The 1929 Murchison earthquake occurred at 10:17 am on 17 June. It struck the Murchison region of the

South Island The South Island ( , 'the waters of Pounamu, Greenstone') is the largest of the three major islands of New Zealand by surface area, the others being the smaller but more populous North Island and Stewart Island. It is bordered to the north by ...

, with an estimated

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

of 7.3, and was felt throughout

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

. There were 17 deaths, mostly as a result of

landslide Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslides ...

s triggered by the earthquake. The rumbling sound of the earthquake was loud enough to be heard at

New Plymouth New Plymouth () is the major city of the Taranaki region on the west coast of the North Island of New Zealand. It is named after the English city of Plymouth, in Devon, from where the first English settlers to New Plymouth migrated. The New Pl ...

, more than 250 km (155 mi) away.

Tectonic setting

New Zealand sits astride the boundary between the Indo-Australian plate and the Pacific plate. In the South Island most of the displacement is taken up on the Alpine Fault passing to the north onto a set of

strike-slip faults In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

, the Marlborough fault system. The sense of displacement across the plate boundary is oblique and most of the faults have a reverse component of slip. Some of the resulting deformation is accommodated within the plates themselves away from the boundary. The 1929 Murchison earthquake occurred on the White Creek Fault, located in the Buller Gorge.

Damage

Although the

epicentre The epicenter (), epicentre, or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Determination The primary purpose of a s ...

was in a sparsely populated area about 65 km (40 mi) north of Murchison, the shaking in the surrounding region was intense enough to cause damage over a wide area. The towns of

Nelson Nelson may refer to: Arts and entertainment * ''Nelson'' (1918 film), a historical film directed by Maurice Elvey * ''Nelson'' (1926 film), a historical film directed by Walter Summers * ''Nelson'' (opera), an opera by Lennox Berkeley to a lib ...

Greymouth Greymouth () (Māori language, Māori: ''Māwhera'') is the largest town in the West Coast, New Zealand, West Coast List of regions in New Zealand, region in the South Island of New Zealand, and the seat of the Grey District Council. The populat ...

and Westport were all affected and roads, buildings and bridges were severely damaged throughout an area estimated at 26,000 km² (10,000 mi²). The worst damage was caused by landslides triggered by the shaking, which destroyed several homes and resulted in 14 of the 17 deaths. Two other deaths were caused by coal mine collapses. Most of the buildings in the Murchison area were built of wood and were warped and moved from their piles, but many brick buildings and chimneys were damaged in Nelson, Greymouth and Westport.

Characteristics

The earthquake was preceded by several

foreshock A foreshock is an earthquake that occurs before a larger seismic eventthe mainshockand is related to it in both time and space. The designation of an earthquake as ''foreshock'', ''mainshock'' or aftershock is only possible after the full sequenc ...

s before the mainshock, accompanied by booming sounds in the mountains around Murchison. There was a large

surface A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...

fault rupture visible in the Buller River, indicating about 4.5 m (14.8 ft) of vertical movement and 2.5 m (8.2 ft) of lateral movement. The earthquake triggered many landslides and these created 38 new lakes, of which 21 still exist, including Lake Stanley, on Stanley River. The Maruia Falls are also a notable by-product of the earthquake. The earthquake was accompanied by unusually loud rumbling noises that were heard as far away as New Plymouth. A maximum

intensity Intensity may refer to: In colloquial use * Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, m ...

of X (''Extreme'') on the

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS) measures the effects of an earthquake at a given location. This is in contrast with the seismic magnitude usually reported for an earthquake. Magnitude scales measure the inherent force or ...

has been estimated for Murchison. Most of the northern part of South Island suffered an intensity of VII (''Very strong'') or greater.

Aftermath

The landslides blocked many roads causing food supplies to run out in some areas near the epicentre. This and the risk of aftershocks and flooding from landslides blocking rivers caused many residents to evacuate the area, mostly on foot. Karamea was completely cut off and received no outside help for two weeks, until a de Havilland Tiger Moth plane landed on the beach. Some of the blocked roads took months to clear.

Importance for studying the Earth's interior

Analysing the seismic waves from this earthquake appearing in the shadow zone, Inge Lehmann demonstrated that there must be a solid

inner core Earth's inner core is the innermost internal structure of Earth, geologic layer of the planet Earth. It is primarily a solid ball (mathematics), ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius. T ...

that reflected

p-waves A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at an ...

References

External links

Landslide blog about the earthquake by Professor Dave Petley, Durham University, UK
* {{Authority control 1929 Murchison New Zealand - Murchison Murchison History of the West Coast Region History of the Tasman District Landslides in Oceania 1920s landslides 1929 natural disasters 1929 disasters in New Zealand