-motion network are often associated with considerable depths of stiff gravels, rather than low Vs30 materials.
NZS 1170.5:2004 Site Subsoil Classification of Wellington City
S. Semmens, N.D. Perrin, G.D. Dellow & R. Van Dissen
ABSTRACT: Wellington has an appreciable seismic risk due to the proximity of its concentrated population and infrastructure to several major earthquake sources. Geotechnical data mainly from 1025 drill holes, along with shear-wave velocity (Vs) determinations specific to this project were used to construct a 3D engineering geological model for Wellington City centre. From this model, the following maps were derived, and are presented in this paper: surficial geology, depth to bedrock, low amplitude site period, and NZS 1170.5:2004 site subsoil classes. The results show that a significant ground shaking amplification hazard is posed to the city, with the waterfront, Te Aro and Thorndon areas having a poorer site subsoil class in terms of NZS 1170.5:2004 than previous studies had estimated.
Modelling Strong Ground Motions for Subduction Events in the Wellington Region, New Zealand
C. Holden & J.X. Zhao
ABSTRACT: The work presented here is focused on simulating ground motions from potential large plate boundary subduction thrust earthquakes at specified locations in the Wellington region in terms of response spectra and acceleration time histories. We employ the methodology of Irikura et al. (2004) and validate their code and procedures using the strong motion dataset from the 2003 Mw 7.2 Fiordland earthquake applying both empirical and stochastic Green’s functions. The method was satisfactorily tested using an intraslab rupture and a record from a nearby aftershock as the empirical Green’s function (EGF). For the stochastic Green’s functions (SGF) approach, we adopt the work of Motazedian & Atkinson (2005) because we consider their assumption of a finite fault source model (instead of a point source) is more appropriate in the Wellington case where events have magnitudes > 5, and distances < 200 km. We are currently applying these techniques to a number of source scenarios that sample a range of plausible subduction interface ruptures underneath Wellington. Key parameters that we vary are hypocentre location, stress drop, and overall rupture area. In addition, recent advances in detailed modelling of the Wellington basin geology will allow us to include site effects in our simulated ground motion results.