Aftershock Cluster, Canterbury

A cluster of moderately strong aftershocks rattled the Canterbury region and residents just before midnight last night.

Tuesday 7th September 2010

Aftershock activity in the Canterbury region eased during Monday morning, the strength and frequency of earthquakes reducing. During the afternoon, the rate picked up before again easing during the evening hours.

As residents relaxed into sleep a cluster of moderately strong aftershocks began, waking sleepers and stressing residents with already frayed nerves. At 11:24 p.m. a magnitude 5.2 earthquake struck 20 km west of Christchurch (20 km south-east of Darfield) at a depth of 9 km. Residents described this as a noisy quake and it was noticeably bigger than other aftershocks that had been experienced in city suburbs during the day. A deeper magnitude 4.0 quake followed at 11:38 p.m. This aftershock was centred at the same location.

At 11:40 p.m. a shallow magnitude 5.4 quake struck further inland. This quake, though stronger, was further from the major population areas and city residents thought the experience was similar to the 11:24 quake. The epicentre was 60 km west of Christchurch (20 km south-west of Darfield) at a depth of 9 km.

The GeoNet website showed the strain of nervous residents querying the site for aftershock information as new events were occurring. The duty seismologist was quick in analysing the quake data and the site was promptly updated, giving Cantabrians an impression of what the renewed activity meant. Through various online forums, many expressed the hope that the 11:24 quake was the dreaded magnitude 6 aftershock that past experience suggests will follow such a big earthquake.

A series of magnitude 3 and 4 aftershocks followed before activity eased around 1 a.m. and residents were able to relax into sleep. Around 3 a.m. some small magnitude 3 aftershocks were felt, followed by another magnitude 5 event at 3:24 a.m. This quake of magnitude 5.4 was centred 30 km south-west of Christchurch (20 km south-east of Darfield) at a depth of 15 km.

Four aftershocks of magnitude 3 have been reported by GeoNet since that time.

McQueen’s Valley Seismic Drum 7 September 2010

[click to enlarge]
This snapshot of the McQueen’s Valley seismograph (courtesy of GeoNet) shows the larger aftershocks experienced in the Canterbury region over the last day. The midnight quakes are the red traces to the right of the 10 marker, showing they occurred about 10 hours before this snapshot was taken. The early morning aftershock appears to the right of the 6 and 8 markers, the quake having struck 7 hours before the image was captured.

The variation in the density (number) of traces on the image shows how the aftershock rate varied through the day. The top half of the image has several traces caused by earthquake shaking, but they are smaller than the midnight events. The lower part of the image shows fewer earthquake traces reflecting the relatively quiet period experienced following the 3 o’clock aftershocks.

The red colouring indicates that the trace has been “clipped” to prevent it overwriting and hiding traces elsewhere on the seismogram. These are the larger aftershocks, whereas the traces that are only black are either smaller quakes or further from the McQueen’s Valley recorder.

Residents’ experience of the larger aftershocks will vary by how close they were to the epicentre, the type of soil they were on at the time, and the condition of their houses. The magnitudes being reported by GeoNet are a measure of the energy released by each earthquake, so being closer to the event means it will be felt more strongly. The noise generated by the house, sounds of items toppling from shelves and behaviour of occupants and pets all add to the seriousness of the experience.

It is hard for those of us outside the region to realise that very shallow magnitude 5 earthquakes are notable events in themselves and, if we were to experience one during the middle of the night, the noise and shaking would have us diving under tables or heading for doorways.

For southern Hawke’s Bay residents, this was amply illustrated late this morning when a magnitude 5.2 earthquake struck the area. The quake, which struck at 10:48 was centred 20 km south-east of Porangahau (90 km south of Hastings) at a depth of 15km. By midday, more than 600 reports of the event had been filed with GeoNet, with strong shaking reported at Hastings, Waipukurau, Dannevirke and Porangahau. Light damage has been reported at Porangahau. The quake was felt from Gisborne to Wellington.

To put up with this constant background of small to moderate earthquakes begins to grate on the nerves after a while. Cantabrians are to be admired for plodding through this while dealing with the damage and mess in their domestic environments and the upheaval in day-to-day life.

Simply turning on the tap to rinse some salad vegetables for dinner requires a new routine. It shouldn’t be done at the present time. Christchurch, once so proud of its tasty water, now has to boil the stuff for three minutes before it can be used safely. Whilst contamination is not universal, it is still possible that local pollution from broken sewer lines is contaminating the water supply to some houses while the neighbour’s supply is fine. Therefore the boiling instruction is wise advice while so much infrastructure remains to be inspected and repaired.

An automatic reaction – I need water, so I turn a tap on – has to be replaced with a new routine. One resident remarked on how many times we wash our hands while preparing a meal. With the possibility of the tap water being contaminated this routine also has to be modified.

Even minor annoyances such as these are part of the new daily routines being experienced in a region where houses are damaged, some people are without toilet facilities, and there is a constant background of aftershocks. There is such widespread variation in people’s circumstances, that some residents have only minor issues to deal with, and their homes are sound and secure.

At the other end of the spectrum is life in the town of Kaiapoi where liquefaction was widespread and many homes are uninhabitable. I was heartened to watch, on television last night, some of the residents pause in their cleanup work yesterday and park their shovels to clap as a truck turned up with portaloos on the back! Thank heavens for small mercies.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST and eyewitness reports from friends in the Canterbury region.]

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