Great Earthquakes In Diverse Places
New Zealand: Aftershocks Continue Following the 2010 Canterbury
Earthquake
By Brian Cross
Published Oct 7, 2010
In the weeks following the 2010 earthquake, Canterbury residents have
learned to endure numerous aftershocks as they rebuild their lives.
At 6.29am on Friday 8 October 2010 an earthquake measuring 4.2 on the
richter scale rattled Christchurch. It came 12 minutes after a 4.4
shake, another early morning wake-up for people forced to become
accustomed to seismic activity since the big one struck on 4 September.
That most recent aftershock was number 325 on the basic data list of
Canterbury aftershocks published by New Zealand’s GeoNet
service.Several Large Aftershocks Rocked Canterbury
The smallest aftershock registered on GeoNet’s website was a 2.5,
barely big enough to be noticed by a person at rest. But there have
been some sizeable ones too, sufficient to cause concern for homes and
commercial buildings damaged in the main shock of maginitude 7.1.
That’s not to mention the psychological effect of the ongoing series of
aftershocks on Christchurch and Canterbury residents who have much to
endure due to loss of property and home comforts in the initial event.
The largest Canterbury aftershocks to date were a couple of 5.4s, one
on 6 September and the other a day later. In the first week after the
initial earthquake there were 74 registered aftershocks, in the
following week 84. Week three saw 29 aftershocks as published by
GeoNet, and there were another 44 in week four.
Gradually the earth beneath the Canterbury Plains was showing some sign
of settling down after the main earthquake that was centred under the
town of Darfield. But the seismic shift along the previously unknown
Canterbury faultline was not done with by any means. For week five the
GeoNet list shows 25 aftershocks - with 20 hours remaining in the
seven-day period for potentially more.
The aftershocks have been predominantly located around Darfield, the
epicentre of the main shock. That’s normal, although movement along the
faultline during main shocks can cause aftershocks some distance away.
So it is, with many aftershocks centred some kilometres to the east,
closer to where the population is greatest.
Many Aftershocks are not Recorded
The above figures are only part of the seismic picture. Geonet’s
website explain that scientists are working through a myriad of data to
identify all the aftershocks that are cluttering their equipment. It
will take months for the true number to accurately be known, but many
more there certainly have been. That is reflected in some of the media
reports, which at first glance appear to exaggerate the real number of
aftershocks.
According to the scientists, one to two thousand aftershocks of
magnitude three could be identified over the course of the entire
aftershock sequence. With a difference of roughly ten-fold for each of
the Richter scale ranges, “that means there are probably up to 20,000
magnitude twos!”, Geonet states on its website. In applying resources
to tracking all the aftershocks, scientists must “strike a balance
between the people we have to do the work and what gives us the best
scientific outcome.”
Documenting an Aftershock Sequence is a Huge Task
The nature of an aftershock sequence is that it can be expected to
gradually diminish over time, with the incidence of larger events more
rapidly reducing. 12 aftershocks of magnitude five or greater have been
recorded in Canterbury, all in the first week after the main
earthquake. (Elsewhere GeoNet records that figure as nine, suggesting
three may have been reassessed downwards).
So the earthquake statistics and the variously reported number of
aftershocks throw up anomolies in the recoded data. That only shows the
immensity of the job on hand for scientists when a large earthquake
strikes. There are some 15,000 earthquakes recorded in New Zealand in a
typical year. That’s a year without an aftershock sequence as currently
being experienced and endured by the people of Christchurch and their
rural Canterbury neighbours.
Read more at Suite101: Aftershocks Continue Following the 2010
Canterbury Earthquake
http://www.suite101.com/content/aftershocks-continue-following-the-2010-canterbury-earthquake-a294468#ixzz11j88t2ve