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Archive September 2010

What’s With Those Aftershocks?! Jesse Dykstra Sep 08

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8 Sept. 2010, 07:49, M 5.1 Aftershock, Ground Shaking Intensity  Source: GNS
8 Sept. 2010, 07:49, M 5.1 Aftershock, Ground Shaking Intensity. Source: GNS

At 7:49 am this morning, Christchurch and western Banks Peninsula were hammered by a particularly vicious aftershock. This latest tremor had a magnitude of  ‘only’ 5.1, but was centered just a few kilometers from Christchurch, at a very shallow depth of only 6km. As can be seen from the Geonet mapping, the ground shaking intensity from this aftershock would have reached up MM 7 near Christchurch.

The short but intense tremor lasted about 10 seconds, but the ground shaking was violent enough to cause more damage (at least to our home), than Saturday’s magnitude 7.1 quake. We had just had the brick chimney and firewall inspected, and deemed sound, yesterday. Both are now in ruin!

Some questions that people in Canterbury are understandably asking;

  • when will these aftershocks diminish?
  • why was this powerful aftershock centered well away from Saturday’s mainshock?
  • was it another earthquake on a different fault?

Dr. Mark Quigley, professor in Geological Sciences at Canterbury University, and one of the lead investigators on the ongoing assessment of the fault has a concise summary of the geology behind the Canterbury aftershocks on his website.

It is far too early in the scientific investigation of the fault to fully understand its geometry or geology. In the coming weeks and months I’m sure that we will learn much more about the nature of this fault. However, from the basic information available now, I have produced this infographic to show this unusual aftershock, the fault trace, and the main earthquake:

 

Google Earth Image w/ Fault Trace, Saturdays' M 7.1 Earthquake & Todays' M 5.1 Aftershock

Google Earth Image w/ Approx. Locations of Fault Trace, M 7.1 Earthquake & Modays' M 5.1 Aftershock

 

 

 

 

 

 

 

 

 

 

 

 

An important point is that aftershocks are caused by slippage on parts of the fault that did not rupture with the main earthquake, or in areas near (but not on) the fault, where the surrounding geology continues to adjust to the recent movements within the earths’ crust. During earthquakes, including large ones, it is common for sections of a fault to rupture, rather than the entire length of the fault. Such piecemeal release of stress can transfer strain to other sections of the fault, or other faults. The sharp jolt felt this morning was likely caused by strain release related to Saturdays large earthquake, but not necessarily on the same fault.  This sharp aftershock was short in duration compared to Saturdays’ event, probably because there was less strain available to release, and less associated ground movement.

The Canterbury aftershocks should be taken as a positive sign, as each aftershock is releasing more built-up strain along and nearby the fault. Although undoubtedly frightening, these aftershocks will, in general, gradually diminish in frequency and severity during the upcoming days and weeks. We must remain vigilant though, as it is highly likely that we will experience at least one more shaking event around magnitude 6, as the earth continues to adjust to this large event.

 

GNS has been working hard on assessing this major earthquake. Check out their website here

GNS have also posted a Utube video containing footage from a flyover of the fault trace:

YouTube Preview Image

Liquefaction Explained Jesse Dykstra Sep 08

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For a nice overview of Liquefaction, and why certain areas of Canterbury were more affected by Saturday’s shaking  (Courtesy of 3news).

 http://www.3news.co.nz/Liquefaction—a-scientific-explanation/tabid/309/articleID/174502/Default.aspx

Also, this post from Visibly Shaken,  by Peter Griffin shows an infographic by ECan

http://sciblogs.co.nz/visibly-shaken/2010/09/06/christchurchs-liquefaction-infographic/

And finally, also from Ecan, an excellent, detailed infographic on the liquefaction hazard in Canterbury, produced before Saturday’s earthquake:

http://img.scoop.co.nz/media/pdfs/1009/Ecan_Brochure_Title__The_Q_files___the_solid_facts_on_Christchurch_liquefaction.PDF

What Lies Beneath the Canterbury Plains? A Fault Revealed Jesse Dykstra Sep 07

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This is the second post of a 3-4 part series on the Canterbury Earthquake.

Canterbury Earthquake, Pt II

Source: GNS

Source: GNS

Was this Canterbury’s ‘Big One’?

