Gisborne Earthquakes

Gisborne residents felt the strongest aftershock since Thursday’s strong earthquake this morning.

A magnitude 4.8 earthquake struck 30 km south-east of Gisborne at 11:35 this morning, Saturday 22nd December 2007. At 30 kilometers depth, the aftershock was shallower and closer to Gisborne than Thursday’s magnitude 6.8 event, and movement was noticeably stronger than yesterday’s quakes.

The two largest aftershocks yesterday, Friday, were a magnitude 4.5 quake at a depth of 60 km located 50 km south-east of Gisborne at 6:47 a.m., and a magnitude 4.2 event at a depth of 35 km located 40 km south-east of Gisborne at 8:33 a.m.

Seismologists advise that, in their experience, quakes of Thursday’s type have weak aftershock sequences. There were 20 aftershocks during the first 4 hours after the main quake, some of which would have been vaguely noticed by residents, but the sequence so far has been weak, and it is thought that earthquakes of Thursday’s type have relatively short aftershock sequences, with few large aftershocks.

Residents report odd instances of items falling over or dropping off walls or notice that things seem to have moved. This will be due in part to the small aftershocks, and is also due to items destablised by the large earthquake finally slipping off hooks or weakened fastenings.

Disaster insurer EQC reports that it has received 768 claims totalling over $3.9 million from Thursday’s earthquake. Most of the claims have come from the Gisborne area, with 20 claims from Wairoa. The domestic cost of the quake has already exceeded the magnitude 6.7 Milford Sound quake of October 16th, for which just under $1.5 million-worth of claims have been lodged.

A State of Civil Emergency still exists in Gisborne, and part of the commercial centre of the city is still inaccessible to shop-owners and the public. Steady progress was made yesterday with inspecting damaged buildings in the Central Business District, with more than half of the locked-down seven city blocks checked by midday.

Progress has slowed as damage is discovered to be worse than first thought. Just as in 1966, following an offshore magnitude 6.2 quake, parts of Peel Street have been hard hit, and it is generally the older heritage buildings that have suffered. Nevertheless, some business premises in the hardest hit zones have been cleared for re-use, and shop-owners have been allowed in to clear up and begin trading.

Life is pretty much back to normal, and the city is functioning, with individual pockets remaining to be inspected and either cleared for re-use or flagged for repair or demolition. Essential services such as electricity, water, sewerage and roading are operating, again with some areas of trouble being attended to.

In a media release this morning, the Gisborne District Council advised that 23 buildings in the CBD have been classified “red” and may not be entered. Eleven have been classified “yellow” and may only be entered temporarily to gain valuables. Cordons and barriers remain in place around dangerous buildings in the inner city.

Forty buildings were marked clear earlier on Friday, and another 40 have been allocated a “green” status, having passed inspection. Peel Street remains closed between Gladstone Road and Palmerston Road. Business premises outside the cordoned area have been operating normally.

The civil emergency status was to have been reviewed at midday today, but a decision has been deferred until 5 p.m.

Seismologists have prepared a short article describing the processes that caused the main earthquake and its aftershocks.

[Compiled from data provided by the Geonet project and its sponsors EQC, GNS Science and FRST, EQC website data, Gisborne District Council media releases, and eyewitness accounts]

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