By Erica Mather 06/05/2016


BREAKING: Scientists from New Zealand and overseas have recorded a slow-motion earthquake below the seafloor off the coast of Gisborne. This unprecedented study contributes crucial information to the understanding of earthquake and tsunami risk for New Zealand and worldwide.

For the first time, scientists have recorded detailed, centimeter-level movement beneath the seafloor in area called the Hikurangi subduction zone. The study demonstrates that slow-motion earthquakes or ‘slow-slip events’ can occur in the shallow region of faults that also have the potential to produce significant, tsunami-generating earthquakes.

This sheds light on the relationship between slow-slip events and regular earthquakes by showing that these two events can occur on the same part of the plate boundary.

The findings, published in the prestigious journal Science, describe how scientists from New Zealand, Japan and the US used a network of seafloor pressure recorders that were able to detect a slow-slip event 2-10 kms beneath the seafloor in September 2014.

The slow-slip event lasted two weeks and caused the interface between the two colliding tectonic plates – the Australian Plate and the Pacific Plate to move 15-20 cm. This plate boundary is known as the ‘Hikurangi megathrust’. The extent of motion detected over this time is equivalent to steady plate motion on the megathrust over three to four years.

Had the plates shifted to this degree suddenly, it would have resulted in a magnitude 6.8 earthquake.

 

Screen shot 2016-05-06 at 2.09.08 PM
Image of the slow slip slip event recorded by seafloor pressure sensors in September/October 2014. The main ‘slip patch’ is shown in red with an area of lesser movement in yellow. There have been more than a dozen of these events to the east of Gisborne since they were first discovered there in 2002.

Slow-slip events release the strain between tectonic plates gradually, over days to weeks rather than in seconds. They have been found to set off destructive earthquakes, such as the magnitude 9.0 earthquake off the coast of Japan in 2011. The present study shows that the slow-slip event recorded in 2014 occurred in the same location as the tsunami-generating, magnitude 7.2 earthquake in 1947 near Gisborne.

Project leader Laura Wallace, a research scientist at The University of Texas at Austin’s Institute for Geophysics said:

“The link [between slow-slip events and earthquakes] 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 emphasises that a greater understanding of this relationship is required to help forecast the likelihood of damaging earthquakes and tsunamis. The offshore Gisborne region is a particularly attractive location for international investigation of slow-slip events as they not only occur regularly, but at extremely shallow depths (2-10km) beneath the seafloor. The International Ocean Discovery Programme plans a major drilling project for 2018 in this area.

Assoc Prof John Townend, Director of the EQC Programme in Seismology and Fault Mechanics at Victoria University of Wellington comments:

“The area of Gisborne is internationally significant because the slip appears to take place at very shallow depths, potentially within the reach of scientific drilling equipment. This study is part of a much larger effort to understand how earthquakes are generated along the eastern coast of the North Island and in similar settings elsewhere.”

The results published from this study will contribute greatly to inform future drilling and other surveys. Professor Townend explains the research that is planned for the future:

“The members of this research team are involved in a larger study aimed at installing permanent monitoring equipment beneath the seabed east of Gisborne, using scientific drilling ships to install instruments that measure fluid pressure, temperature, fluid chemistry, and seismic waves.”

Study co-author Yoshihiro Ito, a professor at Kyoto University, supports this approach. He stresses that the recent findings increase the need to continuously monitor shallow, offshore slow-slip events both in New Zealand and elsewhere, using equipment similar to what has already been established off the coast of Japan.

 

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.)

Featured image: CC YouTube

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