Murchison 1929 Earthquake (Earthcache) EarthCache

Murchison 1929 Earthquake

WHAT IS AN EARTHQUAKE?

The Earth's entire outer surface is like a hard shell, which is broken into smaller pieces called 'tectonic plates'. New Zealand is on the boundary between two of these plates, called the Australian Plate and the Pacific Plate. These plates are constantly grinding into each other, which causes stresses to build up in the brittle, upper layers of the plates. When the brittle rock finally breaks, it generates an earthquake. Under New Zealand, the Pacific Plate is moving to the west-south-west at about 50 mm a year. We have about 15,000 earthquakes in and around New Zealand per year. Most of these are small but around 250 are big enough to be felt without instrumentation¹

An earthquake (also known as a quake, tremor, temblor or seismic activity) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami or tidal wave. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.²

There are two main types of seismic waves, body waves and surface waves. P waves (primary waves) are longitudinal or compressional waves. In solids S waves and can travel through any type of material. In air, these pressure waves take the form of sound waves, hence they travel at the speed of sound. Typical P wave speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite. S waves (secondary waves) are transverse or shear waves, which means that the ground is displaced perpendicularly to the direction of propagation – generally rolling along in the same way as ocean waves. Their speed is about 60% of that of P waves in a given material. S waves arrive second in a seismic station because of their slower speed.^4,1

MURCHISON - JUNE 17, 1929

1929 was a year of earthquakes in New Zealand - 678 in total. Earthquakes on 9 March (Arthur's Pass) and 8 May (Central North Island) were felt through large areas of the country. At 10:17 am on Monday, 17 June, an earthquake shook New Zealand from Auckland to Dunedin, and was measured at 7.8 on the Richter Scale. The earthquake centre was in the Lyell Range near White Creek, just west of Murchison.

The ground rose and swayed, making chimneys fall and the land slide in huge slips down the surrounding hillsides, blocking roads and rivers. A piece of land 75 kilometres long and 30 metres wide was lifted by almost half a metre. 17 people died in the earthquake, many as a result of the landslips and the floods they created. The surrounding hillsides were water-logged with mid-winter rain.

The first reports of serious damage came from Greymouth, Nelson and Westport. In Greymouth most chimneys were destroyed and gas, water and sewage pipes had broken. In Nelson, shop windows had broken and chimneys had collapsed. At Nelson College the tower had fallen and dormitory roofs had been smashed by falling stonework. Due to the broken phone and telegraph system, it was not until days later that the news of the destruction around Murchison was relayed to the rest of New Zealand. ³

AFTERMATH

This mound of sand and gravel was created when sediment liquefied during the Murchison earthquake. The sand spurted out at the surface, creating a mini-volcano. Although such features formed by sand and mud often result from earthquakes, this appears to be a unique example where gravel boulders have been ejected, indicating the force of the eruption.⁵

It was the middle of winter, and heavy rain had soaked the hills and made the conditions worse. Earthquake tremors continued for the next fortnight, four of which a respectable size 7, also adding to the confusion and terror. With their houses unliveable or completely destroyed, the residents of Murchison camped in tents or took shelter in sheds.

Because of the damage to the Buller Gorge road it took 18 months to open the road between Westport and Reefton. When the Governor-General, Sir Charles Fergusson, inspected the disaster area, he had to travel part of the way by horseback. It was later found that the earthquake had been centred under a fault which had been thought to be dormant. The White Creek Fault had shifted in the earthquake, moving 2.1 metres horizontally, and 4.5 metres vertically.

Where the fault ran across the Buller Gorge road a wall 2 metres high was formed. At one point where the faultline crossed the Maruia River, and the land on one side was raised up by the quake, a new waterfall was created.³

REQUIREMENTS TO LOG THIS EARTHCACHE

The coordinates will take you to a site in the Matakitaki valley where the remains of a landslide or slip can still be seen. Once there, perform the following task and answer these questions:

DO NOT POST THE ANSWERS ON-LINE! EMAIL THEM TO THE CACHE OWNER HERE.

• The chunks or boulders are clearly made of two different strata (layers). Go to 41°50.744 172°18.012. The largest boulder you are standing next to has a harder dark layer on top. Measure the thickness of this layer.
• Describe what the softer layer under the dark harder layer consists of.
• Have a think and describe in your own words why you think these kinds of strata may have contributed to the large number of landslides in this valley during the 1929 quake.
• Tell us where you live (or if you are a visitor to New Zealand, where you arrived in the country), and how long it would have taken for “P” waves from the Murchison Earthquake to travel to you.
• In your public on-line log, simply share your general experience of your visit together with a photo with your GPS visible with the horizontally striped cliffs behind you.

YOU CAN LOG YOUR VISIT STRAIGHT AWAY, BUT YOUR ANSWERS MUST BE EMAILED TO THE CACHE OWNER AT THE SAME TIME.

ATTRIBUTIONS

1. New Zealand GeoNet project: EQC, GNS Science and LINZ
   http://www.geonet.org.nz/earthquake/
2. Wikipedia
   http://en.wikipedia.org/wiki/Earthquake
3. Christchurch City Libraries
   http://christchurchcitylibraries.com/kids/nzdisasters/eqmurchison1929.asp
4. Wikipedia
   http://en.wikipedia.org/wiki/Seismic_wave
5. Te Ara - the Encyclopedia of New Zealand
   http://www.teara.govt.nz/

OTHER REFERENCES

1. http://ehive.com/account/3348/object/1828
2. http://owenrivertavern.co.nz/murchison_earthquake
3. http://murchisonquake.shampayne.org.nz/
4. http://homepages.ihug.co.nz/~glaive/nz/pages/murchis.htm
5. http://www.teara.govt.nz/en/historic-earthquakes/5/3
6. http://www.gns.cri.nz/what/earthact/earthquakes/index.html
7. http://www.theprow.org.nz/the-murchison-earthquake/