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 Belt'.
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.