Alpine fault in New Zealand not your average fault

08/18/04

Ents, orcs and hobbits may have trod upon New Zealand soils, but beneath the Southern Island lies a giant earthquake fault that may help seismologists understand how the Earth moves and bends, according to a Penn State seismologist.

"One of the issues that makes the Alpine fault interesting is that while it is a strike slip fault for most of its length, it begins in a transition from a subduction zone to a strike slip fault," says Dr. Kevin Furlong, professor of geosciences. "Most of the major faults in the world that are strike slip faults San Andreas, Anatolian in Turkey -- initiate quite differently, but this is different because the subduction is ripping off part of the Australian plate before it joins the Alpine fault."

Subduction occurs when one of the Earth's tectonic plates slides beneath another plate. The subduction area or zone is usually the location for earthquakes and volcanic activity. Mt. St. Helens formed in a subduction zone. The Alpine fault does not have volcanic activity. Strike slip faults occur when two tectonic plates slide past each other.

The origin of the Alpine fault is located in the transition area between Fiordland and the Southern Alps on the South Island of New Zealand near Milford Sound. Here, the Australian plate meets the Pacific plate. While most other strike slip faults begin as an area of many small faults and eventually coalesce into one fault that dominates, the Alpine fault begins as a single fault.

"The fault is moving at about an inch and a quarter (30 mm) a year, tectonic plate speed, right from the beginning, which is very fast for a new fault," Furlong told attendees at the 2004 Western Pacific Geophysics Meeting today (Aug. 18) in Honolulu. "We would like to get at the physics of what is happening on that fault."

At the origin of the Alpine fault, the Pacific plate and the Australian plate have a small jog or notch forming a tiny subduction zone of about 60 miles (100 km) by 120 miles (200 km). On Aug. 20, 2003, an earthquake of magnitude 7.2 occurred in this area.

"We are interpreting this earthquake as a reflection of the tearing of the plate when it transitions from subducting to a strike slip zone," says Furlong. "To match the observations, we need a tear in the plate."

There have been no major earthquakes on the main portion of the Alpine fault in the last 100 years. Paleoseismologists believe that there was an earthquake in the 1700s, but their calculations are still uncertain.

"We know that there is at present significant seismicity at the southern end of the fault and at the northern end where the fault changes directions, but the central portion is relatively aseismic," says Furlong. "We do not know if this is normal or not."

While earthquake monitoring on the Alpine fault is not as dense as on the San Andreas, records show that earthquakes of magnitude 3 occur quite frequently, but not in the central portion of the fault.

"When we take into account the detection level of the monitoring, there in fact appear to be more earthquakes in New Zealand than on the San Andreas in California,""says Furlong.

One indication that things have not always been quite so calm on the Alpine fault is the presence of a rock type called psuedotachylite, which is thought to form either during an earthquake or with a meteor impact. In New Zealand they are formed along the Alpine fault by earthquakes. The rock contains bands of melted rock that seeps into fractures. The melted rock forms by the frictional heating during an earthquake. To form psuedotachylite, the right conditions of temperature and pressure must occur, and along faults, this implies that there must be very high stresses during the earthquake.

"The Alpine fault generates large amounts of psuedotachylite," says Furlong. "Why should the fault have such a high stress level? Something about the mechanical behavior is still puzzling. Perhaps the fresh edge of the plate that joins the fault is rough and that is the reason for the melting during earthquakes."

Because the Alpine fault begins so cleanly, Furlong believes it can tell us something about strike slip faults in general. While the Alpine fault does not impact large populated areas, the other major strike slip faults do, including the San Andreas in California, the Anatolian in Turkey and faults in China and Central Asia.

Source: Eurekalert & others

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