The fault recurrence interval is the average time between surface ruptures on a fault. The Institute of Geological and Nuclear Sciences consider it is the best measure to use when evaluating the hazard risk of an active fault.
Historical and geological evidence shows that faults rupture repeatedly along the same narrow fracture. For example, there is evidence of two major fault ruptures on the Wellington Fault within the last 700 years, each with a horizontal offset of about four metres. There is also evidence of a total offset of almost one kilometre on the Wellington Fault in the last 140,000 years, indicating at least 200 major earthquake ruptures during this time. Along the Wairarapa Fault, up to 130 metres has been displaced along the same fault scarp that first ruptured in 1855. This indicates that multiple surface ruptures have occurred in the same location along the same fault scarp.
Figure 7.1: Wairarapa Fault – repeated rupture on same fault
Faults with short recurrence intervals are generally more likely to rupture in the near future than faults with a longer recurrence interval. It is important to remember that this is a statistical measurement only, and may not be an accurate predictor of future movement on a fault. For example, although the White Creek Fault has a long recurrence interval of more than 20,000 years it ruptured in the 1929 Murchison earthquake.
Detailed investigation, usually involving trenching, is needed to determine the fault recurrence interval.
Recurrence intervals of surface rupture on New Zealand faults range from several hundred years (for example, the Hope and Alpine faults) to tens of thousands of years (for example, the Waverly, Whitemans and White Creek faults).
Fault recurrence intervals can be divided into six Fault Interval Classes:
I. Less than or equal to a recurrence every 2,000 years
II. Recurrence between 2,000 and up to and including 3,500 years
III. Recurrence between 3,500 and up to and including 5,000 years
IV. Recurrence between 5,000 and up to and including 10,000 years
V. Recurrence between 10,000 and up to and including 20,000 years
VI. Recurrence between 20,000 and up to and including 125,000 years.
The fault recurrence interval measure can also be related to accepted levels of risk in the current Building Code. Appendix 1 gives details of most of New Zealand's known active faults, and indicates which regional council jurisdictions these faults fall within. It also gives a confidence rating of these faults' average recurrence intervals.