When I was shaken out of a deep slumber at 4:36 am last Saturday, I couldn’t help but think that ‘this is the big one’.   The intensity of the shaking was certainly more than anything that I have experienced before. As I initially struggled to come to my senses, to even know if I was in the middle of a particularly vivid dream, it seemed that everything that wasn’t nailed down was jumping around. Over the next 20-30 seconds, the violent shaking and crashing escalated until it was nearly deafening.  But not deafening enough to drown out the roar of the wave that was bearing down on us from the west at tremendous speed.  A wave travelling through solid earth. When it arrived, the wave bore the entire house up upon its crest, and dropped us down the other side, as if we were afloat on some tempestuous solid sea. The 107 year old house flexed and groaned as the wave passed, protesting the immense strains that must have tested its solid wood-frame construction. The wave and its accompanying roar sped off towards central Christchurch, and the violent shaking and crashing resumed for a few more seconds.  I don’t know exactly how long the main event lasted for, but at a guess, I would say that it was something like one minute. By then, as the shaking diminished, I knew that it wasn’t an Alpine Fault earthquake, which when it ruptures in the not-too-distant future, will generate strong shaking for several minutes.  I will examine the likely impacts of the future Alpine Fault earthquake, and the implications of Canterbury’s earthquake for planning and preparing for the real ‘Big Event’ in another post in the near future.

A Previously Unknown Hazard Lurks Beneath the Canterbury Plains

Saturday’s magnitude 7.1 earthquake was centered near Kirwee, approximately 44 km west of Christchurch, at a depth of only 10 km. So why didn’t we know that this potentially damaging fault existed beneath the Canterbury plains, quietly building up enormous energy, so close to Christchurch?

The media has been portraying this as a ‘new fault’, which has not ruptured in the past. This is partly true in that this particular section of fault has not ruptured historically (i.e. within the last 200 years), and had not been previously identified.  However, that does not necessarily mean that this fault hasn’t been active in the past.  As noted by Dr. Mark Quigley, professor in Geological Sciences at Canterbury University, and one of the lead investigators on the ongoing assessment of the fault, it is possible that the recurrence interval on this fault is less than a few thousand years.  Indeed, the modern day surface of the Canterbury plains is relatively young, having been steadily built out over the last 16,000 years, as the great braided rivers of Canterbury transport vast quantities of material from the Southern Alps.  The Canterbury plains and the braided rivers that nourish them are, by nature, a very active landscape.  So while this may indeed be a “new fault”, it is also very possible that it has ruptured in the past, but the surficial evidence of that rupture is no longer evident.

Seismically active faults are often initially identified by visual cues.  The presence of a fault may be given away by its surface expression, such as fault traces (visible line of disturbance), or offset/displaced topography (such as river valleys).  However, where these faults occur beneath or within very young and active surfaces, such as the Canterbury plains, their surface expressions may be periodically erased by active surface processes, such as deposition of gravels by braided rivers. Whenever the fault ruptures, its surface expression may be rewritten.  Here is a typical view of the 20+ km surface expression of the fault following Saturday’s earthquake in Canterbury:

 

Source: GNS (text & arrows added)

Source: GNS (text & arrows added)

Geophysical Investigation Methods

Surface expression is not the only way to identify an earthquake fault. Geophysical investigation methods, including seismic refraction surveying, and ground penetrating radar (GPR) can show the subsurface structure of the earth.  The seismic reflection profile shown below suggests a series of faults, many of which are blind (i.e. do not show any surface expression) in part of the northwest Canterbury Plains:

Seismic Reflection Profile, Northwest Canterbury Plains

Seismic Reflection Profile, Northwest Canterbury Plains

 While geophysical survey methods allow scientists to look into the earth, these techniques are time consuming, labour intensive, and require specialized, often expensive equipment. For these reasons, areas of interest (e.g. surface fault traces) are often identified prior to any major geophysical investigation. It would take a tremendous concerted effort and a great deal of resources to undertake a detailed geophysical investigation of a region the size of Canterbury.  Consequently, it is very possible (perhaps even probable), that faults buried beneath the ephemeral surface of the Canterbury Plains may not have been discovered yet.

Faults Beneath The Canterbury Plains Predicted by Scientists?

This is not to say that the presence of such faults hasn’t been predicted by scientists.  An offshore fault system has been identified in Pegasus Bay, and may be related to the relatively well-established Marlborough fault system. The Pegasus Bay faults trend NE-SW, and likely extend under the Canterbury plains.

