Preliminary broadband modelling of an Apline Fault earthquake in Christchurch

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Holden, C.; Zhao, J.X. 2011 Preliminary broadband modelling of an Apline Fault earthquake in Christchurch. Lower Hutt, N.Z.: GNS Science. GNS Science report 2011/28 15 p.

Abstract: We have computed broadband synthetic seismograms in Christchurch for a large possible Alpine fault earthquake. In this preliminary study, we chose conservative values for all source parameters based on our current understanding of large crustal earthquake source mechanics. By using conservative parameters, we are attempting to model maximum possible ground shaking intensity. We computed the ground motion for generic rock sites in Christchurch. We subsequently superimposed the effects of soft soil condition on the modelled ground motions. Calculations for synthetic seismograms are based on a validated algorithm for large crustal earthquakes. Large accelerations are generated from localized asperities while the ground motions resulting from the rest of the fault rupture area are negligible. We distributed asperities where large surface fault displacements have been inferred from paleoseismic studies. For each asperity, records from a small or moderate earthquake were used as proxies for the Green’s functions. As such, they account for path effects incurred during propagation of the waves from the earthquake to the receiver site. Site effects for soft ground conditions were also added to account for possible amplification of ground shaking by soil layers in Christchurch. The preliminary estimates for peak horizontal acceleration are less than 4% g. These results are reasonably consistent with recorded values from recent large earthquakes (Mw > 7) and distances of 150 km+. (auth)