Whakatāne Graben

The extremely active andesitic Whakaari / White Island is situated in the middle of the off shore limits of the Whakatane Graben and the dacite Putauaki (Mount Edgecumbe) which last had a significant eruption about 300 BCE[9] is towards the top of the on shore section of the graben. Just off shore is Moutohora Island, an andesitic complex volcano that last erupted in the Pleistocene and still has geothermal activity.[10] The main area of land geothermal activity is used for geothermal power at the Kawerau Power Station. The widespread decay earthquakes after the 1987 Edgecumbe earthquake were absent from this area and the nearby Putauaki volcano (see areas of absent yellow shading near Kawerau in seismic activity map at top of page).[11]. Nearby caldera volcanism has produced rhyolitic tephra and ignimbrite deposits that are very deep towards the south western end of the graben.

Subsidence in the center of the graben had commenced before 600,000 years ago and currently is at the rate of up to 2 mm (0.079 in)/year.[4] On both sides of the graben uplift has occurred. To the west Castlecliffian (mid Quaternary) marine sediments have been elevated at a rate of 1 mm (0.039 in)/year to more than 300 m (980 ft) above sea level.[4] To the east in the area interacting with the active faults of the North Island Fault System uplift has been half this for the last 120,000 years.[4]

On present land there have been significant earthquake ruptures in the last 800 years in each of the three most active fault zones associated with the graben. These are the Rotoitipakau fault, the Onepu and Edgecumbe faults, and towards the north west the Matata boundary faults.[4] Off shore the Rangitaiki fault has been widening at 1.26 ± 0.69 mm/year for 17,500 years and slipping at up to 3.7 mm (0.15 in)/year.[2] The off shore boundary faults of the Tarawera fault to the north and the White Island Fault to the south are active and define the modern off shore Taupō Rift.[1]