December 24, 2011

Relentless quakes in Christchurch

Posted by Austin Elliott

Christchurch, New Zealand, has suffered its fourth day (and 5th, 6th, and 7th big quakes) in barely over a year of what continues to be a brutal onslaught of seismicity. It began in September, 2010, with a magnitude 7.0 earthquake outside of town that caused billions of dollars in economic losses. The vigorous aftershock sequence of that quake was punctuated in February by a magnitude 6.3 earthquake east of the September rupture patch. This quake struck a direct hit on the city, killing 181 people and devastating the central business district. The liquefaction caused by these two quakes [pdf] has flooded and destabilized vast swaths of neighborhoods around Christchurch. After months of additional aftershocks, a 5.5 rattled the city in June, followed within hours by another M6.0, both yet farther east. The two of these caused additional destructive liquefaction throughout the city. They have been having their own series of aftershocks, compounding the tremors being felt as a result of the largest prior two quakes.

Earthquakes in the Canterbury sequence September 2010 - November 2011, colored by intervals between the dates of the largest quakes. Note the eastward migration of the concentration of seismicity. Green represents the September 2010 M7.0 Darfield quake and aftershocks; Red represents the February 2011 M6.3 Christchurch earthquake and aftershocks; Blue represents the June Port Hills quakes and aftershocks.

Cumulative number of earthquakes near Christchurch plotted over time. After each of the major quakes, the aftershocks die off in frequency at very predictable rates... but keep getting interrupted by new big quakes!

Just before 2pm on December 23, as locals were no doubt greeting out of town relatives for a warm, summer Christmas, they were struck by a hefty magnitude 5.8 offshore, east of previous quakes. A couple moderate aftershocks followed on its heels, and 8 minutes later there was a 5.3. About an hour and a half later, in the midst of a whole slew of modest aftershocks, one of these turned into a 6.0 and rocked the city hard. There have been literally hundreds of aftershocks since then, including another M5.1. Cantaburians and their Christmas guests have undoubtedly managed very little sleep with the incessant jostling, and it takes no stretch of the imagination to understand how exasperated they must be with the relentless quakes. There’s a nice post at Highly Allochthonous about the tectonics of these quakes, and The Landslide Blog discusses some of the notable features of these two new quakes and their implication for quake-related politics and policies in Christchurch. I’ll jump into the mix here with a collection of footage from the latest events!

The following videos capture what each of these latest quakes was like, and the gasps, sighs, and exclamations as residents reveal their exhaustion from the continuous trauma of not knowing what to expect from each new tremor that hits. ALL of these videos show beautiful separation of the P- and S-wave arrivals, in other words the rumbling versus the shaking. The warning from the extra few seconds of rumbling is thanks to the quakes’ location offshore and thus their relative distance from the city.

This family’s camera was running when the December 23 sequence started; it captured the 5.8, and they arrive home at the tail end of an early modest aftershock (after 02:15 in the video):


The camera’s still running, and they’re trying to calm the nerves with a beer when the M6.0 hits. Not a great way to help cool the nerves–this guy’s surprise is evident.


Next, one of the clearest videos showing the strong shaking of the 6.0:


Here’s the raw footage of a clip commonly replayed on the news, from the newsroom itself. Again, this is a scene of the M6, so they’ve already been shaken–physically and emotionally–by a few. You can see the exasperation:


Another clip commonly replayed on the news:


This next clip documents a far greater–but still pretty much justifiable–sense of panic. It’s actually heartbreaking to see how it begins with a news report in the background about the 5.8, optimistically trying not to jinx themselves by saying the quakes have died down over the hour. It helps illustrate why the M6, the largest of the three that day, was so demoralizing. Careful, there’s a hefty dose of adult language. Who can blame him?


A rattling kitchen (these folks have things well secured!):


Here’s a set of videos from security cameras in various places. There’s a whole lot of lurching during passage of the surface waves.

Home security camera


Electronics store


Another shot widely replayed on the news shows a woman darting from a shaking storefront.

The following video is from a supermarket, and shows the chaos of the quake as well as the exasperation of the beleaguered residents.


This family captures the quake from outside as they warn their children of more danger. It may sound like bleak consolation for the kid, but Dad’s right: nature has been nothing if not ruthlessly unpredictable here.