Is global seismic activity increasing?

by Rebecca Priestley / 20 August, 2013
Rebecca Priestley examines the apparent frequency of contemporary earthquakes.
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.

Remains of a shop in Riccarton Rd, Christchurch. Photo/Thinkstock


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.”

Comparison of earthquake size New Zealand to world
Source: Adapted from United States Geological Survey and Geonet


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.

Send questions to: science@listener.co.nz
MostReadArticlesCollectionWidget - Most Read - Used in articles
AdvertModule - Advert - M-Rec / Halfpage

Latest

A post-mortem on Todd Barclay and Matt McCarten's fiascos
76497 2017-07-24 00:00:00Z Politics

A post-mortem on Todd Barclay and Matt McCarten's …

by Jane Clifton

In the catalogue of disaster, is a Todd Barclay worse than a Matt McCarten?

Read more
The Trump family's Kremlin connection
76655 2017-07-24 00:00:00Z World

The Trump family's Kremlin connection

by Paul Thomas

From “nothing to see here” to a Cold War-era spy story played out in real life, the Trump family’s Kremlin connection is a source of fascination.

Read more
The Journey – movie review
76661 2017-07-24 00:00:00Z Movies

The Journey – movie review

by James Robins

A van isn’t a great vehicle for a drama on how old enemies ended the Troubles.

Read more
Gaylene Preston on the difficulties of filming at the United Nations
76664 2017-07-24 00:00:00Z Movies

Gaylene Preston on the difficulties of filming at …

by David Larsen

Tracking Helen Clark’s tilt for the top job at the United Nations, Gaylene Preston documented the creatures of the diplomatic world.

Read more
Jackie van Beek puts the gags aside for The Inland Road
76815 2017-07-24 00:00:00Z Movies

Jackie van Beek puts the gags aside for The Inland…

by Russell Baillie

Best known for her comedy roles, Jackie van Beek takes a dramatic detour in her feature-directing debut.

Read more
Parisian Neckwear plays the long game, even as its centenary approaches
76427 2017-07-24 00:00:00Z Small business

Parisian Neckwear plays the long game, even as its…

by Rob O'Neill

Parisian Neckwear, founded in 1919, has survived depression, war, deregulation and a deluge of cheap imports. How? Just feel the cloth.

Read more
David Tamihere case: Key witnesses' doubts about murder of Swedish tourists
76738 2017-07-23 00:00:00Z Crime

David Tamihere case: Key witnesses' doubts about m…

by Donna Chisholm

Nearly 30 years after young Swedish tourists Urban Hoglin and Heidi Paakkonen disappeared in the Coromandel key witnesses say the mystery haunts them.

Read more
Modern slavery and tourism: when holidays and human exploitation collide
76728 2017-07-23 00:00:00Z Social issues

Modern slavery and tourism: when holidays and huma…

by The Conversation

With the advent of orphanage tourism, travellers think they're doing good. But they can often just be lining the pockets of the orphanages' owners.

Read more