2010+Canterbury,+New+Zealand+Earthquake

//Matthew Vandersall, BAE/MAE, Penn State, 2011//
 * 2010 Canterbury, New Zealand Earthquake (September 3, 2010) **

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Introduction
On September 4th, 2010 at 4:35 AM local time, a 7.1 magnitude earthquake affected the region of Canterbury, New Zealand. Reports indicate that the epicenter of the earthquake was 25 miles west of Christchurch near the town of Darfield. The initial quake lasted approximately 40 seconds with aftershocks continuing to be felt weeks after the event. New Zealand’s modern earthquake codes and policies helped to reduce major structural damage to infrastructure and life supporting facilities. Two people were reported to have been injured during the earthquake, one from a chimney collapse and another from shattered glass. Total damages were reported throughout the area to have caused a estimated $3 billion in repairs (EERI, 2010).

Keywords: Canterbury, New Zealand, earthquake, seismic, liquefaction

Earthquake Overview
The hypocenter of the quake was approximately 40 km west of Christchurch at a depth of 10 km. The main shockwave was caused by a strike-slip or sliding motion along an East-West running fault. Further analysis showed that there was also a strong reverse or compressional faulting component to the main shock. The main fault was located on a flat agricultural area called the Canterbury Plains (Tonkin & Taylor, 2010). Figure 1 shows the location of the epicenter and intensity experienced by the surrounding region. Within the following two weeks, approximately 550 aftershocks were detected with a magnitude of at least 3.0. The strongest aftershock had occured shortly after the main event recording a magnitude of 5.6. There was no reported loss of life and only two people were injured during the event. The main contribution to the lack of injuries were that most people were asleep at the time of the earthquake (EERI, 2010). The geology of the region is comprised of alluvial or river borne deposits of sand, silt, and gravel from the surrounding rivers. During a seismic event, loose soils such as sand, silt, and gravel are compacted causing the ground to settle. The groundwater table is located about two meters below the surface which affects the top 10 to 20 meters of sediments. As these smaller types of soil collapse, the groundwater between the particles will start to pressurize. The increase of water pressure is the cause of liquefaction as sand and silt are forced through the surface. Liquefaction will continue to occur until the pore water pressure has equalized. An additional consquence is the uplifting of underground utilities such as manholes (Tonkin & Taylor, 2010).

Liquefaction and lateral spreading caused underground pipelines to break, disrupting access to drinkable water to many areas. Approximately 100 km of pipelines were in need of replacement in those areas affected by lateral spreading. A major concern was that sand and other debris was entering the pipelines and pumping stations, delaying access to restored service. It is estimated that 11,000 tons of sand was removed from both the pipes and the pumping stations (EERI, 2010). As a result, the ground water table increased due to earthquake pressure, causing manholes to be uplifted.

Building Performances
Under the modern earthquake codes, recently engineered structures performed well. Within the last 15 years, the New Zealand government has made a push to improve these building codes. A number of older buildings which were constructed using reinforced concrete moment frames experienced minor structural damage. Steel structures are less common in the Christchurch area but generally performed as expected. The building type that sustained the most wide spread damage was with unreinforced masonry structures. Only a few of these structures were retrofitted and those that were have shown to have improved performance than those that were not. Reports indicate that there was a significant amount of nonstructural damage widespread over all building types, including elements such as glass and storage racks.

