Kerepehi Fault

The Kerepehi Fault (also known as the Kerepēhi Fault) is a NeS-to NWeSE-striking normal fault system in the North Island of New Zealand aligned with the Hauraki rift valley that produced the Firth of Thames and the Hauraki Plains. The Kerepehi Fault has a maximum potential of generating earthquakes with magnitudes of Mw7.2 or above.[1]

Kerepehi Fault
Kerepēhi Fault
The active Kerepehi Fault intra-rift fault in context of Hauraki Rift. Segments are labelled A (Awaiti), E (Elstow), W (Waitoa), P (Te Poi) and O (Okoroire). The also known to be active Te Puninga Fault (T) and an active fault only indirectly associated with the Hauraki Rift, the Wairoa North Fault are also shown. The presumed inactive Hauraki and Firth of Thames faults of the Hauraki Rift eastern and western edges respectively are also shown. The Hauraki Rift is shown in light purple shading, the old Taupo Rift in light yellow and modern Taupo Rift in light red shading.
EtymologyKerepehi region central to Hauraki Plains
CountryNew Zealand
RegionWaikato and South Auckland Regions
Characteristics
Length80 km (50 mi)
Displacement0.46 mm (0.018 in)/yr
Tectonics
PlateIndo-Australian
StatusActive
EarthquakesSee 'Historical Earthquakes'
TypeNormal fault
MovementMw7.2+ [1]
AgeMiocene-Holocene
New Zealand geology database (includes faults)

Geology

The Kerepehi Fault in the southern portion of the Hauraki Rift is much more complex than previously thought and as a result has the potential for large earthquakes, although single fault segment events should be less than 7 in magnitude. Previously it was thought to contain 5 fault segments with events separated by many thousands of years of moderate magnitude but the mean event separation anywhere in the fault zone is now known to be only about 1000 years in what is a belt of many faults and at least 6 complex segments on land. Three segments have been identified under the sea.[2] The fault system extends therefore from Waiheke Island to south of Te Poi and is the intra-rift fault structure for the now geologically fairly inactive by New Zealand standards, Hauraki Rift. Work using geolocation on the Hauraki Rift which is a North - South trending, 250 km (160 mi) long and 25 km (16 mi) wide on-shore/off-shore continental rift reveals a widening rate of 0.9 mm (0.035 in)/year although some of the raw data suggests that at the Te Poi end it might be up to 1.5 mm (0.059 in)/year.[3] The recently identified but yet to be fully characterised 25 km long Te Puninga fault is presumably a parallel intra-rift fault much closer to the western borders of the Hauraki Rift.[4]

Hazards

Earthquake Hazards

Recent historic ruptures have involved up to 2 m (6.6 ft) of vertical displacement, which suggests associated contemporary earthquake risk that could be of intensity 7.0 at the nearby major population centres, being Auckland, Hamilton, Tauranga and Thames.[5] Forty percent of New Zealand's population live, and 40% of GDP generation occurs within 50 km (31 mi) of the fault.[3] A major magnitude 8 to 10 event resulting from up to 3 segment rupture would be devastating to Hauraki Plains infrastructure.

Tsunami Hazards

At least a third of the fault structure is likely to be underwater and thus will offer a tsunami hazard to the low-lying areas of the Hauraki Plains and shore areas of the Firth of Thames and Hauraki Gulf. The tsunami hazard was calculated to be low in 1999 but has not been revised as a result of the later on shore work showing that some earthquakes were larger than historically assumed.[1][6]

Historical Earthquakes

In the past, the Kerepehi Fault has experienced significant earthquakes that might have occurred before the advent of written records. However, the following list consists of recently occurred quakes along the fault(s), as far back as the year 1900.

  • 1926 Magnitude 4.6[7]
  • 1927 Magnitude 5.5[8]
  • 1972 Magnitude 4.9[9]
  • 2023 Magnitude 5.1[10]
  • 2023 Magnitude 4.8[11]

References
  1. Persaud, Mira; Villamor, Pilar; Berryman, Kelvin; Ries, William; Cousins, J.; Litchfield, N.; Alloway, Brent (2 Jan 2016). "The Kerepehi Fault, Hauraki Rift, North Island, New Zealand: active fault characterisation and hazard" (PDF). New Zealand Journal of Geology and Geophysics. 59 (1). doi:10.1080/00288306.2015.1127826.
  2. Fromont, Ashleigh (2017). "A seismic investigation of the evolution and structure of the Kerepehi Fault, in the context of the Hauraki Rift, New Zealand".
  3. Eccles, Jennifer; Hreinsdottir, Sigrun; Rowland, Julie (2019). "Structure and Activity of the Hauraki Rift" (PDF).
  4. "Investigating Te Puninga fault on the Hauraki Plains". Retrieved 2022-08-29.
  5. "Research shows Kerepehi Fault more complex than previously thought - 12/05/2015".
  6. Chick, L. M. (1999). "Potential tsunami hazard associated with the Kerepehi Fault, Hauraki Gulf, New Zealand (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand".
  7. "GeoNet - Magnitude 4.6, Fri Nov 12 1926 10:21 AM". Retrieved 13 April 2023.
  8. "GeoNet - Magnitude 5.5, Mon Nov 7 1927 12:30 PM". Retrieved 13 April 2023.
  9. "GeoNet - Magnitude 4.9, Sun Jan 9 1972 9:32 AM". Retrieved 13 April 2023.
  10. "GeoNet - Magnitude 5.1, Wed Jan 4 2023 5:39 AM". Retrieved 13 April 2023.
  11. "GeoNet - Magnitude 4.8, Fri Feb 3 2023 2:02 AM". Retrieved 13 April 2023.
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