Increased Earthquake Activity, Canterbury

Aftershock activity has increased in Canterbury overnight, continuing into the morning.

Sunday 26th December 2010

Six felt earthquakes have struck the Canterbury region since two o’clock this morning, all centred within 10 km of Christchurch.

Activity began at 2:08 a.m. with many residents woken by a magnitude 3.8 quake centred within 5 km of Christchurch at a depth of 8 km. A shallower (6 km) magnitude 3.7 quake followed at 2:32 a.m.

A very shallow magnitude 2.7 quake centred 10 km east of Christchurch at 5:34 a.m. was followed by another very shallow magnitude 3.8 event at 8:03 this morning. This quake was centred within 5 km of Christchurch at a depth of 4 km.

GeoNet’s seismic network 26 December 2010

GeoNet’s seismograph network shows the extent of the shaking from the largest aftershock in Canterbury this morning.

The largest event of the series was a magnitude 4.9 earthquake which struck at 10:30 this morning. The quake was also centred within 5 km of Christchurch but slightly deeper at a depth of 12 km. Instruments show that shaking reached MM7 (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.”) at Papanui High School, Christchurch Resthaven, the Botanic Gardens and Cashmere High School.

MM6 shaking, (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.”) was recorded by instruments at Canterbury Aero Club and North New Brighton School.

Damage reports are being filed from Avonhead, Cashmere, Hoon Hay, Middleton, Papanui, Redwood, Riccarton and Somerfield. The 10:30 earthquake took out power in some central Christchurch suburbs, and local radio has reported building damage in Moorehouse Avenue and Sydenham which is yet to be confirmed.

Within an hour of the quake, the public had filed more than 850 reports with GeoNet. Three reports claimed heavy damage at Avonhead and Christchurch, equivalent to MM8 shaking which is defined as: “Alarm may approach panic. A few buildings are damaged and some weak buildings are destroyed.”

Light aftershocks have continued, most of them tiny events that would only be felt close by or not at all. The largest event since the 10:30 earthquake struck 5 minutes afterward. The magnitude 3.6 quake was also centred within 5 km of Christchurch.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST.]

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