Geologically faults that are old but pose very small threat of and earthquake activity. Faults like this that have being active and have moved once or more in the 120,000 years are likely to move again in the future. This is what is classified as an active fault. active faults are repeated breaks in the earths surface.
Scarps and other Landscape Features
Indications found here show the presence of an active fault where the land along the fault has ruptured it produces breaks in the Earths surface this is know as the Fault Scarp. A good example is the 1929 Murchison Earthquake where the fault scarp moved 4.5 metres high (Up thrust) and formed along part of the White Creek Fault. Repeated upward movements of land along a fault can create steep hillsloops and over geological time build mountains. This is how many of the mountains ranges around New Zealand have being formed,by thousands of separate offset movements along an active fault - a few example are the Rimutaka and Tararua ranges along the Wairarapa Fault, the Inland and Seaward Kaikoura ranges along the Clearance and Hope faults and the western side of the Southern Alps along the Alpine Fault. The bedrock along a faultline is often shattered by the repeated movement of earthquakes over the years and can be eroded easily by streams and glaciers, which produce distinctive linear valleys. Fault movement can also offset landscape features and this can also cause streams to develop right angle bends where they cross the fault line.
Determining fault activity
To determine and estimate the likelihood of future earthquakes along a given section of a fault you must take the following variables and determine how often, on average earthquakes have occurred here in the past and the dates of past earthquakes that have occurred here and the last time an earthquake has happened here. To determine the past dates of earthquakes there are serval methods, as earthquakes trigger small slips and larger landslides they bury vegetation and strip trees and soil from the hillsides. Trenches are dug across the faultline and plant materials that are recovered are taken away and tested using radiocarbon dating. Trees also reveal dated as a new stand of trees begin to grow and replace where a landslips has occurred and tree rings are a good way of estimating the date. When the trench is dug an estimate of the movement of past earthquakes can be determine from the offset of the surface features or from breaks in the layers of sediment exposed across the fault. From the extent of the movement during a single earthquake you can estimate the likely magnitude of the quake.
There is a number of active faults in New Zealand that have occurred in the last 200 years. The Awatere Fault in 1848, The Wairarapa Fault in 1855, Hope Fault 1888, Poulter Fault and White Creek Fault 1929 and the latest one was the Edgecumbe Fault in 1987. There is only one known fault in New Zealand that has moved twice within the period off written records, this is the Kaiapo Fault near Taupo which moved during earthquake swarms in 1922 and 1983.
These faults are considered to present the greatest earthquake hazard and are those that move the most frequent. Faults can happen at any time, they can range from a frequency of several hundred of years to several thousands of years. There is more than 50 active faultlines that are in New Zealand that have occurred in the last 2,000 years. Some faults moved in historical times and are unlikely to cause an earthquake in the near future. The White Creek Fault that caused a 7.8 magnitude Murchison earthquake in 1929, showed no evidence of having moved previously for 20,000 years. It will be probably be centuries before it happens again.
Offshore active faults
About a third of all New Zealand’s shallow earthquakes occur offshore. The continental shelf that lies east of the country is a region of that is being deformed as the Pacific Plate descends beneath the Australian Plate. This zone is up to 150 kilometres wide and includes many active faults. A number of the country’s major faults also extend into Cook Strait, and the Alpine Fault has offshore segments. These submarine faults present a particular hazard for coastal cities and towns, because offshore earthquakes can cause tsunamis, either directly by the movement of land along the fault or by triggering underwater landslides.
The Wellington–Mohaka Fault
The North Island has the longest active fault in New Zealand that runs from Cook Strait to the Bay of Plenty and is named The Mohaka Fault. Over 75% of Wellington people live within 10 kilometres of this fault. The distinctive landscape features around Wellington are due to this fault. At least two major earthquakes have occurred in the last 1,000 years and the most recent was 400 years ago. During these earthquake section of land on opposite sides of the fault moved past each other by 4 metres Large earthquakes on this section of the fault are estmated to occur about every 500 to 700 years.
Cache site and Questions
This earthcache is placed in the Upper Hutt Suburb of Totara Park. Totara Park is being specially planned and designed as it is sitting on top of a faultline and this is to the local residence. No residential houses are allowed to be built within 20 metres of this fault.
At this location you will have to do the following to be able to claim the cache. Failure to comply with the logging requirements may result in your online log been deleted. Please forward your answers by email, and wait for confirmation before logging. Do NOT place any answers on the web page
Q1: Standing at the GZ looking South you are standing on which part of the faultline (Geological term please)
Q2: What is the third word that is on the nearby sign
Q4: Go to S41 06.322 E175 05.191. Facing 230 degrees true - (Optional)Please take a photo of your GPS at the GZ facing towards the given direction(Please DONT include the sign in the photos - or your log may be deleted)
Q5: Name two methods that are used to establish the dates of past Earthquakes
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