Source: Walters et al., 2006. NZ Journal of Marine & Freshwater Research, Volume 40, Issue 1

Source: Walters et al., 2006. NZ Journal of Marine & Freshwater Research, Volume 40, Issue 1

As mentioned by professor Quigley on his website, the presence of such hidden, or ‘blind’ faults were predicted by University of Canterbury scientist Dr. Jarg Pettinga and colleagues from GNS and Geotech Consulting Ltd in a report produced for the Canterbury Regional Council in 1998. However, predicting the existence of such faults is only the first step towards identifying their actual locations, and the likely recurrence intervals of associated earthquakes. Perhaps, Saturday’s powerful

The Marlborough Fault System & Pegasus Bay Fault (19)

The Marlborough Fault System & Pegasus Bay Fault (19)

earthquake in Canterbury will provide the impetus to get this important task underway, so that Cantabrians have a better idea of what to expect next time.

Haiti: 230,000 Deaths. Canterbury: 0 Deaths. Why? Canterbury Earthquake (Pt I) Jesse Dykstra Sep 06

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This is the first post of a 3-4 part series on the Canterbury Earthquake.

Canterbury Earthquake, Pt I

 

CHCH0890_Quigley

Photo: Professor Mark Quigley, University of Canterbury

motd

Source: USGS

 

 

 

 

 

 

 

 

When I was contemplating a name for this blog a few weeks ago, I wanted something that would convey the following:  with planning, preparation, community support, and perhaps a bit of luck, it is possible to come through a major natural disaster, without major loss of life, and with the resiliency to build again, better than before.  

Little did I know that Christchurch and the smaller communities of central Canterbury would be tested this last Saturday, with the most damaging earthquake since 1931 in Napier.  So how has Canterbury coped with this latest disaster? Remarkably well, it would appear. But there are lessons to be learned.

Saturday morning’s earthquake in Canterbury had many similarities to the January 2010 earthquake that struck Haiti. Both events occurred at shallow depth (~10-13km), with a magnitude of around 7, near major population centres.  Canterbury’s earthquake was centered about 40 km west of Christchurch, and Haiti’s approximately 25 km west of the capital city of Port-au-Prince.

Haiti Compared to Canterbury

Location Date Magnitude (Mw) Max Shaking Intensity (MM) Number of Deaths Economic Impact ($NZ)
Port-au-Prince,  Haiti 16:53, 12 Jan 2010 7.0 X ~230,000 + 20 Billion
Christchurch, New Zealand 04:36, 4 Sept,2010 7.0 IX 0 + 2 Billion

 

Peak Ground Acceleration

The Mercalli Intensity Scale is a measure of Peak Ground Acceleration (PGA), which can exceed 1g in the most severe earthquakes. When the upward forces exerted by an earthquake exceed the gravitational force that normally keeps things grounded, structures, cars and people can get tossed into the air. The Haitian earthquake produced maximum  ground shaking intensities of around MM X, with MM IX producing most of the damage in Port-au-Prince.  Similarly, early indications from the Canterbury event suggest maximum ground shaking intensities of around MM IX, with some areas possibly experiencing MM X.  In both cases the earthquake was centered on land, so there was no accompanying tsunami, although there may have been a localized tsunami caused by a submarine landslide during the Haiti event. 

Clearly, the two earthquakes must have had similar destructive power. So why, incredibly, was no one killed in Canterbury, while 230,000 people lost their lives in Haiti?

Better Building Standards

Learning from past earthquakes (especially the magnitude 7.8 Napier earthquake in 1931), New Zealand has implemented stringent building codes. Modern homes are generally timber-frame construction, which flex and absorb the energy of an earthquake. Modern commercial and office buildings are generally constructed with isolated foundations, while many historic buildings have been retrofitted with earthquake dampening devices. New Zealand is now a world leader in earthquake engineering.  Still, there was significant damage in Christchurch, most often to older un-reinforced brick structures, and in areas where liquefaction amplified the ground shaking.  And of course, there was major damage to the water and sewerage infrastructure, and disruptions to power supply and transportation networks.  

 

Source: nzherald.co.nz

Source: nzherald.co.nz

Destruction of Historic Deans 

Homestead, Christchurch 

 

 

 

 

 

 

The rebuilding cost in Canterbury is currently estimated at over 2 billion (NZ) dollars, compared to over 20 billion dollars for the rebuilding efforts in Haiti. Haiti is one of the poorest countries in the world, and does not benefit from stringent building codes.  Construction practices are substandard and earthquake-proof buildings are few.  An estimated 250,000 residences were destroyed or severely damaged in Haiti, leaving nearly 1 million people homeless.  Even such important buildings as the Presidential Palace and National Assembly did not withstand the severity of the shaking. The collapse of buildings in Haiti led to tens of thousands of people being buried under rubble, or trapped inside unsafe structures.  Essential services were decimated.  Infrastructure vital to the disaster response was severely damaged, meaning that people could not get the help that they needed in time. The loss of hospitals, major roads, rail links, harbours, and communication networks  severely hampered rescue and relief efforts.  Without sufficient aid, thirst, famine, looting, and eventually disease took a terrible toll.

