Unlocking the secrets of slow slip using next-generation seismic experiments and IODP drilling

Dr Rebecca Bell, Imperial College London. Part of the IGT Seminar.

Subduction plate boundary faults are capable of generating some of the largest earthquakes and tsunami on Earth, such as the 2011 Tōhoku, Japan and the 2004 Sumatra-Andaman earthquakes, together responsible for over 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 occurs faster than the plate motion rate but too slowly to produce seismic waves. 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. Most well-studied SSEs (e.g. Cascadia and SW Japan) occur at depths exceeding 20 km; too deep for direct sampling and high-resolution seismic imaging. A notable exception to this lack of access is the north Hikurangi margin, 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 from 3D seismic and conduct near-surface monitoring of the area of the fault undergoing slow slip. For this reason the north Hikurangi margin has been the focus of a number of large international experiments in 2017-2018, the objectives of which and preliminary findings will be discussed in this presentation. 

2D seismic reflection data collected in 2005 revealed thick high-amplitude reflectivity zones coinciding broadly with the source areas of SSEs at depths of 5 km below the seafloor. Without high-resolution velocity models of this zone and an understanding of the type of material that makes up the subducting plate the interpretation of these high-reflectivity zones is ambiguous. Constraining the origin of this reflectivity could provide important clues as to the processes responsible for slow slip. In Dec 2017 – Jan 2018 a 3D seismic experiment was conducted in New Zealand in an attempt to image the structure and recover the physical properties of the plate boundary in areas of slow slip. These experiments funded by NSF, NERC, Japan and New Zealand involved the deployment of 100 ocean bottom seismometers, 200 onshore seismometers and the collection of 3D seismic reflection data. The instruments were deployed with high density to allow the collection of data suitable for the application of Full-Waveform Inversion techniques to produce high-resolution velocity models.

In Nov-Dec 2017 and March-May 2018 two International Ocean Discovery Program (IODP) expeditions, 372 and 375, used scientific drilling in New Zealand for the first time to target slow slip events providing valuable calibration data for seismic models and revealing the lithology of material being subducted.

In this talk I will discuss how these expeditions aim to reveal the geophysical environments of slow slip events and get closer to understanding what factors lead to slow slip. I will discuss preliminary findings regarding the lithostratigraphy and physical properties of the incoming sedimentary section from shipboard measurements and discuss potential implications for the source of high-amplitude reflectivity in the slow slip zone.