Serious Aftershock, Canterbury

Some Canterbury residents have felt their strongest shaking since Saturday’s magnitude 7.1 earthquake this morning.

Wednesday 8th September 2010

A very shallow magnitude 5.1 earthquake struck 10 km south-east of Christchurch at 7:49 this morning. The aftershock was the closest quake of the Darfield earthquake series experienced by residents of New Zealand’s second largest city.

The quake was centred within 5 km of Lyttelton, 10 km north-west of Diamond Harbour roughly placing it close to the suburbs of Beckenham, Somerfield and Spreydon. At only 6 km depth, it was one of the shallower aftershocks to date.

This location and depth of this morning’s earthquake meant that it caused the strongest shaking experienced by city residents since the magnitude 7.1 earthquake struck early on Saturday morning. Residents say this morning’s earthquake was noisy and threw items from shelves and cupboards, broke glassware and crockery and caused trees to shake. Many thought it was the feared magnitude six aftershock because of the vigorous shaking.

Most notable were items not disturbed by Saturday’s earthquake and the magnitude 5 aftershocks experienced since. Previously undisturbed items such as ornaments were thrown to the floor. This is due to the direction of shaking from the much closer earthquake being in a different direction from previous events.

This morning’s aftershock has caused the Lyttelton Tunnel to be closed for inspection and yet another review of the state of weakened structures in Christchurch city.

Whilst this morning’s aftershock has not been the strongest of the series in terms of energy released (the magnitude) another measurement system, the Modified Mercalli (MM) scale, better illustrates the effects of the quake on city residents.

Shaking reached MM7 (damaging) in south-eastern suburbs of Christchurch, Diamond Harbour and Lyttelton. This is defined as – “General alarm. People experience difficulty standing. Furniture and appliances are shifted. Substantial damage to fragile or unsecured objects. A few weak buildings are damaged.”

MM6 shaking was reached in most of the rest of Christchurch city. This is defined as – “Felt by all. People and animals are alarmed, and many run outside. Walking steadily is difficult. Furniture and appliances may move on smooth surfaces, and objects fall from walls and shelves. Glassware and crockery break. Slight non-structural damage to buildings may occur.”

There have now been ten aftershocks of 5th magnitude, most centred close to the town of Darfield. The quakes have ranged between magnitude 5.0 and 5.4 at depths between 4 and 15 km. Three of the aftershocks have been magnitude 5.4.

This morning’s big shake comes after yet another period of relative quiet for the region. Last evening and the morning hours were relatively calm compared to recent days, and residents managed to get some much-needed sleep. Magnitude 3.8 and 3.9 quakes were recorded at 9:07 and 9:08 last night and a magnitude 4.6 at 41 minutes past midnight. Magnitude 3.7 and 4.5 events struck at 3:47 and 3:59 and it was relatively quiet until a couple of magnitude 3 events were recorded after 6 o’clock this morning.

Activity has again picked up with six earthquakes with magnitudes between 3.5 and 5.1 being experienced since 7 o’clock this morning.

[Compiled from earthquake data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST, and eyewitness accounts from family and friends.]

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