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Scientists gather in Gisborne to focus on "Silent Earthquakes" - 29/07/2011

About 70 scientists from 10 countries will gather in Gisborne this week for a three-day workshop to discuss the best ways to study ‘silent’ earthquakes which happen at subduction zone plate boundaries around the world.

A subduction zone is where one tectonic plate dives under or “subducts” beneath the other plate, such as the one off the North Island’s east coast. Subduction zones are known to produce the largest earthquakes and tsunami on the planet, such as the recent magnitude 9 earthquake in northern Japan.

Also known as slow slip events, these silent earthquakes are similar to an earthquake in that they involve more rapid than normal movement between two pieces of the Earth’s crust along a fault line. However, in a normal earthquake the slip occurs in a matter of seconds, suddenly releasing seismic energy, while slip in a silent earthquake can take weeks to months to occur.

uakes Graphic

Slow-slip events were first discovered on the west coast of Canada about15 years ago and have since been recorded at about a dozen locations around the world, including New Zealand.

No-one feels anything or sees anything different at the surface in a slow-slip event. The phenomenon might have gone unrecognised if it wasn’t for the advent of highly accurate GPS instruments.

Scientists have proposed numerous theories to explain why this phenomenon occurs, but testing the theories is challenging as these events occur many kilometres underground, so physical evidence is difficult to obtain.

Locally, scientists first recognised silent earthquakes in Gisborne in 2002, and since then they have been observed in Hawke’s Bay, Manawatu, and on the Kapiti Coast.

A large slowslip event began under the Manawatu region in mid-2010, and probably will continue for the next several months. If slip in the current Manawatu event had occurred in seconds - as in a normal earthquake - rather than over many months, it would be equivalent to about a magnitude 7 earthquake, similar in size to the September 2010 Darfield earthquake.

About eight slow-slip episodes have occurred under Gisborne since 2002, with events clustering at roughly two-year intervals.

Workshop convener, Laura Wallace of GNS Science, said the purpose of the gathering in Gisborne was to discuss how ocean drilling techniques could be used to study this phenomenon.

“The best way to understand the true cause of slow-slip events is to drill into and sample the area on the plate boundary fault where they are known to occur, and monitor a whole range of physical and chemical properties at the plate interface,” Dr Wallace said.

Offshore Gisborne offers one of the best opportunities in the world to do this as the silent earthquakes there are occurring at depths of 5-15km below the seafloor, making the shallow part of the slip zone accessible with modern ocean drilling methods.

In addition to offshore Gisborne, there are possibilities for studying slow-slip events with drilling techniques in central Japan and Costa Rica, and scientists will also discuss these locations at the workshop.

At most other places in the world, silent earthquakes occur at depths of 30km to 40km, which is much too deep for direct investigation by drilling.

Workshop participants will discuss the key components needed in an ocean drilling programme to investigate slow-slip events. One of the workshop outcomes will be a report that provides a framework on how ocean drilling can be used to understand the origin of silent earthquakes.

If scientists decide to go ahead with a proposal to drill off the Gisborne coast, they will need to garner international support and funding. Such a project would take place over many years and be done in stages. It would also depend on the availability of a scientific ocean drilling ship. These ships are typically booked for several years in advance.

Dr Wallace said a drilling programme off the Gisborne coast represented a fantastic opportunity to learn more about the processes leading to silent earthquakes.

“Findings from such a project would have global significance as it would have the potential to significantly boost our understanding of the mechanics of subduction zone faults and the earthquakes that occur on them.”

The workshop is being funded from three sources – the Ministry of Science and Innovation, the Integrated Ocean Drilling Program (of which New Zealand is a member), and the National Science Foundation in the United States.

More information about the workshop is available here: http://www.gns.cri.nz/slowslip