Not a Time for Complacency

The relatively small amount of damage in Christchurch (at least compared to Haiti) allowed emergency services to mobilize and respond quickly to the earthquake. Hospitals remained operational throughout.  Some essential services were damaged, but the Christchurch City Council responded quickly to water and sewerage disruptions, while electricity and communications providers worked effectively to get their systems back online. Most of Christchurch already has running water, working sewers, electricity and communications restored. This is a tribute to the preparation of the region, and reflects New Zealand’s strong commitment to disaster planning and preparedness.

However, it must be acknowledged that luck played a part here as well. If the Canterbury earthquake had occurred at 4:53 in the afternoon, as it did in Haiti, the number of deaths and serious injuries would surely be much higher.  People may have been crushed or trapped by collapsing buildings, chimneys and brick facades. Traffic accidents would have occurred all over the city, as traffic lights stopped working, fissures opened up in road surfaces, and various structures collapsed onto busy streets.

Canterbrians can be proud of how their region and communities have coped so far with this disaster. But we must not lapse into complacency. We have the best possible scenario here; a major disaster, but with minimal impact, that we can learn from, in order to better prepare ourselves for the next one. As seen in Haiti, it could have been so much worse. And let us not forget, the Alpine Fault is still there, and the elastic band is stretching a bit further each day, storing up energy.

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Pt II will be posted tomorrow

From the Frying Pan into the Flood: Pakistan’s Worst Natural Disaster Unfolds (Pt II) Jesse Dykstra Sep 03

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Unprecedented Precipitation?

So why has this monsoon season caused the worst flooding in Pakistan’s history? The overall impression given by the media is that this year’s flood is unprecedented. But is it?

On 29 July, 2010, nearly 300 mm of rain fell in parts of the upper Indus catchment. As should be expected during the summer monsoon season, this very heavy rainfall was followed by additional precipitation in the headwaters of Indus catchment. Over one month later, flood waters in the lower reaches of the Indus (where most people live), have only begun to recede.

So is such a wet summer monsoon season unheard of? Knowing that it is not unheard of for 10,000 mm of rain to fall over a period of approximately 4 weeks during July and August in some parts of the southern Himalaya, a back of the envelope calculation suggests an average daily rainfall of nearly 170 mm, for 60 days straight! So it would appear that at least in the wettest regions of the Himalaya, 300mm in 24 hours is a somewhat regular occurrence.

 

Source: Unitar/Unosat

Source: www.allvoices.com

Science Provides a Warning

Looking for more concrete evidence, I did a quick search of the literature, which brought up a recent paper appropriately titled “Flood risk assessment of the River Indus of Pakistan” (Khan et al., 2010, Arabian Journal of Geosciences). The authors estimated the future risk of flooding in the Indus valley by expanding upon the mathematical distribution of the last 60 years of historical river gauge data for peak river discharge, in order to simulate future flood events. The Indus river system is the primary source of hydroelectric generation in Pakistan, with several crucial dams and reservoirs in the lower catchment. The authors’ results showed that the design capacity of many of the various dams and spillways on the River Indus were barely sufficient to handle a 1 in 20 year flood event. For example, at the GUDDU dam site, which is designed to handle a maximum discharge of 1.2 million cubic feet per second (cfs), the authors estimated that the 1 in 18 year flood event would have a peak discharge of nearly 1.1 million cfs. The authors concluded that “there is an urgent need to construct new dams/barrages on the River Indus and to increase the spillway capacity of reservoirs”.

A few months on, these warnings have a prophetic air. On the 8th of August, 2010, the gauge at the GUDDU dam site peaked at 1.16 million cfs, or approximately a 1 in 20 year flood event. Other gauges on the Indus tell a similar story – this does not appear to be an exceptionally large flood event for the Indus River. In fact, based on historical evidence, we should expect similar peak discharge at least once every 20 years. Despite this, the results of the latest flooding have been catastrophic. Why?

A Country of Extremes

Pakistan is a country of geographical extremes, from the extensive deserts and plains of the Indus valley delta in the south, to the lofty heights of the Karakoram Himalaya in the north. With an area of approximately 800 thousand square kilometres, Pakistan encompasses an area nearly three times the size of New Zealand. The River Indus and its’ tributaries are the lifeblood of Pakistan and 170 million people depend upon the river for clean water, agriculture and hydroelectricity. The River Indus provides the water to irrigate vast tracks of agricultural land that would otherwise be parched for 9-10 months of the year.

