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What is a Fault?
Rocks respond to stress (being squeezed or pulled apart) near the Earth’s surface by breaking. When rocks break and there is no offset along either side of the break, the break is called a fracture or joint. Rocks can also break as a result of thermal expansion and contraction, the effects of fluids freezing, or when rocks are squeezed together or pulled apart. When the rocks move past each other along fracture surface, it is called a faulting. Fault surfaces are often nearly planar, and that planar surface is referred to as a “fault plane.”
The four types of faulting:
Normal fault: the rocks above the fault plane, or hanging wall, move down relative to the rocks below the fault plane, or footwall.
Reverse fault: one in which the hanging wall moves up relative to the footwall.
Strike-slip: caused when rocks on either side of a nearly vertical fault plane move horizontally.
Oblique-slip fault: forms when movement is not exactly parallel with the fault plane. Oblique movement occurs when normal or reverse faults have some strike-slip movement and when strike-slip faults have either some normal or reverse movement.
It’s easy to recognize a fault when you look at an outcrop or an exposure. Usually, different rock types or rock features (such as quartz veins, mineral layers, or beds) are broken and offset along the fault plane. Faults are commonly marked by debris that forms when there is movement along the fault plane. Grinding of rock along the fault plane may also produce a clay-like, pulverized rock called gouge. Sometimes when the fault plane is exposed, you may see grooves, striations (scratches), and asymmetric fractures, called slickensides that provide visual evidence of movement.
In a major earthquake the ground may rupture, with ruptures typically occurring along existing fault traces.
Faults within the Kapiti district
The Institute of Geological and Nuclear Sciences (GNS) has mapped all the known active fault traces within the Kāpiti District and have identified Fault Avoidance Zones - buffer zone either side of the known active fault traces. These are identified in the District Plan maps.
At MacKay's Crossing, in the region of waypoint 1, the State Highway is on top of the fault. In order to ascertain the Fault direction, Scientists dug a trench across the Fault near the State Highway overbridge .
What we know about the earthquake faults in the Kāpiti Coast District
GNS has provided information on the average recurrence interval, i.e. the estimated average interval between ruptures on an individual section or length of a fault. Below are its estimates on when the District's faults last ruptured.
Ohariu Fault: The Ohariu Fault, a strike slip fault, is one of the major faults in the Wellington area. Average recurrence interval of surface rupture of 1,300 to 3,800 years. The last three large earthquakes on it were about 1,000, 4,000 and 4,800 years ago. Each time the foothills jolted south about 4 metres and rose about a metre. It is expected that an individual surface rupture along the fault could generate three to five metres of right-lateral displacement at the ground surface, with a lesser and variable amount of vertical displacement. There is no risk from fault rupture at present but based on the best information available there is a low risk.
Northern Ohariu Fault: could generate three to four metres of right-lateral displacement at the ground surface, with a lesser and variable amount of vertical displacement.
Gibbs Fault: anticipated that the Gibbs fault would have a longer recurrence interval than the Ohariu or Northern Ohariu faults. A 3,500–5,000 year recurrence interval has therefore been estimated.
Ōtaki Forks Fault: a 3,500–5,000 year recurrence interval has been estimated and the potential surface rupture has been estimated to be approximately one metre.
Southeast Reikorangi Fault: A fault about which the least is known. It is estimated that the recurrence interval is 5,000–10,000 years.
Earthquakes, not just those on the Ohariu Fault, have raised the coastal plain here about 5 metres in the past 5,000 years.
Source: Institute of Geological and Nuclear Sciences' report 'Earthquake Fault Trace Survey Kāpiti Coast District' (August 2003, updated August 2007).
(An Aerial view of the Ohariu Fault - Photo courtesy of GNS Science Lloyd Homer)
Go now to waypoint 3 for views of the direction of the Ohariu Fault
And now for some questions about the Ohariu Fault. Please send your completed answers to the cache owner; do not post your answers with your on-line log.
1. Standing at Waypoint 1 (MacKay's Crossing) what evidence can you see, if any, that you are virtually standing on the fault?
2. During the last 3 large earthquakes in each case what direction were the foothills jolted, and by how much.
3. The foothills also rose during these 3 large earthquakes – by how much?
4. At waypoint 3 please take a photo of the distant foothills, with your caching name or something caching related included in the photo. You do not have to be in the photograph. This photograph is required for your on-line log.
I hope you enjoyed learning about the Kapiti faults - I look forward to receiving your answers.