Rock fall following the Christchurch 6.3 earthquake. Source http://blogs.agu.org/landslideblog/2011/02/23/on-the-causes-of-the-high-levels-of-loss-in-the-christchurch-earthquake/
Posted 28 February 2011
Last week 11 students and staff from the School of Earth & Environmnetal Sciences (SEES) returned from a geological fieldtrip to the South Island of New Zealand to investigate active tectonic processes including the fault rupture from the magnitude 7.0 earthquake in Christchurch last September. Little did we know that a second large earthquake (magnitude 6.3) would devastate much of Christchurch only 5 days after our return highlighting the unpredictability associated with seismic hazards.
The fieldtrip was organised by two SEES PhD students, Steph Kermode and Nathan Jankowski, who head up the student social group – GROUNDSWELL and was supported by SEES staff Brian Jones and Solomon Buckman. The students included a mix of postgraduates and undergraduates from all levels. The purpose of the trip was to observe active tectonic and glacial processes that have sculpted the landscape in New Zealand that are not readily observable in the relatively stable Australian continent. The long-term aim is to run this fieldtrip each year as an intensive field-based summer subject in which students can get first hand experience of active geological processes including volcanoes, geothermal power stations, glaciers and faults associated with active mountain building.
The landscapes and mountains of New Zealand are incredibly young with most of the relief having formed in only the last 5 million years. This is in stark contrast to the Australian continent that has not experienced any major mountain building activity for the past 200 million years and subsequently been eroded down to a vast, flat continent. Despite the contrast, Australia and New Zealand share a common geological origin as they were joined together 85 million years ago as a part of the supercontinent Gondwana. Between 85-45 million years ago New Zealand rifted away from Australia creating the Tasman Sea that now seperates the two continents. New Zealand is situated directly on the boundary between the Australian and Pacific plates making it a particularly active in terms of volcanic and seismic activity. In the North Island the Pacific Plate is moving to the east and subducting (sinking) beneath the North Island resulting in the development of an active volcanic arc and a deep sea trench to the east which extends all the way to Tonga. To the south subduction has flipped with the Australian Plate subducting beneath the South Island to form the Macquarie Ridge and Southern Alps. In between the North and South Islands is an intense zone of faulting where the New Zealand continent is being wrenched apart by the Alpine Fault. This is a major transform (strike-slip) plate boundary and has been active for the past 25 million years. The Alpine Fault consists of many subsiduary fault splays along its length. The big surprise with the Christchurch earthquakes has been the fact that Christchurch has not experienced large or regular earthquakes in historic times and that the fault line has not been identified due to it being buried by thick sequences of river sediment that has been eroded off the Southern Alps. Christchurch is also quite a distance from the Alpine Fault which may have built a collectively false sense of security. New Zealand is referred to as the Shakey Isles for good reason. It sits on the Pacific Rim of fire and is subject to regular, intense seismic activity as the tectonic plates jostle and collide with each other.
It was clearly evident when we visited Christchurch that it was still rebuilding from the September 3, 2010 magnitude 7.0 earthquake that struck 45 km west of the city in the rural outskirts of Roleston. It is important to realize that the Richter scale used to measure the magnitude of earthquakes is a base-ten logarithmic scale so an earthquake measuring 5.0 has a shaking amplitude ten times that of an earthquake of magnitude 4.0. However, the total energy released is 33.3 times the amount for a difference of 1. Put simply a difference of 2 on the Richter scale results in about 1000 times the amount of total energy released. Most movement on faults is accommodated by large earthquakes. Unfortunately earthquakes remain difficult to predict in the short time-scales useful to people due to the numerous variables – build up of stress, time since last rupture, water saturation of the fault plane and most importantly the fact that earthquakes generally occur 10’s to 100’s km below the surface where we cannot make direct observations of the physical conditions. Geologists rely solely on geophysical and seismic data to interpret conditions and structures deep in the lithosphere.
We visited the fault rupture and although the roads had been repaired the 4 m offset of roads, fences, hedges and canals was clear to see as well as numerous cracks and compressional mounds along the fault trace. It was also evident that many of the locals weren’t happy with the attention they were getting from passers by like us who wanted to stop and view the fault. There was a real sense amongst Cantebrians that they were lucky to get away without any loss of life after the first earthquake. Unfortunately that was not the case with the recent earthquake in which the death toll has just passed 100 and there are still over 200 people missing.