All this irrigated land requires a massive system of dams, reservoirs, levees and canals. Now one of the worlds’ premier agricultural producers, Pakistan has the largest contiguous irrigation system in the world. All these structures have been engineered to capture, store and divert the precious waters of the Indus and its’ tributaries. The resulting conversion of flood channels, grasslands, wetlands and even desert into arable land has fundamentally changed the natural flow regime of the Indus. Quite simply, the river system no longer has the capacity to convey significant flood events out to sea. As has been tragically illustrated over the past few weeks, even a 1 in 20 year flow events can result in catastrophic flooding. Unfortunately, this means that the increasing millions of people who occupy and farm the floodplains of the lower Indus river valley will continue to be vulnerable to future flood events.

From the Frying Pan into the Flood: Pakistan’s Worst Natural Disaster Unfolds (Pt I) Jesse Dykstra Sep 02

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Source: nationalgeographic.com

Source: nationalgeographic.com

“Amid Drought, Pakistan Prays for Rain”. That was the title of a National Geographic Daily News story published on 1 July, 2010. Since then, Pakistan’s scorching drought has indeed been remedied. Heavy monsoon rains over the past month have instigated widespread flooding, leading to the worst natural disaster in Pakistan’s history. As flood waters finally show signs of receding, the exceptional toll of the disaster is becoming clear:

  • over 17 million people displaced (approximately 10% of Pakistan’s population)
  • 1600 lives lost
  • 4.6 million people left homeless
  • Up to one fifth of the country inundated
  • Initial relief efforts will cost ~$460m US dollars
  • Recovery costs will be in the billions (US dollars)
  • Devastation of millions of hectares of fertile crop land and grazing land will have long term effects on Pakistan’s ability to produce food when they need it most
  • The food shortage in Pakistan will also have a global impact, as Pakistan is one of the world’s largest agricultural exporters
pakistan 2

Source: Unitar/Unosat (click on image to enlarge)

The people of Pakistan have been having a rough go of it in recent years. Parts of northern Pakistan and Kashmir are still rebuilding following the 2005 Kashmir earthquake which resulted in over 80,000 deaths, and left 3.5 million people homeless. Many of these deaths occurred after the initial earthquake, as aftershocks, heavy rain, steep mountainous terrain and landslides ensured that critical transport links were blocked, slowing relief efforts. As is all too often the case with vulnerable populations, a lack of clean water, food, emergency shelter and medical supplies eventually took the greatest toll.

Earthquakes and flooding are simple facts of life in northern and western Pakistan, where the greatest mountain range in the world, the Himalaya, are actively being thrust up by the collision between the Indian subcontinent and the Eurasian plate. More than 50 million years of uplift has created the great topographic barrier of the Himalaya, which dramatically affects the climate of the region. Dry, cold Arctic winds are prevented from crossing the Himalaya into South Asia, keeping the Indian subcontinent relatively warm. During the northern hemisphere summer, moisture laden winds from the Indian ocean are gradually drawn northwards over the warm plains of southern India. Eventually these warm moist air currents collide with the Himalaya and cool as they are forced upwards, generating the summer monsoon rains that nourish the agricultural regions of India, Bangladesh, Nepal and Pakistan. The summer monsoon season in South Asia lasts for only a few weeks, but can generate some of the highest precipitation rates on earth. Some areas near the southern boundary of the Himalaya receive over 10 m of annual rainfall, the vast majority of which falls during the summer monsoon.

The Great Rivers

Several of the world’s great rivers, including the Ganges, Indus, Brahmaputra, Yangtze and Mekong originate in the Himalaya region. Approximately half of the world’s population (3 billion people) live within river drainages that are nourished by snowmelt and monsoon rains in the Himalaya.

A Hazardous Place

Scientists know that flooding, drought, earthquakes, and landslides have been occurring in South Asia for millions of years. There is nothing about these events that should catch us by surprise. The evidence of past events is recorded in the very landscape itself. Before our eyes is the greatest mountain range on earth, still being rapidly uplifted by astounding tectonic forces where two continental plates collide. Some of the world’s greatest rivers systems cut entirely through the immense topographic barrier of the Himalaya, in areas subject to some of the highest seasonal rainfalls on earth. Great gorges and rugged mountains give way to the largest river deltas in the world, reminding us that phenomenal volumes of sediment are being actively moved from the mountain highlands to nourish great floodplains, where half the world’s population lives, and makes a living off of the fertile land.

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Pt II will be posted tomorrow

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