Strong ground motion characteristics observed in the 13 June 2011 Mw6.0 Christchurch, New Zealand earthquake

Abstract

This paper examines the salient characteristics of near-source strong ground motions recorded from the 13 June 2011 Mw6.0 Christchurch, New Zealand earthquake. 26 strong motions were recorded within Rrup=30km of the source, with maximum geometric mean horizontal shaking exceeding peak accelerations and velocities of PGA=1.5g and PGV=100cm/s, respectively. The polarity of strong motion recordings with Rrup<6km illustrate the complexity of the seismic source, supporting the two-fault model of Beavan et al. [4]. Directivity in the observed ground motions in eastern and central Christchurch is dominated by the secondary rupture of a right-lateral ENE-trending fault plane, which produces polarization in a predominantly NS direction. As seen in previous events in the Canterbury earthquake sequence, significant basin-generated surface waves led to strong long period ground motions with relatively long significant durations. The installation of strong motion instruments in the Port Hills following the 22 February 2011 Mw6.2 Christchurch earthquake resulted in five new observed ground motions on rock or shallow soil sites, relative to past events, with the PARS and GODS stations on top of the causative faults recording very large amplitudes, inferred as a result of site response and topographic effect coupling. Significant nonlinear site response is evident at several locations in eastern Christchurch, resulting in truncation of acceleration amplitudes, with liquefaction-induced cyclic mobility clearly evident at two locations. Pore pressure recordings at five locations in eastern Christchurch illustrate that significant excess pore water pressure (EPWP) generation in a Mw5.3 event, which occurred 80min prior to the Mw6.0 event, resulted in residual EPWP's that had a measurable influence on high frequency ground motion amplitudes at soft soil sites in the latter event. Finally, the observed ground motion amplitudes are compared with empirical models for response spectra and significant duration.