Is seismic activity increasing? Because of the way our memories work – new experiences are remembered more vividly than repeated experiences – people often recall their childhood as containing more distinctive and memorable events. When we were younger, it seems, the summers were longer and hotter, the storms were wilder, the earthquakes were bigger.
Or not. The 6.5-magnitude earthquake on July 21 was the biggest quake I’ve felt and the first to scare me. I was in my local supermarket in Wellington. As the shelves started shaking and the noise surrounded me – like a sudden hailstorm hitting a tin roof – I was uncomfortably aware that I was standing between heavily stacked 6m-high shelves, my family were somewhere else and I was on a low-lying isthmus in a potential tsunami inundation zone.
I wasn’t in Christchurch for the big 2010 and 2011 earthquakes, but on one of many visits following the quakes, my family and I spent a memorable Christmas holiday where regular earthquakes jolted our Lyttelton house. What with the devastating Canterbury quakes and now this lively Cook Strait sequence, it seems to me we’re having an unusual amount of seismic activity. Is this so?
The answer is both yes and no. “Earthquakes of this size are not unusual at all,” says Annemarie Christophersen, a GNS Science seismologist. Geonet’s statistics show that, on average, the New Zealand region experiences a magnitude 7 to 7.9 quake every two and a half years and two magnitude 6.0 to 6.9 quakes each year. What has made the recent quakes so significant and so damaging, however, is their shallow depth, and with Christchurch, the record-high ground acceleration and proximity to an urban centre. “Christchurch was a very unusual earthquake in the sense that it was an aftershock that just happened to occur very close to the city centre.”
But what about globally? In the past decade we’ve had the massive Tohoku earthquake in Japan and the Sumatra-Andaman quake, each with huge tsunamis. Is seismic activity increasing internationally? No, says the United States Geological Survey on its website. “Although it may seem we are having more earthquakes, earthquakes of magnitude 7 or greater have remained fairly constant.”
What has changed is that with improved global communications and an increase in the number of seismographs, seismologists can now locate more earthquakes than ever before. But there’s no increase in seismic activity. “According to long-term records … we expect about 17 major earthquakes (7.0 to 7.9) and one great earthquake (8.0 or above) in any given year.”
Although earthquakes of the magnitude we’ve been experiencing lately might be expected by geologists, I’m not wrong in feeling I’ve experienced more and bigger earthquakes recently than when I was a child. If people of my generation feel the same, it’s because we were born in a period of relatively low seismic activity.
There was a long gap between the 1968 Inangahua earthquake (7.1) and the 2003 Fiordland quake (7.2), with the most significant tremor in this period being the shallow and destructive 6.5 Edgecumbe quake in 1987. Christchurch’s 2011 event was the most destructive earthquake since the 1931 Napier quake. Each occurred as part of a cluster of seismic activity.
“There was a cluster of earthquakes starting in 1929, and another that began in 2003,” says Christophersen. These magnitude 7 quakes “tend to cluster closely in time. We don’t yet know if this is by chance or if there’s some kind of triggering mechanism.”
Suffice to say in the long term what we’ve been experiencing recently is perfectly normal and to be expected.
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