Depth Study 1- CHCH 2011 Earthquakes

#12 Describe the Magnitude, Duration, Frequency, Probability, and Scale of Spatial Impact of the hazard

• Magnitude:

  • 6.2 MW (moment magnitude)

  • IX MMI (devastating- modified mercalli scale)

• Duration:

  • 12:51 pm 22/02/2011

  • 10 seconds of shaking

  • Aftershocks = 361 first week

  • 7000 in first 12 months

  • 30 aftershocks over 5MW in following year

= EQ Cluster

• Frequency:

  • 30 large EQ since 1848 (2per decade)

  • 75 EQ in last 30 days

    • 12 × 3<5

    • 1 between 5>6

  • Napier (Christchurch), 1931 + Murchison (1929) killed people

• Probability:

  • Alpine Fault (main)

    • Port Hills Fault (Secondary) had never ruptured

  • 1991, New Zealand Earthquake commission had predicted possibility of moderate EQs in Canterbury region.

• Scale Of Spatial Impact:

  • EQs was felt as moderate (MMI) across the South Island and into the south of North island

  • ‘Strong EQ’ experienced in Canterbury region, with intensity decreased with distance from epicentre

#13 Explain the cause/s of the hazard

• New Zealand sits on boundary of the Pacific and Australian tectonic plates, which are constantly colliding and building up massive geological stress

• A hidden, deep-underground fault (port-hills fault) suddenly ruptured under immense pressure. Directly triggered by stress from a larger EQ five months earlier (nearby).

• The rupture happened directly underneath the city and was incredibly shallow (5km deep). This meant the seismic energy hit the surface instantly, creating some of the most violent ground shaking ever recorded.

#14 Discuss the Economic, Environmental, and Social impacts of the hazard

Environmental:

• Topographic change

  • Rolleston moved 3m north-west, toward Dartfield

• Liquefaction

  • Conversion of soil into fluid-like mass 900,000 tonnes removed CHCH

• Rockfalls and Landslides

  • SHI and railway near KaiaPoi house-size rocks fell at Redcliffs and Summer

• Changes to hydrology

  • Hororata river bed raised Avon and Heathcote rivers - lateral spread led to increased flood risk

#15 Explain the means by which the activities of people intensified the impacts of the hazard

Poor land use and urban planning:

• Building suburbs on low-lying flood plains and marshly ground with high water tables

  • Result: the shaking triggered severe liquefaction, turning the ground into over 10,000 homes, ruptured underground pipes, and rendered entire neighbourhoods permanently uninhabitable.

Unreinforced historical buildings

• Keeping historic brick-and-mortar buildings without retrofitting them with steel reinforcement due to high costs and heritage laws

  • Results: brick walls collapsed outward onto sidewalks and roads, crushing cars and pedestrians

Engineering and construction failures

• Substand and design, construction, and safety-checking of modern office buildings

  • Result: these designs flaws led to sudden, catastrophic pancake collapses of multi-story buildings, causing the majority of earthquakes 185 fatalities

Poor-Quake panic and traffic gridlock:

• Thousands of people simultaneously attempting to flee the city centre by car

  • Result: massive gridlock on damaged roads blocked emergency vehicles and scratch-and-rescue teams from quickly reaching collapsed building.

Unprepaired damage from a previous quake

• Delays in insurance assessments and construction meant many buildings damaged by a prior earthquake in September 2010 had not yet been repaired

  • Result: already weakened structures were highly vulnerable and collapsed immediately when the February 2011 EQ struck

#16 Evaluate two hazard risk management strategies implemented to reduce the impacts of the hazard, including mitigation and preparedness

Preparedness:

• Shake-Out

  • Successfully built muscle memory across the population for the ‘Drop, Cover and Hold’ action. This instinctive reaction saved countless lives-particularly among school kids by preventing injuries from falling objects

• Positive-

  • Highly effective for immediate survival, but limited in managing post-disaster human behaviour

• Negative-

  • It only prepares people for the actual duration of the shaking. It didn’t prevent the post-quake panic

  • E.G people running in path of falling exterior masonry or causing the immediate traffic gridlock that blocked emergency service.

Mitigation:

• The Ground Improvement Programme

  • A highly effective engineering solution designed to stop liquefaction. By physically reinforcing weak soils with grave pillars, cement packing, or deep foundation piles, the land is hardened to prevent it from turning to liquid mush in future EQ

• Positive-

  • An excellent engineering fix, but restricted by high costs and practical geographic limits

• Negative-

  • Extremely expensive and slow to implement due to the high cost. It wasn’t economically viable to fix every neighbourhood; Instead, the worst-affected residential areas had to be completely abandoned