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Subduction zones are located where one of the Earth's tectonic plates slides beneath another - this motion is controlled by the plate boundary fault zone. These plate boundary faults are capable of generating some of the largest earthquakes and tsunami on Earth, such as the 2011 Tōhuku-oki, Japan and the 2004 Sumatra-Andaman earthquakes, together responsible for ~250,000 fatalities.

However, in the last 15 years a new type of seismic phenomena has been discovered at subduction zones: slow slip events (SSEs). These are events in which slip on the fault occurs faster than the tectonic plates usually move, but too slowly to produce seismic waves and shaking. Slow slip events may have the potential to trigger highly destructive earthquakes and tsunami on faults nearby, but whether this is possible and why slow slip events occur at all are two of the most important questions in earthquake seismology today. 

There is some evidence that slow slip preceded two of the largest earthquakes this decade, the 2011 Tohuki-oki and 2014 Iquique, Chile earthquakes. Therefore, there is an urgent societal need to better understand slow slip events and their relationship to destructive earthquakes.

Most well-studied SSEs (e.g. in Cascadia, NW North America and SW Japan) occur at depths exceeding 20 km; which makes them difficult to study. A notable exception to this lack of access is the north Hikurangi subduction zone, New Zealand, where well-characterised SSEs occur every 1-2 years, over periods of 2-3 weeks at  depths of <2 -15 km below the seafloor. 

The large magnitude and close proximity of the SSEs to the seafloor makes it feasible to precisely locate, drill into, collect logs, sample, image and recover physical property information using man-made seismic waves and conduct near-surface monitoring of the area of the fault undergoing slow slip. 

For this reason the north Hikurangi subduction zone has been the focus of a number of major international experiments in 2017-2018, including the deployment of a large 3D array of seismometers (the NZ3D FWI project) and deep-ocean drilling by the International Ocean Discovery Program (Expeditions 372 and 375). The objectives of these new experiments that attempt to unlock the secrets of slow slip will be discussed in this presentation, together with a first look at preliminary findings.

Speaker

Rebecca Bell, Imperial College London

Dr. Rebecca Bell is a senior lecturer at Imperial College London, where she lectures in Plate Tectonics and seismic reflection techniques. She conducted her undergraduate degree at the University of Oxford and graduated with a PhD from the National Oceanography Centre Southampton, University of Southampton. 

Her PhD investigated early-stage continental rifting of the Gulf of Corinth in central Greece. Rebecca then furthered her interests in studying plate boundaries using seismic methods with a position as an Active Source Seismologist at GNS Science in New Zealand. Her work here involved investigating the Hikurangi subduction zone, focusing on the characteristics of slow slip zones. 

In 2008 Rebecca joined Imperial College London as a Statoil funded Post-doctoral Research Associate and later became a Research Fellow. She joined the academic staff of the Earth Science and Engineering Department at Imperial College in 2015. Rebecca has a strong interest in public outreach and in 2014 was the Earth Science columnist for the Guardian Observer. She is currently the principle investigator of the NZ3D FWI project, which has deployed 200 seismometers across the North Island of New Zealand to image zones of slow slip (follow the project on twitter at @NZ3D_FWI).

Videos of past lectures can be viewed online in our Past Meeting Resources

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