Hope Fault

The junction between the Hope fault and the Alpine Fault is complex. The Kelly Fault forms a major splay of the Hope Fault from just west of Harper Pass; it splays again to the west into the Newton and Hura faults. From repeat GPS surveying on the region near Arthur's Pass, the Kelly Fault marks a sharp reduction in velocities to the northwest of the Kelly and Hura faults.[2]

This 42 kilometres (26 mi) long segment runs from Harper pass to the junction of the Hope and Boyle rivers. The maximum slip-rate estimated from the offset of a stream is about 13 millimetres (0.51 in)/yr. A minimum slip-rate has also been estimated from offset of channels in a late Holocene alluvial fan complex of 8.1 to 11.0 millimetres (0.32 to 0.43 in)/yr. The average recurrence interval for earthquake slip events on this segment is 310–490 yr.[3]

The Hope River segment runs about 30 kilometres (19 mi) from the confluence of the Boyle and Hope rivers as far as the Hanmer Basin. From the offset of dated river terraces, a late Holocene horizontal slip-rate of about 10 millimetres (0.39 in)/yr and a vertical slip-rate of about 0.6 mm/yr have been calculated for this segment. The displacement is thought to have occurred by repeated slips which, if similar to that which caused the 1888 earthquake, indicate a return period of about 140 years.[4]

The Conway segment is the longest and straightest of the segments with a strike of about N73°E. It extends 70 kilometres (43 mi) from the eastern part of the Hanmer Basin to just west of the junction with the Jordan Thrust near Kowhai River. The slip-rate for this segment, estimated from offset alluvial features, is about 23 mm/yr. The fault dips steeply to the northwest at about 70° and forms the southern termination of the Seaward Kaikoura, Hawk and Amuri ranges. This topography is a result of the significant component of reverse displacement on this fault zone. An average recurrence interval of 180–310 yr has been calculated for earthquakes along this segment.[1]

The Seaward segment extends from near the Kowhai River to the coast and has been recognised on the continental shelf from seismic reflection data, extending for at least another 13 kilometres (8.1 mi).[5] Part of this segment ruptured in the 2016 Kaikoura earthquake.

Satellite photograph of the Hanmer Basin, the area of agricultural land in the centre of the picture

Hanmer Basin is a small (10 x 20 km) pull-apart basin formed at the overlap between the Hope River and Conway segments of the Hope Fault. The right-stepping geometry of the offset between the two segments has caused local extension to occur and a basin to form. The hot springs of Hanmer Springs are one of the results of the basin formation. Other smaller basins of similar type have formed at minor offsets along the Hope River segment. The basin began to form in the Pleistocene and contains more than 1000 m of sedimentary fill in the main depocenter. The eastern end of the basin is currently being deformed by transpression associated with the southwestern end of the Conway segment. The destruction of the basin at its eastern end is matched by transtensional deformation at the western end that continues to enlarge the basin, giving rise to a roughly steady-state geometry. The basin fill is being constantly reworked as it is uplifted at the basin's eastern end and redeposited to the west.[6]