The epicenter of the February 21, 2011 magnitude 6.3 earthquake was only 5 km from the centre of Christchurch with the epicenter centred on Lettelton. Because of proximity to the epicenter and the shallow depth (5 km) of the hypocentre, ground shaking in Christchurch was much more severe for this latest earthquake than for the larger magnitude 7 event in September. Ground accelerations were unusually high for this event, probably due to the shallow depth of the earthquake hypocenter and the thick unconsolidated substrate of wet mud and sand that much of Christchurch is built on. Compared to solid rock, sands and muds have the effect of slowing and amplifying seismic waves as they travel through the earth resulting in greater shaking. Wet sediments are also prone to liquefaction when shaken which means they suddenly change from behaving as a solid during normal conditions to a liquid during an earthquake. During an earthquake liquid sand or mud can spew out of cracks in the ground and flow down roads and collect in depressions and drainage networks. Heavy buildings and structures will tend to sink and become unstable during liquefaction if they do not have adequately engineered foundations. Typically ground shaking is in the order of 25%g for a magnitude 6.3 earthquake but the Christchurch earthquake produced shaking of up to 188%g. To put this in perspective, any shaking above 100%g is enough to overcome the acceleration of gravity and start throwing objects up in the air! Although New Zealand has a very strong and strictly enforced earthquake building code, this level of shaking resulted in severe and widespread damage. The Modified Mercalli Intensity scale (I-XII) is used to measure damage based on observations and interviews. Levels of IX to X were recorded around the epicenter which means intense to violent damage of well-built stuctures and damage or destruction of some well built wooden structures. Most houses are built of wood in New Zealand because it is much more flexible and resistant to earthquakes than brittle brick structures. Unfortunately, aftershocks can occur for many months after an event creating dangerous conditions in already weakened structures. The other aspect is that where stress is released by an earthquake it can result in increased stress along other faults segments resulting in an “unzipping” effect as stress in the crust is redistributed and comes to a new equilibrium. This appears to be the case with this second magnitude 6.3 earthquake following the magnitude 7.0 earthquake last year some 45 km further west.
Part of my research involves investigating evidence of ancient earthquakes (Paleoseismology) in areas of Australia that are prone to seismic activity and I have a PhD student – Chulantha Jayawardena, investigating active faults in the Adelaide region. Although Australia is relatively stable compared to New Zealand it is still affected by earthquakes as evident by the magnitude 5.6 Newcastle earthquake in 1989 and the magnitude 5.4 Adelaide earthquake in 1954. Earthquakes in Australia are referred to as intraplate earthquakes as they do not occur on plate boundaries and are much less understood and certainly less predictable in terms of their distribution. The danger with these intraplate earthquakes is that they may have long recurrence intervals of 100’s or 1000’s of years before the crustal stresses build up enough to rupture and generate an earthquake and they can strike areas that are underprepared for such events. We are investigating active faults along the margins of the Mount Lofty and Flinders ranges in South Australia by way of trenching, mapping and using ground penetrating radar to identify previous ruptures. Some of these faults have rupture lengths and offsets of single events that suggest magnitudes in the order of 5-7 on the richter scale. Part of our research involves dating these paleoseismic events by sampling the sediments that have accumulated adjacent to the fault rupture using luminescence dating techniques (OSL) to further constrain the timing of past earthquakes. Identifying hidden fault lines and constraining the timing of past seismic events is of fundamental importance in understanding how mountains such as the Flinders Ranges form in intraplate settings and of course it has important practical implications in terms of planning and implementing appropriate building codes in earthquake prone regions of Australia.
Earthquakes are a global hazard that knows no political boundaries. Earthquake response and rescue efforts are often globally assisted and require the expertise of many disciplines including engineers, geologists, planners, medics, police and emergency response personnel. Earthquake mitigation is an ongoing process from the initial identification of faults and historic seismic activity, through to developing appropriate building codes, to the rescue efforts when these hazards strike through to planning for the next event. The tectonic processes so evident in New Zealand provide an important modern-day analogue in terms of understanding how older continents like Australia have been shaped and formed in the past.
For further information please contact Dr. Solomon Buckman solomon@uow.edu.au in the School of Earth & Environmental Sciences
