LATE HOLOCENE PALEOSEISMICITY OF THE AUSTRALIA-PACIFIC PLATE BOUNDARY IN CENTRAL SOUTH ISLAND: THE ALPINE TO HOPE FAULT TRANSITION

In the southern half of South Island, New Zealand the NE-striking Alpine Fault forms the major structure that releases strain between the Australia and Pacific plates, while in the northern half, the plate boundary strain is released along several faults which comprise the Marlborough Fault System. The transition occurs adjacent to Hokitika and Greymouth on the West Coast. In this area, about half of the c. 27 ± 5 mm/yr dextral-reverse motion of the Alpine Fault is transferred to the Kelly and Hope faults, which splay to the ENE into Pacific plate rocks. We have undertaken new paleoseismic studies on the Alpine Fault SW of this transition at Toaroha River, NE of it at Inchbonnie and Lake Poerua, and along the western end of the Hope Fault at Matagouri Flat. Previous long-term data for the recurrence of surface-rupturing earthquakes from these sites are on the order of 300-500 yr.

At the Toaroha River, colluviums derived from the scarp of the Alpine Fault have been shed into an abandoned channel that has been accumulating peat. Preliminary results suggest 3 surface faulting events during the last 600 years (perhaps correlated with the AD 1717, 1620 and 1425 events). An earlier event is postulated on the basis of the transition from an alluvial terrace to silt-filled channel.

To the NE at Inchbonnie and Lake Poerua, the slip rate of the Alpine Fault is c. 13 ± 2 mm/yr. An understanding of the paleoseismic record there relies heavily on geomorphology, soils, dendrochronology and dating of remnant and drowned tree stumps in combination with trenches. These data confirm a very young rupture (probably AD 1717) and 3 older events during the last c. 1200 years.

At Matagouri Flat, trenches and soil pits were excavated to understand the recent rupture record of the Hope Fault. The trench shows evidence for 2 surface faulting events during the last c. 700 yr. The most recent event occurred since c. AD 1458. Based on dendrochronology and landscape change dates, this event occurred between the last Alpine Fault event and the widespread European settlement of New Zealand from AD 1840.

Collectively, these results confirm that the geologic slip rate and GPS strain data are mirrored in the history of large earthquake release along the Alpine Fault to Hope Fault transitional zone through central South Island.