700 aftershocks - and counting

Aftershock number 700 was a long time in coming - after number 699 - but when it came, it was a memorable one.

After an interval of over 24 hours, the 700th tremor to shake Canterbury came just after midnight this morning, and it was a real jolt.

The 4.6 magnitude shake, at a depth of only 9km, happened a half hour past the stroke of midnight.

A seismologist said this morning that the aftershocks appeared to be dying down.

"We said at the outset we're looking probably at weeks rather than days, and I think that probably is pretty accurate now," GNS Science seismologist Warwick Smith said today.

"It is tailing off. They don't get smaller in a hurry, they do get less frequent and that's what we're seeing."

Experts initially suggested the main quake on September 4 would be followed by an aftershock measuring about 6 on the Richter scale.

However, the risk of that was now looking less likely, Mr Smith told Radio New Zealand.

"About 20 minutes after the main shock two weeks ago there was a 5.6 and it is indeed beginning to look as though that might turn out to be the largest aftershock."

700 is a far greater number of aftershocks than usual, in the event of a major quake - including San Francisco in 1989 and Haiti earlier this year.

As of the end of February, a total of 55 aftershocks were recorded by the US Geological Survey (USGS) since the magnitude 7.0 earthquake that shook Haiti on January 12. (It must be noted, however, that this American survey only counted aftershocks at the level of 4.5 magnitude or higher. Many of the 700 aftershocks in Canterbury were between 3.0 and 4.5 magnitude.).

An earthquake on Mexico's Baja California fault line in April this year was followed by in excess of 500 aftershocks.

A massive 7.3 quake hit Kobe, in Japan, in 1995 and killed 6000 people. Around 240 aftershocks happened in the days following that cataclysmic event.

However, two years later, records show around 2,500 aftershocks in total affected the region - although only 400 of those were strong enough to be felt on the surface.

The Queen sent a message of support to the people of Canterbury just days after the Canterbury disaster, when barely 300 aftershocks had been recorded.

Now many are wondering if the passing of the 700 mark will prompt the Pope to send a message.

The 699th tremor hit before midnight on Saturday night, leaving seismologists and quake watchers everywhere an unusually long wait before number 700.

NOTE: We acknowledge there are different "aftershock counts" out there. For example, on the Christchurch Quake Live website, it states this morning, "As of Monday September 20, 2010 8:00am there has been a total of 689 shocks recorded."

- NZ HERALD STAFF and NZPA