Unreinforced Masonry Structures
Early construction in New Zealand consisted of unreinforced masonry (URM) as the primary building method. It is estimated that around 900 URM buildings are located within the Canterbury region. A sample of 595 URM buildings investigated showed that approximately 21% of these were determined to be unsafe to occupy (EERI 2010). The main modes of failure typically seen involve out-of-plane facade wall failures, parapet failures, chimney failures, and some in-plane damage. The lack of anchorage and reinforcement was the cause for number collapses of chimneys and gable end walls. An example of such failures can be seen in Figure 2, where a portion of the masonry wall and parapet had collapsed onto the adjacent sidewalk. Poor mortar quality also contributed to these collapses causing weathering in the mortar joints. Building construction between 1880 and 1930 utilized a lime mortar bond. Lime mortar is more susceptible to water ingress due to its porosity allowing for weakened strength between masonry units (Dizhur, 2010). Churches are an example of structures typically designed using unreinforced masonry. St. John's Hororata Church was constructed in the early 1900's using stone masonry. Effects of the earthquake caused the bell tower to collapse (see Figure 3) causing additional damage to the nave (Anagnostopoulou, 2010). Through a goverment effort to improve public safety, retrofitting of URM buildings had been voluntarily encouraged in recent years. URM buildings which had been retrofitted performed better and suffered far less damage than similar unretrofitted structures. Some of the retrofitting techniques include the application of through bolts or adhesive anchors, grout injections, addition of steel moment or braced frames, concrete moment frames and walls, and external steel rods, angles, and plates (EERI, 2010).

Reinforced Concrete Structures (Pre-1970)
Reports have indicated pre-1970 reinforced concrete buildings suffered only minor damages. The main failure modes typically observed include cracking to masonry infill walls, column hinging, and joint shear failures. Generally taller structures suffered more structural damage than one or two story buildings. This is mainly attributed in part to the flexibility of the structure and that taller buildings exhibit greater roof displacements. Stiffening elements such as masonry infill walls have shown to prevent significant structural damages. Due to strong ground movement in the North South direction as a result of the earthquake, more structural failures were found to occur in those elements along the North South direction (Pampanin, 2010).

Residential Structures[[image:Canterbury_manhole.jpg width="240" height="157" align="right" caption="Figure 4: underground manhole uplifted. Source: Jim Harding via Wikimedia Commons."]]
Typical residential construction in New Zealand consists of light timber framing on a concrete slab or piled foundations. Residential buildings constructed within the last 20 year have concrete slabs. These are generally 4 inches in depth and have either limited or no reinforcement. Older residences primarily use timber or concrete piles with either reinforced or unreinforced concrete perimeter foundations. Exterior walls are built with traditional wood siding, stucco, or unreinforced brick veneer. The majority of damage caused by the earthquake to residential buildings was due to soil liquefaction. Soil liquefaction resulted in the damage to a large portion of residential buildings. Repairs to these residences is difficult if not impossible due to poor soil conditions. Geotechnical engineers must perform an individual analysis for each building to determine the appropriate course of action. Observations showed that structures built on piled foundations did perform better than those with concrete slabs. This can be attributed to the piled foundations ability to allow for more differential settlement than concrete slabs. The addition of reinforcement to concrete slabs or increased thicknesses could have reduced structural damage to floor slabs. Settlement issues led to additional damages to windows, doors, and other non-structural elements. Chimney collapses were common in many residential buildings, often falling and damaging other areas (Buchanan, 2010).

Steel Structures
Overall, steel structures in the Canterbury area suffered minor damages. Preliminary reports showed that only minor inelastic behavior and plastic hinging started to occur within a week of the main event. Steel members exhibiting inelastic behavior were not in direct need of repair. Replacement was advised for some high strength connection bolts where connection slipping had occurred only for cautionary purposes. In cases involving combined steel and concrete lateral systems, damage had occured in some of the concrete elements. An example of this is a hospital parking garage which utilized eccentrically braced frames (EBF) consisting of concrete columns in conjunction with steel beams and bracing (see Figure 4). The concrete columns that supported a ramp built on the top story suffered damage due to shear failure. Additionally, a few of the steel plates used in the EBF links demonstrated lateral-torsional buckling until being restrained. (Bruneau, 2010).

