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New findings help in understanding offshore quake threat - 06/05/2016

Scientists have found that slow-motion earthquakes or ‘slow-slip events’ can rupture the shallow portion of a fault that also moves in large, tsunami-generating earthquakes.

The discovery was made by conducting the world’s first detailed investigation of centimetre-level seafloor movement during a slow-slip event at the Hikurangi subduction zone off the North Island’s east coast.

“This observation has revealed the true extent of slow motion earthquakes at an offshore subduction zone for the first time,” said project leader Laura Wallace, a research scientist at The University of Texas at Austin’s Institute for Geophysics.

The finding increases understanding of the relationship between slow-slip and normal earthquakes by showing that the two types of events can occur on the same part of a plate boundary.

Geophysicist Stuart Henrys, of GNS Science, with some of the ocean-bottom seismometers prior to their deployment off the coast of Gisborne in  mid-2014. They typically spend about 12 months on the seafloor recording tremors before being retrieved and replaced by fresh instruments.

Geophysicist Stuart Henrys, of GNS Science, with some of the ocean-bottom seismometers prior to their deployment off the coast of Gisborne in mid-2014. They typically spend about 12 months on the seafloor recording tremors before being retrieved and replaced by fresh instruments.

“The link has been difficult to document in the past because most slow-slip monitoring networks are strictly land-based and are located far from the offshore trenches that host tsunami-generating earthquakes,” Dr Wallace said.

Insights into activity along subduction plate boundaries, such as off the North Island’s east coast, are important as this is where the world’s largest earthquakes and tsunamis are generated.

An international team of researchers from the US, Japan and New Zealand (GNS Science) collaborated on the research, which has just been published in the latest issue of Science.

Using a network of highly sensitive seafloor pressure recorders, the team detected a slow-slip event, in September 2014 beneath the seafloor off the Gisborne coast.

The 2014 slow-slip event lasted two weeks, resulting in 15-20 centimetres of movement along the ‘Hikurangi megathrust’ that forms the plate boundary between the North Island and the Pacific Plate. This amount of motion is equivalent to three to four years’ worth of steady plate motion on the megathrust – the interface or surface between the two colliding tectonic plates.

If the movement had occurred suddenly, rather than slowly over two weeks, it would have resulted in a magnitude 6.8 earthquake. The seafloor sensors recorded up to 5.5 centimeters of upward movement of the seafloor during the event.

The slow-slip event was also recorded by continuously operating GPS instruments in the GeoNet network (www.geonet.org.nz), which showed up to 3cm of eastward movement onshore in the Gisborne region.

Slow-slip events are similar to earthquakes, but instead of releasing strain between two tectonic plates in seconds, they do it over days to weeks, creating quiet, centimeter-sized shifts in the landscape. In a few cases, these small shifts have been associated with setting off destructive earthquakes, such as the magnitude 9.0 Tohoku-Oki earthquake that occurred off the coast of Japan in 2011 and generated a tsunami which caused the Fukushima Daiichi nuclear power plant disaster.

The slow-slip event that the team studied in 2014 occurred in the same location as a magnitude 7.2 earthquake in 1947, which generated a large tsunami in the coastal area near Gisborne. “Our results clearly show that shallow slow-slip event source areas are also capable of hosting seismic rupture and tsunami generation,” said study co-author Yoshihiro Ito, a professor at Kyoto University.

“This increases the need to continuously monitor shallow, offshore slow-slip events at subduction zones in New Zealand and elsewhere, using permanent monitoring networks similar to those that have already been established off the coast of Japan.”

Earthquakes are unpredictable events, Dr Wallace said, but the linkage between slow-slip events and earthquakes could eventually help in forecasting the likelihood of damaging earthquakes and tsunamis.

“To do that we will have to understand the links between slow-slip events and earthquakes much better than we currently do,” she said.

The research team installed the seafloor instruments in May 2014 using the NIWA research ship Tangaroa. The network consisted of 24 seafloor pressure gauges, and 15 ocean bottom seismometers. The team collected the devices in June 2015 using the US research ship Roger Revelle


Slow slip map

The East Coast of the North Island is one of about a dozen areas worldwide where slow earthquakes occur regularly. However, the offshore Gisborne region is unique in that they occur at extremely shallow depths (2-10km) beneath the seafloor. The unusually shallow depth of the events makes Gisborne a very attractive area for international scientists to investigate this phenomenon.

GeoNet GPS instruments have detected many slow-slip events in New Zealand since the first one was discovered off the coast of Gisborne in 2002, but the one in 2014 is the first slow-slip event worldwide to be investigated in detail using instruments deployed on the seafloor.

The shallow slow-slip events off Gisborne are the subject of a major scientific ocean drilling project planned by the International Ocean Discovery Program for 2018. As part of this project, sensors will be placed inside two drillholes off the coast of Gisborne to monitor changes during slow-slip events and earthquakes. Scientists believe that continuous monitoring of slow earthquakes off Gisborne with both seafloor and borehole instruments is vital to build up knowledge of the relationship between slow-slip events, megathrust earthquakes, and tsunamis globally.

Scientists involved in this project are from University of Texas, Columbia University, Kyoto University, University of Tokyo, Tohoku University, GNS Science, the University of California-Santa Cruz, and the University of Colorado Boulder. The research was funded by the United States’ National Science Foundation; the Japan Society for Promotion of Science; Japan’s Ministry of Education, Culture, Sports, Science and Technology; and by funding from participating universities, and New Zealand Direct Core Funding to GNS Science. Some of the ship time for the project was supported by the New Zealand Government’s Marine Funding Allocation. (An allocation by MBIE to support the operations of the Tangaroa.)


What is the significance of these findings for the earthquake and tsunami risk of the North Island's east coast?

The prevailing view of megathrust earthquakes is that subduction interplate faults slowly and steadily accumulate stress from tectonic forces until a threshold is overcome and then they fail catastrophically in a large earthquake. Traditional efforts to forecast the likelihood of quakes are based on this simple model. Scientists now know that faults don’t behave that way and non-traditional earthquakes (like slow-slip) occur throughout the earthquake cycle. This leads to more complex patterns of stress accumulation that potentially can alter the strength of the fault plane between the two tectonic plates. Importantly, understanding and monitoring the range of seismic behaviour at plate interfaces may provide a valuable tool to forecast the likelihood of damaging earthquakes and tsunamis. This not only applies to the North Island's East Coast, but also globally.

What are the next steps in this field of investigation?

1. Scientists have successfully detected slow-slip with ocean-bottom instruments through one 'cycle' and they now see the importance of extending this work to record multiple cycles of slow-slip to increase the understanding of this phenomenon. They hope this will be achieved by combining seafloor observations with placing instruments in boreholes in 2018 through the International Ocean Discovery Programme (IODP). This will involve drilling up to three boreholes into the boundary zone between the two tectonic plates east of Gisborne. 

2. So far scientists have detected shallow slow-slip occurring off the Gisborne coast and would like to know if other parts of the East Coast 'deform' in the same way. In June 2016 they will deploy four instruments on the seafloor off Hawke’s Bay for this purpose. In addition, they believe there is a compelling case to extend the GeoNet national network of earthquake monitoring instruments offshore.

3. Scientists want to test if slow-slip deformation can rapidly change stresses on the subduction interface and if those changes can be detected with current instrumentation and observations. Seismologist Dr Bill Fry of GNS Science was awarded a Marsden Fund grant in 2015 to further this research.