The plate activity that created the Taupo Volcanic Zone is
responsible for the region's fractured surface crust. These faults
are found in two extensive belts.
The smaller faults, found in the Taupo Volcanic Zone, are
collectively called the Taupo Fault Belt (TFB). In the Taupo Fault
Belt the land is being steadily pulled apart at about an average
rate of 5mm a year. When earthquakes occur in the Taupo Volcanic
Zone they are likely to be 'normal', that is, the faults pull
directly apart. The Edgecumbe earthquake of 1987 (magnitude 6.3)
was an example of this kind of earthquake.
The table shows the average return period in years for moderate
to strong earthquake shaking for principal settlement in the Bay of
Plenty region. After model of Smith & Berryman (1992)
The Modified Mercalli Scale (MM) is the measurement of intensity
of earthquake shaking that would be felt based on a scale of 1 -
12, where 12 is the maximum value. MM6 is a shaking intensity that
would be felt by all, causing alarm, but probably creating only
minor damage in the area most affected.
Tauranga can expect to experience such shaking on average every
10 years, Rotorua every 8 years and Whakatane every 5 years.
MM8 shaking causes serious damage to poor-quality buildings and
some damage to well-built constructions. Such shaking can be
expected to affect each of the three main towns in the region about
once every 150 - 180 years.
To the east of the Taupo Volcanic Zone a major belt of faults
and folding is obvious in the steep ranges that make up the spine
of the North Island. This is called the North Island Shear Belt,
and extends from East Cape to Wellington. Earthquakes generated in
the Shear Belt are likely to be significantly more powerful,
although also less frequent, than those from the Taupo Fault Belt.
The movement of the Shear Belt faults tends to be more a sliding
action than a pulling apart, which gives rise to the term 'Shear
Note: Fault means a fracture in the earth's crust.
Several damaging earthquakes within or very near to the region
have been recorded over the last 85 years. The average return time
for moderate to strong ground shaking is less than 50 years for all
major settlements in the region. The Edgecumbe earthquake of 1987
was the most damaging earthquake that the Bay of Plenty
Region had experienced in decades.
Information on other regions' earthquake
hazards is available on the New Zealand Shakeout