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3 Understanding Earthquakes and Active Faults

3.1 Introduction to terminology

A fault is a fracture in the Earth's crust. The opposite sides of the fracture are held together by pressure and friction, but as stress builds up a fault may suddenly rupture. In a large rupture, shock waves cause the earth to shake violently and produce an earthquake.

Figure 3.1: Relationship between faults and earthquakes

The point at which a fault plane starts to rupture is known as the focus or origin. The point on the surface directly above the focus is called the epicentre.

Earthquakes occur along a rupture referred to as a fault plane. The fault plane may extend some distance below the earth surface. The point at which the fault plane starts to rupture is known as the focus or origin. The location on the surface of the earth immediately above the focus is called the epicentre.

In a large earthquake, the fault rupture may extend up to the ground surface, and suddenly form a fault scarp (the disrupted land form created by the rupture). Because the fault plane is not always vertical, areas of surface faulting or fault scarp may occur some distance away from the epicentre of the earthquake that generated them.

All buildings close to the epicentre of a large shallow earthquake will be strongly shaken, and this shaking causes most of the earthquake damage. Any building sited across a fault scarp is likely to suffer more damage, especially if the foundations are offset.

Figure 3.2: Examples of fault displacement 

White Creek Fault – The 1929 Murchison earthquake resulted in over 4 m of vertical displacement of the ground surface at the White Creek fault (Berryman 1980). Note the cyclist standing on the upthrown side of road that is displaced by the fault

Edgecumbe Fault – The 1987 Edgecumbe earthquake resulted in about 7 km of surface rupture along the Edgecumbe fault, and up to about 2 m of vertical displacement of the ground surface at the fault (Beanland et al 1989). Arrows mark the location of surface fault rupture.

Hope Fault – The 1888 earthquake on the Hope fault resulted in about 3 m of right lateral displacement of the ground surface at the fault. The offset fence-line shows the amount of displacement across the fault (Cowan 1991).

Examples of damaging fault scarp displacement around New Zealand are not difficult to find. For example the 1987 Edgecumbe earthquake resulted in about 7km of surface rupture along the Edgecumbe fault, and up to 2m of vertical displacement of the ground surface at the fault (Beanland et al 1989). The 1929 Murchison earthquake resulted in over 4m of vertical displacement of the ground surface at the White Creek fault (Berryman 1980), creating a terrace which was insurmountable to traffic on a local road.

An active fault is a fault that has ruptured repeatedly in the past, and whose history indicates that it is likely to rupture again. An active fault creates a fault hazard risk. The level of that risk depends on the fault recurrence interval (section 7), fault complexity (section 8), and nature of development in the area.

New Zealand geological maps use a distinctive colour for faults that have moved in the last 120,000 years. This is generally regarded as the upper limit for a fault to be classified as active. Most of New Zealand's major active faults have been identified and mapped, at least on small-scale maps.

Faults may show horizontal offset, vertical offset, or a combination of the two. The following is a list of historic surface fault ruptures that have accompanied major earthquakes over the last 160 years:

  1. 1848: the Awatere earthquake in Marlborough created strike slip displacement with a maximum surface offset of 7 metres
  2. 1855: the Wairarapa earthquake created strike slip displacement with a maximum surface offset of 13 metres
  3. 1888: the Hope earthquake in North Canterbury (Glenn Wye) created strike slip displacement with a maximum surface offset of 3 metres
  4. 1929: the White Creek earthquake at Murchison created "reverse and strike slip" displacement with a maximum surface offset of 4 metres
  5. 1931: the Napier earthquake created reverse and strike slip displacement with a maximum surface offset of 2 metres
  6. 1934: the Pahiatua earthquake created a reverse displacement with a maximum surface offset of 4 metres
  7. 1968: the Inangahua earthquake created a reverse displacement with a maximum surface offset of 1 metre
  8. 1987: the Edgecumbe earthquake created a normal displacement with a maximum surface offset of 2 metres.

A map of active faults in New Zealand would show that faults with a fault rupture recurrence interval of less than 2000 years are:

  • grouped in a series of short fault traces to the southwest of Turangi through the National Park area
  • grouped in a broad band of fault traces running northeast from Taupo township through to the Bay of Plenty near Matata and Whakatane
  • follow the mountain ranges of the lower North Island, through western Hawke's Bay to the Huirau Ranges near Lake Waikaremoana
  • follow a line north east from near Milford Sound along the Southern Alps, splitting into four or more branches near Otira with one branch following along the Wairau River valley, with others fanning out along alignments similar to the inland and seaward Kaikoura Ranges
  • follow a line northeast from Lake Coleridge toward the northern end of Pegasus Bay.

A map of active faults in New Zealand would show that faults with a fault rupture recurrence interval of greater than 2000 years are:

In the North Island:

  • scattered along the west coast of the lower North Island, with short traces visible running parallel, and inland from, the coast from Wellington to the Kapiti; traces along the foothills of the Tararua ranges and northern Manawatu / Rangitikei; and a series of short traces running from the coast up to 20km inland between Hawera and Wanganui;
  • a short fault in Taranaki running from the coast, inland to near Rahotu, north of Opunake
  • two short, inland, faults near Inglewood
  • concentrated in a broad band of short traces running from Cape Kidnappers to Waipawa
  • follow the eastern side of the Tararua Ranges, with two traces originating near Masterton and Carterton running northeast towards an area near Castlepoint
  • in the eastern Bay of Plenty, running in several lengthy faults from near Lake Waikaremoana northwards to an area between Whakatane and Opotiki; and a series of short faults running from the coast, inland, between Opotiki and Te Kaha
  • a series of short faults inland along the East Coast of the North Island, spread from an area inland of Waipiro southwest to Wairoa
  • a lengthy fault running from the Firth of Thames inland towards an area west of Matamata
  • a fault running from the southeastern edge of the Auckland urban area southwards to an area east of Pukekohe
  • two faults running from parallel to Lake Wairarapa.

In the South Island:

  • a series of more than six faults running approximately north-south, inland from Westport, the southern-most being from an area near Reefton to Ahaura, with the others running parallel to the coast inland from around Murchison as far north as Tadmor
  • a series of faults forming an irregular line from an area near the Lewis Pass northwards towards Richmond
  • a band of faults contained in an area between the Smyth Range, Mayfield, Puketa (south of Kaikoura) and Hanmer Springs
  • a single fault running along the Awatere River valley
  • an irregular dog-legged fault running from an area near Lake Pukaki southward through Twizel, Omarama and the Dunstan Mountains
  • a fault running along the eastern edge of lake Wanaka, southwards in an irregular line towards an area near Lumsden
  • a short fault on the east coast, running north-south from, approximately, the Teieri River mouth to the Tokomairiro River
  • a short fault running north-south near Mosgiel
  • a loose scattering of faults in an area around Cromwell, Clyde, Alexandra, Roxburgh and south of Naseby.

Figure 3.3: Active faults map of New Zealand