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SUMMARY:Unlocking the secrets of slow slip using next-generation seismic e
 xperiments and IODP drilling at the north Hikurangi subduction zone\, New 
 Zealand - Rebecca Bell (Imperial\, London\, UK)
DTSTART:20190306T160000Z
DTEND:20190306T170000Z
UID:TALK117214@talks.cam.ac.uk
CONTACT:Nienke Blom
DESCRIPTION:Subduction plate boundary faults are capable of generating som
 e of the largest earthquakes and tsunami on Earth\, such as the 2011 Tōho
 ku\, Japan and the 2004 Sumatra-Andaman earthquakes\, together responsible
  for over 250\,000 fatalities. However\, in the last 15 years a new type o
 f seismic phenomena has been discovered at subduction zones: slow slip eve
 nts (SSEs). These are events in which slip occurs faster than the plate mo
 tion rate but too slowly to produce seismic waves. Slow slip events may ha
 ve the potential to trigger highly destructive earthquakes and tsunami on 
 faults nearby\, but whether this is possible and why slow slip events occu
 r 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 seismi
 c imaging. A notable exception to this lack of access is the north Hikuran
 gi margin\, New Zealand\, where well-characterised SSEs occur every 1-2 ye
 ars\, 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 make
 s it feasible to precisely locate\, drill into\, collect logs\, sample\, i
 mage and recover physical property information from 3D seismic and conduct
  near-surface monitoring of the area of the fault undergoing slow slip. Fo
 r this reason the north Hikurangi margin has been the focus of a number of
  large international experiments in 2017-2018\, the objectives of which an
 d preliminary findings will be discussed in this presentation.\n\n2D seism
 ic reflection data collected in 2005 revealed thick high-amplitude reflect
 ivity zones coinciding broadly with the source areas of SSEs at depths of 
 5 km below the seafloor. Without high-resolution velocity models of this z
 one and an understanding of the type of material that makes up the subduct
 ing plate the interpretation of these high-reflectivity zones is ambiguous
 . Constraining the origin of this reflectivity could provide important clu
 es 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 ima
 ge 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\, 20
 0 onshore seismometers and the collection of 3D seismic reflection data. T
 he 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.\n\nIn Nov-Dec 2017 and March-May
  2018 two International Ocean Discovery Program (IODP) expeditions\, 372 a
 nd 375\, used scientific drilling in New Zealand for the first time to tar
 get slow slip events providing valuable calibration data for seismic model
 s and revealing the lithology of material being subducted. In this talk I 
 will discuss how these expeditions aim to reveal the geophysical environme
 nts of slow slip events and get closer to understanding what factors lead 
 to slow slip. I will discuss preliminary findings regarding the lithostrat
 igraphy and physical properties of the incoming sedimentary section from s
 hipboard measurements and discuss potential implications for the source of
  high-amplitude reflectivity in the slow slip zone.
LOCATION:Marine/Wolfson Building lecture hall\, Bullard Labs.
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