|
EARTHQUAKE HAZARDS
Earthquakes are measured in terms of their released energy according
to the magnitude (M) scale, while the human effects of earthquakes
are assessed according to the Modified Mercalli
Intensity Scale (MM).
Manukau City lies in one of the low earthquake activity regions
of New Zealand. Most earthquakes recorded within the Auckland region
have magnitudes less than M 4 and are neither felt widely nor cause
damage. Large earthquakes occurring distant from Manukau City are
often felt, but rarely with intensities exceeding MM IV (see
accompanying table).
Over the last 150 years only two earthquakes with magnitudes greater
than M 5 have been centred within 100 km of Manukau City. The 1891
Waikato Heads earthquake (M=5.5-6.0) is known to have caused some
damage, with recorded intensities of MM VI to
MM VII near the west coast and about MM
VI in Manukau City. In January 1972 the Te Aroha earthquake
(M=5.1) was felt throughout South Auckland, and with an intensity
of MM IV in Manukau City.
 New
Zealand seismic hazard model (Stirling et al. 1998, 2000)
The seismic hazard model for New Zealand is based on the distribution
and long-term recurrence behaviour of active faults and the spatial
distribution of earthquakes observed in historic time. It can be
used to estimate future ground motions that will occur throughout
the country in any given time period. The example shown (left)
gives the peak ground accelerations expected at 10% probability
in 50 years, on stiff soil (site category B). Generally seismic
hazard decreases with distance from active faults and areas of high
seismicity. Manukau City is about 100 km from the nearest known
active fault and is away from areas of high seismicity rates. It
is not in an area of high seismic hazard.
Activity on faults
The greatest potential for future fault activity and consequent
large earthquakes exists along faults in the Hauraki Plains and
in South Auckland. The closest known active fault is the Kerepehi
Fault located near the centre of the Hauraki Plains. A recurrence
interval of 3600 years has been estimated, with future events likely
to generate earthquakes of about M 7 and result in moderate shaking
(MM VII) in Manukau City. Wairoa North Fault
in the east of Manukau City may also show evidence of movement in
the recent geological past, but no data are presently available
to determine past earthquake recurrence and magnitude. It may have
potential for future activity and large earthquake generation. Drury
Fault, immediately south of Manukau City, may also have potential
for future activity, but the data required to assess this is not
presently available.
Manukau City ground shaking susceptibility
Earthquake ground shaking hazard at any site is modified by the
earth materials beneath the site. The ground shaking susceptibility
map (above) shows the distribution of materials belonging
to three site susceptibility categories. Areas of unconsolidated
to very soft Holocene and Late Pleistocene sediments (site category
C) are likely to amplify ground-shaking during an earthquake, such
that felt intensities in a single event could be up to two MM units
higher than on adjacent rock sites. Liquefaction of loose, water-saturated
sand and silt layers is likely when shaking intensities exceed MM
VII. Moderate amplification of ground shaking is probable on
firmer Pliocene and Pleistocene sediments and most volcanic materials
(site category B), with felt intensities up to one MM unit higher
than on adjacent rock sites. The least amplification will occur
on moderately hard to hard, Mesozoic and Tertiary sedimentary rocks,
and basalt lavas that overlie them (site category A).
Manukau City seismic hazard
This map (below) shows the peak ground accelerations expected
for Manukau City at 10% probability in 50 years. It ranges from
0.1g to 0.24g, equating to a felt intensity of MM
VII over most of the city area, but reaching MM
VIII in areas of greatest ground shaking susceptibility. Unlike
the seismic hazard model for New Zealand,
this map shows the effect of variable ground conditions on the hazard.
Modified Mercalli
Intensity Scale - what does it mean?
(In part; summarised from Downes 1995)
MM II Felt by persons at rest,
on upper floors or favourably placed.
MM III Felt indoors; hanging
objects may swing, vibration similar to passing of light trucks.
MM IV Generally noticed indoors
but not outside. Light sleepers may be awakened. Vibration may be
likened to the passing of heavy traffic. Doors and windows rattle.
Walls and frames of buildings may be heard to creak.
MM V Generally felt outside,
and by almost everyone indoors. Most sleepers awakened. A few people
alarmed. Small unstable objects are displaced or upset. Open doors
may swing. Some windows and toilet fixtures cracked.
MM VI Felt by all. People
and animals alarmed. Many run outside. Objects fall from shelves.
Glassware and crockery broken. Unstable furniture overturned. Slight
damage to some types of buildings. A few cases of chimney damage.
Loose material may be dislodged from sloping ground.
MM VII General alarm. Furniture
moves on smooth floors. Unreinforced stone and brick walls crack.
Some pre-earthquake code buildings damaged. Roof tiles may be dislodged.
Many domestic chimneys broken. Small slides such as falls of sand
and gravel banks. Some fine cracks appear in sloping ground. A few
instances of liquefaction.
MM VIII Alarm may
approach panic. Steering of cars greatly affected. Some serious
damage to pre-earthquake code masonry buildings. Most unreinforced
domestic chimneys damaged; many brought down. Monuments and elevated
tanks twisted or brought down. Some post-1980 brick veneer dwellings
damaged. Houses not secured to foundations may move. Cracks appear
on steep slopes and in wet ground. Slides in roadside cuttings and
unsupported excavations. Small earthquake fountains and other instances
of liquefaction.
|