New Zealand Building Code
The New Zealand Building Code has been updated to account for the 2010 Canterbury earthquake. Many of these ammendments are based on the majority of damages assessed. Modification were made specifically to sections mentioning concrete, drains, glazing, and chimneys. Clauses of the concrete section were changed to include applications "to external walls which could collapse inwards or outwards from a building as a result of internal fire exposure," not just "outwards" as originally stated in the clause. Changes were also made to the minimum area of shear reinforcement for concrete elements. Additional reference documents were added the section on drains. These documents include "Methods of testing soils for civil engineering purposes," and the "New Zealand Geomechanics Society Guidelines" (Department of Building and Housing, 2011). As a result of a large number of glazing failures, ammendments were made to improve public safety from broken glass. The use of glazing "within 2000 mm horizontally or vertically, from any part of a stairway or landing shall be Grade A safety glass." Glazing shall also meet certain barrier requirements when at a location 1000 mm or more above floor level. Due to the number of collapsed chimneys, the code amended that reinforcing steel used in chimneys shall conform to AS/NZS 4671. Brick chimneys shall be restrained using closed stirrups in the stack and "U" shaped bars elsewhere. Vertical reinforcing should also be use at all corners of the chimney. Forces due to liquefaction and lateral spreading on foundations were a large concern after the earthquake. Ammendments were made to the lateral strength of "free head" piles in cohesive soil, which have no restriction against head rotation when lateral displacement occurs (Department of Building and Housing, 2011). Although revisions were made to the structural building code as a result of the Canterbury earthquake, other modifications will need to be made to account for future information and events such as the 2011 Christchurch earthquake.

Similar Events
On February 22, 2011 at 12:51 PM local time, a earthquake with a magnitude of 6.3 hit the city of Christchurch, NZ. The epicenter was closer to the populated city of Christchurch than the 2010 Darfield earthquake was. Figure 5 shows the locations of the epicenters and aftershocks of both the 2010 Canterbury earthquake and the 2011 Christchurch earthquake. The time of the event and the close proximity to the city attributed to the deaths of 184 people. The total damage had been estimated to cost $15 billion in repairs. Liquefaction was widespread, similar to the September 2010 event, affecting more than 50% of the Christchurch area. As a result, extensive damage was done to water lines, roads and highways, and electrical power lines. Approximately two to three times the number of unreinforced masonry structures were damaged than in the 2010 Canterbury earthquake. One explanation is that buildings which passed inspection during the September earthquake, were weakened enough to partially collapse during the February event. Overall, the New Zealand Building Code will require updated requirements for seismic performance of all building types to prevent a similar occurance (EERI, 2011).

Lessons Learned
As a result of improved building codes over the last few decades, many newer structures were left relatively undamaged. About 15 years prior, improvements were made to minimize the impact of natural disasters to infrastructure and other lifeline facilities. There are three major areas of concern for structures within the Canterbury region. Due to the regions composition of soft soils and sand, serious structural damage was caused by liquefaction of soil and lateral spreading. There was also considerable amounts of damage caused specifically to non-retrofitted URM buildings, many of which are historic structures. Lastly, there was widespread damage to non-structural building components, specifically glazing systems. Previous laws established in 1968, proclaimed local governments establish policies for buildings which are "earthquake prone." Due to the region being considered a moderate seismic zone, the local government enacted retrofitting policies only after major events. After the both the 2010 and 2011 seismic events, the government now requires retrofits to be made within the next 30 years (EERI 2010).

Conclusion
The 2010 Canterbury earthquake hit the Canterbury region in the early morning on September 4th, 2010, registering a magnitude of 7.1. Liquefaction played an important part in causing major damage to both structures and infrastructure. Operation of underground utilities were disrupted as a result of increased soil pressures, causing manholes to lift out of the surface. Structurally, unreinforced masonry structures did not perform as well as other building types for a number of reasons. The lack of anchorage for masonry and poor mortar quality resulted in the collapse of numerous facades. As a result of modern building codes, minimal damage had affected newly constructed buildings. Investigations into the predominant failure types will help to further improve the New Zealand building code, in efforts to prevent a more catastrophic event.