Showing papers by "Fred Davey published in 2011"

Natural hazards—the Christchurch earthquakes

Abstract: ‘Expect the unexpected in natural hazards’, is the theme of this editorial. Before the Darfield earthquake of 4 September 2010, Christchurch was not considered to be at high risk from local earthquake shaking. No active faults had been mapped in its vicinity, and the main risk was considered to be from a rupture of the Alpine Fault, the plate boundary fault, lying about 140 km to the northwest on the other side of the Southern Alps. The earthquake occurred at 0435 h local time and was located about 37 km west of central Christchurch (Fig. 1). Although significant damage by shaking to older buildings occurred in central Christchurch, the major damage was to buried infrastructure and to residential buildings due to massive liquefaction of the near surface sediments particularly in the vicinity of the rivers and estuaries. The mechanism of the main, 11 km deep, magnitude (Mw) 7.1 shock was initially controversial, as a distinct 24 km long surface rupture, now termed the Greendale fault, indicated an east-west orientated right lateral strike slip displacement of up to 4.6 m, whereas initial seismological models indicated a high angle reverse thrust dipping to the northeast. Further modelling demonstrated a rupture originating at 11 km depth first occurred along a high angle northeast trending thrust fault and about 10 seconds later along an east-west strike slip trending fault that broke the surface and had up to 4.6 m of strike slip and 1 m of vertical movement. Subsequent modelling, both seismological and geodetic (ground displacement), is indicating an even more complex initial faulting pattern (Fig. 1) involving the Greendale fault and as many as three blind reverse faults. Aftershocks were initially clustered around the east-west fault line, but subsequently spread beyond the ends of the Greendale fault which is consistent with a more complex net of subsurface faults. The earthquake was well recorded by the New Zealand’s national-scale GeoNet networks and the Canterbury regional strong-motion network (CanNet). The highest peak vertical acceleration recorded by strong motion seismographs was 1.26 g. As with many earthquakes, damage associated with the surface rupture was very small compared with that caused by shaking and particularly by local ground conditions giving rise to liquefaction, sand boils and lateral ground spreading. A notable feature of the aftershock sequence before the Christchurch earthquake of 22 February 2011, was the concentration of aftershocks offset to the southeast of the Greendale Fault (Fig. 1) and aligned approximately east-west, extending under the northern flank of the 5.8 Ma Lyttleton volcano that forms Banks Peninsula. The magnitude (Mw) 6.3 Christchurch earthquake occurred towards the eastern end of this zone of seismicity. It was located about 5 km from central Christchurch towards Lyttleton and at a depth of about 5 km. Damage was much more extensive than for the Darfield earthquake, as its epicentre was closer to central Christchurch, shaking was unusually strong, liquefaction and rock falls were extensive, and buildings had already been weakened by the earlier Darfield earthquake. Unfortunately, it occurred in the middle of the working day and fatalities were high. Technically, the earthquake was remarkable for several reasons. No surface rupture has been detected so far, even though the hypocentre was only 5 km deep. Peak ground accelerations were unusually high. Peak vertical accelerations reached 2.2 g and horizontal accelerations exceeded 1 g near the epicentre, where fortuitously a strong motion recorder was located. Initial analysis of seismological data suggests that the earthquake had an oblique thrust mechanism, dipping to the south at 608 to 658 and with a dextral strike slip component, on a fault plane extending from about 5 km to 1 2 km depth. It had a maximum rupture of 3.5 m with the bulk of the deformation close to the surface. The direction of propagation of the rupture *up dip* and the orientation of the fault focussed most of the energy released by the earthquake towards Christchurch giving rise to the unusually high shaking that occurred there. The propagation of the fault trace to the surface may have been stopped by the overlying volcanic rocks of the Lyttleton volcano. Geodetic modelling also indicates an oblique thrust fault model, with little deformation apparently extending offshore. In New Zealand, the identification and mapping of active fault traces have formed major components of defining the earthquake risk of a region. In both the recent major Christchurch earthquakes no causative active fault was known prior to the events, and hazard estimates have relied solely on the regional distribution of historical earthquakes. Damage was relatively minor along the 4 September 2011 rupture of the Greendale fault and no surface fault trace has been detected for the 22 February 2011 Christchurch earthquake. Liquefaction, however, has had a major impact on the environment built and natural. The characteristics of the rupture on 22 February 2011 and its New Zealand Journal of Geology and Geophysics Vol. 54, No. 2, June 2011, 149 150